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The atmospheric Green's function method is a technique for modeling the response of the atmosphere to changes in the spatial field of surface temperature. While early studies applied this method to changes in atmospheric circulation, it has also become an important tool to understand changes in radiative feedbacks due to evolving patterns of warming, a phenomenon called the "pattern effect." To better study this method, this paper presents a protocol for creating atmospheric Green's functions to serve as the basis for a model intercomparison project, GFMIP. The protocol has been developed using a series of sensitivity tests performed with the HadAM3 atmosphere‐only general circulation model, along with existing and new simulations from other models. Our preliminary results have uncovered nonlinearities in the response of the atmosphere to surface temperature changes, including an asymmetrical response to warming versus cooling patch perturbations, and a change in the dependence of the response on the magnitude and size of the patches. These nonlinearities suggest that the pattern effect may depend on the heterogeneity of warming as well as its location. These experiments have also revealed tradeoffs in experimental design between patch size, perturbation strength, and the length of control and patch simulations. The protocol chosen on the basis of these experiments balances scientific utility with the simulation time and setup required by the Green's function approach. Running these simulations will further our understanding of many aspects of atmospheric response, from the pattern effect and radiative feedbacks to changes in circulation, cloudiness, and precipitation. Plain Language Summary: Many properties of the atmosphere are affected by the temperature of the ocean surface. Knowing how strong these effects are would help us to better predict global warming. The response to a given surface warming depends on where the warming occurs. To account for this, researchers sometimes simulate the response to individual patches of warming and then assume the response to an arbitrary warming pattern can be summed together from these patch responses. This is sometimes called the atmospheric Green's function method, and it works well at recreating the atmospheric response to historical temperature changes. We are organizing a Green's Function Model Intercomparison Project (GFMIP), in which participants will apply the method consistently for many climate models. This paper presents the GFMIP protocol. In the course of developing this protocol, we found that the atmospheric response to warming is not proportional in all cases: the response to surface warming is not the opposite of the response to surface cooling; warming twice as much doesn't cause twice as much of a response; and making a patch of warming twice as large doesn't cause twice as large a response. GFMIP will help us figure out how to account for this nonlinearity. Key Points: The Green's Function Model Intercomparison Project (GFMIP) explores the atmospheric response to surface temperature patch perturbationsThis paper presents the GFMIP protocol, which was generated using insights from past studies and new sensitivity testsGreen's functions reconstruct the response to historical temperatures, but nonlinearities can affect responses to other warming patterns [ABSTRACT FROM AUTHOR]

Over‐estimation of summer precipitation over the Tibetan Plateau (TP) is a well‐known and persistent problem in most climate models. This study demonstrates the impact of a Gaussian Probability Density Function cloud fraction scheme on rainfall simulations using the Weather Research and Forecasting model. It is found that this scheme in both 0.1° and 0.05° resolutions significantly reduces the wet bias through both local feedbacks and large‐scale dynamic process. Specifically, increased cloud water/ice content with this scheme reduces surface shortwave radiation, and consequently surface heat fluxes and evapotranspiration. This, in turn, dampens the large‐scale thermal effect of the TP and weakens the exaggerated monsoon circulation and low‐level moisture convergence. It is this large‐scale dynamic process that contributes the most (∼70%) to the wet bias reduction. Although this paper presents a modeling study, it highlights the cloud radiative feedback to the large‐scale dynamics and precipitation over the TP. Plain Language Summary: Despite numerous attempts to correct the overestimation of summer precipitation over the Tibetan Plateau (TP) in current global and regional climate models, the issue persists. This study applies the Gaussian Probability Density Function (GPDF) cloud fraction scheme in the Weather Research and Forecasting model at two different resolutions (0.1° and 0.05°) during a summer over the TP. The results show that the GPDF scheme significantly mitigates the precipitation overestimation, particularly in the high‐resolution modeling. We explored the physical processes, both local and remote, that contribute to this improvement. Specifically, an increase in cloudiness reduces the amount of radiation reaching the land surface. This decrease in surface radiative heating not only reduces local evaporation but also weakens the thermal effect of the TP. The latter is a major driver of the South Asian monsoon that conveys moisture to the TP, and its weakening reduces moisture convergence over the TP. Both the decreases in local evaporation and remote moisture convergence contribute to the alleviation of the precipitation overestimation, and the latter plays a dominant role. These findings provide a unique perspective for reducing the wet bias over the TP, focusing on the surface available energy and associated remote moisture processes. Key Points: The use of the Gaussian Probability Density Function cloud fraction scheme in high resolution greatly reduces wet bias over the Tibetan Plateau (TP) during summerMore cloud water/ice with the scheme lessens TP's thermal effect, causing a weaker South Asian monsoon and moisture convergenceWet bias reduction is mainly governed by the decrease in remote moisture rather than local evapotranspiration [ABSTRACT FROM AUTHOR]

The construction of sub‐daily pressure series is described for the cities of London (GB) and Paris (FR). The series extend back 1692 and 1748, respectively, and as such they represent two of the longest sub‐daily series of barometric pressure available. These series are updated from the previously documented London and Paris daily series and offer more homogeneous series, and in the case of the London series a more temporally complete sequence of data. A pairwise homogenization procedure has been applied to the two series alongside the long series of pressure that exists for De Bilt (NL). The De Bilt series has been available for some time in the International Surface Pressure Dataset (ISPD), but further quality control and homogeneity‐checking procedures have been applied to the data in this paper and therefore the three series are released together in this dataset. The series are of immediate interest for understanding changes to storm activity across the English Channel and North Atlantic region over an extended timeframe but may also be assimilated into reanalysis datasets such as the 20th‐century reanalysis. [ABSTRACT FROM AUTHOR]

In this work, we consider a Shallow‐Water Quasi Geostrophic equation on the sphere, as a model for global large‐scale atmospheric dynamics. This equation, previously studied by Verkley (2009, https://doi.org/10.1175/2008jas2837.1) and Schubert et al. (2009, https://doi.org/10.3894/james.2009.1.2), possesses a rich geometric structure, called Lie‐Poisson, and admits an infinite number of conserved quantities, called Casimirs. In this paper, we develop a Casimir preserving numerical method for long‐time simulations of this equation. The method develops in two steps: first, we construct an N‐dimensional Lie‐Poisson system that converges to the continuous one in the limit N → ∞; second, we integrate in time the finite‐dimensional system using an isospectral time integrator, developed by Modin and Viviani (2020, https://doi.org/10.1017/jfm.2019.944). We demonstrate the efficacy of this computational method by simulating a flow on the entire sphere for different values of the Lamb parameter. We particularly focus on rotation‐induced effects, such as the formation of jets. In agreement with shallow water models of the atmosphere, we observe the formation of robust latitudinal jets and a decrease in the zonal wind amplitude with latitude. Furthermore, spectra of the kinetic energy are computed as a point of reference for future studies. Plain Language Summary: We conducted a study on a model that represents the movements of planetary flows. This model has important physical and mathematical properties that are related to its long‐term behavior, which is essential for understanding geophysical turbulence. In this work, we developed a numerical method for simulation that preserves the key mathematical structure of the model through a two‐step process. We applied our method to simulate global atmospheric flow and investigate the impact of varying strengths of planetary rotation. Our findings demonstrate the expected formation of wind patterns known as zonal jets, where stronger winds occur near the equator and weaker winds near the poles. We also present energy spectra that illustrate the influence of planetary rotation on the transfer of turbulent energy, which aligns with existing theoretical predictions found in literature. These results highlight the potential of our numerical method for studying fundamental problems in geophysical fluid dynamics. Key Points: We develop a numerical method preserving Casimirs to simulate balanced shallow water flow on the sphereWe perform global high‐resolution simulations while accurately accounting for latitude‐dependent effectsOur simulations show the formation of robust zonal jets and provide key insights into quasi‐geostrophic turbulence [ABSTRACT FROM AUTHOR]

This study assesses the midsummer precipitation prediction over eastern China by the dynamic downscaling method. Based on the Climate Forecast System version 2 of the National Centers for Environmental Prediction and the Weather Research and Forecasting Model, the prediction performance of global and regional models on the July precipitation over eastern China is further analyzed by hindcast experiments from 1982 to 2010 and prediction experiments from 2011 to 2021. The results suggest that the regional model forced by the global model can noticeably improve the prediction skill for precipitation in eastern China, especially in the region from the South of North China to the Yangtze River Basin, referred as the northern China in this paper. In addition, we perform a diagnostic analysis of the reason for the improvement of the model prediction skill. The results indicate that the high resolution of the regional model and the refinement of physical process parameterizations contribute to improving the simulation ability for the East Asian atmospheric circulation pattern, heat flux, especially for the meridional teleconnection pattern in East Asia and the sensible heat flux in the northern China, thus further improving precipitation prediction. [ABSTRACT FROM AUTHOR]

Observations have revealed that the deadly "hot eastern‐pluvial western" climate pattern in Asia in summer 2022 is by no means an exception. In recent decade this pattern is becoming much more frequent owing to the notable westward extension and enhancement of the western Pacific subtropical high (WPSH). The anomalously enhanced WPSH fills the gap in geopotential heights between the Arabian Sea‐Iranian Plateau and the western Pacific, produces a heat dome over eastern Asia and intensifies the moisture convergence over western Asia by changing the water vapor transportation path. Projections of 19 CMIP6 models show that both the temperature in eastern Asia and precipitation in western Asia will remarkably increase under Shared Socioeconomic Pathway (SSP) 2‐4.5, SSP 3‐7.0, and SSP 5‐8.5 scenarios, with temperature anomalies 0.8°C, 1.5°C, and 2.4°C higher and the percentages of precipitation anomaly 6.3%, 9.4%, and 12.5% larger than the reference period in the near, middle, and long terms under the moderate emission scenario (SSP2‐4.5). Therefore, the "hot eastern‐pluvial western" Asia is becoming a new normal and exacerbating the risk of compound disasters. Plain Language Summary: In summer 2022, eastern Asia experienced the most extensive and long‐lasting heatwave since 1951, while exceptional floods hit western Asia during the same period. It is found that this "hot eastern‐pluvial western" pattern has occurred more and more frequently since 1981. This could be attributed to the linear trend of the westward extension and enhancement of the western Pacific subtropical high. In this paper, we assess the expected changes by using the latest generation of global climate model projections. The "hot eastern‐pluvial western" climate pattern and the associated atmospheric circulations are projected to increase from 2020s to the end of the 21st century under different greenhouse gas emission scenarios, suggesting more fatal heatwaves in eastern Asia and floods in western Asia in the future. Key Points: The catastrophic climate pattern of "hot eastern‐pluvial western" Asia occurred frequently in recent decades especially in summer 2022The increasing trend of this pattern could be attributed to the westward extension and enhancement of the western Pacific subtropical highProjections of CMIP6 models show that the "hot eastern‐pluvial western" will occur normally in future [ABSTRACT FROM AUTHOR]

It is shown that a recently developed hybrid modeling approach that combines machine learning (ML) with an atmospheric global circulation model (AGCM) can serve as a basis for capturing atmospheric processes not captured by the AGCM. This power of the approach is illustrated by three examples from a decades‐long climate simulation experiment. The first example demonstrates that the hybrid model can produce sudden stratospheric warming, a dynamical process of nature not resolved by the low resolution AGCM component of the hybrid model. The second and third example show that introducing 6‐hr cumulative precipitation and sea surface temperature (SST) as ML‐based prognostic variables improves the precipitation climatology and leads to a realistic ENSO signal in the SST and atmospheric surface pressure. Plain Language Summary: This paper introduces and tests schemes for efficiently enabling significant expansion of the utility and scope of a recently introduced hybrid modeling technique that combines machine learning with an atmospheric global circulation model (AGCM). Simulation experiments are carried out with an implementation of the approach on a low resolution simplified AGCM. An examination of the simulated atmospheric circulation suggests that the hybrid model can capture dynamical process not captured by the AGCM. Moreover, the addition of precipitation and sea surface temperature (SST) as machine learning predicted physical quantities to the model improves the precipitation climatology and leads to a realistic El Niño‐La Niña signal in the SST and atmospheric surface pressure. Key Points: A hybrid system combining an atmospheric global circulation model (AGCM) with a machine‐learning component can capture processes not captured by the AGCMMachine learning provides a flexible framework to introduce additional prognostic variables into the hybrid modelThe prototype hybrid model tested in the paper is stable and has a realistic climate in decades‐long simulation experiments [ABSTRACT FROM AUTHOR]

This paper expands on work showing that the winter North Atlantic Oscillation (NAO) is predictable on decadal timescales to quantify the skill in capturing the North Atlantic eddy‐driven jet's location and speed. By focusing on decadal predictions made for years 2–9 from the sixth Coupled Model Intercomparison Project over 1960–2005 we find that there is significant skill in jet latitude and, especially, jet speed associated with the skill in the NAO. However, the skill in the NAO, jet latitude and speed indices appears to be sensitive to the period over which it is assessed. In particular, skill drops considerably when evaluating hindcasts up to the present day as models fail to capture the recent observed northern shift and strengthening of the winter eddy‐driven jet, and more thus positive NAO. We suggest that the drop in atmospheric circulation skill is related to reduced skill in North Atlantic sea surface temperatures. Plain Language Summary: Climate models have the capability to predict the evolution of mean atmospheric circulation over long timescales, from annual to decadal and longer. However, models are overestimating the chaotic, unpredictable component of the climate's variability and, although model predictions follow the observed oscillations of the climate, the strength of these oscillations is critically underestimated. While climate models have demonstrated skill in predicting the North Atlantic Oscillation (NAO), this paper assesses model skill in predicting the evolution of the wintertime North Atlantic eddy‐driven jet, with the aim to highlight how much of the predictive skill of the NAO is derived from accurate predictions of the jet's state. We find levels of skill similar to that for the NAO, with slightly higher skill for the jet's strength (speed) over its location (latitude of maximum speed). We also notice a drop in skill over the last decade, as models fail to capture the latest trends in the NAO and jet's evolution, and suggest that it might be related to degradation in skill at predicting surface temperature variability. Key Points: The winter North Atlantic eddy‐driven jet is predictable on decadal time‐scales with skill in jet speed comparable to that for the North Atlantic Oscillation (NAO)Anomalies in the jet are substantially smaller than expected from skill alone and so suffer from noise overestimationSkill drops significantly in the last decade, as hindcasts do not capture the return to positive NAO conditions post‐2010 [ABSTRACT FROM AUTHOR]

The use of state estimation techniques offers a means of inferring rotor effective wind speed from standard measurements of wind turbines. Typical wind speed estimators rely upon a pre‐computed quasi‐steady aerodynamic mapping, which describes the relationship between pitch angle and tip‐speed ratio and the power coefficient. In practice, the static mapping does not capture the influence of turbine structural dynamics and atmospheric turbulence, inevitably resulting in poor performance of the wind speed estimation. In addition, the turbine aerodynamic properties might not be easily accessible. Thus, this paper presents a rotor effective wind speed estimation method that obviates the requirement for prior knowledge of turbine power coefficients. Specifically, the proposed method exploits a simple actuator disc model, where the aerodynamic power and thrust coefficients can be characterized in terms of axial induction factors. Based on this insight and standard turbine measurements, real‐time estimation of rotor effective wind speed and axial induction factors can then be achieved using a simplified turbine drive‐train model and an extended Kalman filter. In addition, the actuator disc model can be updated easily over time by calibrating solely two correction factors. Thus, the proposed algorithm presents an alternative for estimating the rotor effective wind speed, which is valuable for numerous applications, for example, LiDAR‐assisted control and coherence studies. [ABSTRACT FROM AUTHOR]

This study investigates inherent numerical dissipation due to upwind fluxes and reconstruction strategies for collocated Finite‐Volume integration of the Euler equations. Idealized supercell simulations are used without any explicit dissipation. Flux terms are split into: mass flux, pressure, and advected quantities. They are computed with the following upwind strategies: central, advectively upwind, and acoustically upwind. This is performed for third and ninth‐order‐accurate reconstructions with and without Weighted Essentially Non‐Oscillatory limiting. Acoustic‐only upwinding for pressure and mass flux terms and advective‐only upwinding for advected quantities is the most flexible simplification found. It reduces data movement and computations. Assuming a constant speed of sound in acoustic upwinding gives similar results to using the true speed of sound. Dissipation from upwind adapts automatically to grid spacing, time step, reconstruction accuracy, and flow smoothness. While stability is maintained even at 21st‐order spatial accuracy, there is a limit to the spatial order of accuracy for which upwinding alone can create a realizable solution in the conditions of this study. Convex combinations of upwind and central solutions for flux terms also reduced dissipation, but as the central proportion grows, solutions become physically unrealizable. The range of length scales of the kinetic energy spectra can be extended along k−5/3 to smaller spatial scales by reducing dissipation either with higher‐order reconstructions or using convex combinations of upwind and central fluxes. However, not all extensions of the length scale range along k−5/3 exhibit physically realizable solutions, even though the spectra appear to be physical. Plain Language Summary: Integrating the Euler equations that govern certain regimes of fluid motion forward in time must be performed carefully. Subtle choices in the numerical algorithm can lead to instability or solutions that are unphysical. "Upwinding" is an algorithmic choice where the developer prefers information that comes from the upwind direction, and it is known to lead to smoother and more stable solutions in general. This study explores the effects on stability and physical realism of using different upwind preferences for different variables in the fluxes of fluid properties between adjacent cells in a discrete 3‐D grid. Sound waves are a part of the equations of atmospheric motion. They propagate through time on the edge of an expanding sphere, which makes determining a single "upwind" direction more complex. One can transform the equations into an equivalent form called "characteristic" form, though, that simplifies this definition. That transformation couples the variables together tightly and requires extra computations and data movement across a large parallel machine. This paper shows that there are simplifications to this fully upwind characteristic approach that are just as physically realistic and stable but reduce the number of computations and volume of data transfers significantly. Key Points: There are simplifications to a fully upwind Riemann solver that require fewer computations and less data movementUpwinding according to advective versus acoustic dynamics makes a large difference in stabilityOne can use convex combinations of central and upwind fluxes to obtain physically realizable solutions with less dissipation [ABSTRACT FROM AUTHOR]

The Western Pacific jet stream (WPJS) is an essential part of atmospheric circulation in winter, which significantly influences the weather and climate of the North Pacific and North America. In this paper, the characteristics and mechanism of WPJS subseasonal variation in winter are investigated. The upstream atmospheric disturbances in the East Asian polar‐front jet and subtropical jet merge over the Northwestern Pacific to form the subseasonal variability in WPJS, which has a significant period of 40–60 days. During the positive phase events of subseasonal WPJS, the convergence position of the upstream atmospheric disturbances shifts southwardly accompanied with the local enhancement and eastward extension of subseasonal WPJS. On the other hand, the subseasonal WPJS divides into the southern and northern westerly branches during the negative phase events. By the horizontal propagation of local Eliassen‐Palm fluxes in the upper atmosphere, the northward drift of the upstream atmospheric disturbances convergence dominates the delayed acceleration of the northern upper westerly branch. However, the intensification of atmospheric baroclinicity and upward baroclinic energy caused by the leading strong subtropical frontal zone determine the acceleration of the southern upper westerly branch. Plain Language Summary: As an important part of the atmospheric circulation in the Northern Hemisphere, the Western Pacific jet stream (WPJS) has significant impacts on the climate and weather of the North Pacific and North America. However, previous studies more focused on the interannual or synoptic variability of WPJS. The characteristics and mechanism of the subseasonal variability of WPJS have not been appropriately explained, which are very important for the persistent disastrous weather. In this paper, the subseasonal variability characteristics and mechanism of WPJS in winter are investigated by using the daily average reanalysis data from year 1979 to 2022. About 130 WPJS persistent positive and negative phase events are detected, each of which can sustain more than 20 days. Further results show that the subseasonal variability of WPJS is determined by the synergistic effects of upstream atmospheric disturbances and subtropical frontal zone (STFZ). With the southward (northward) convergence of the strong upstream atmospheric disturbances and weaker (stronger) STFZ, the lagging positive (negative) subseasonal phase events of WPJS is more likely to occur and persist. This study is beneficial to the weather and climate forecasts of North Pacific and North America. Key Points: About 130 subseasonal events of winter Western Pacific jet stream (WPJS) are detected from year 1979–2022, each of which persists more than 20 daysWPJS shows local enhancement and extension during the positive subseasonal phase, but meridional separation in the negative oneThe synergistic effects of the upstream atmospheric disturbances and oceanic subtropical front on the subseasonal WPJS are revealed [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the relationships between soil moisture, cloud cover, solar radiation, air temperature and humidity, and precipitation during the warm half of the year in East‐Central Europe over the years 1971–2020. The temporal and spatial variability of these meteorological elements is presented in association with the occurrence of anticyclonic blocking events over the study area. It demonstrates that changes in soil moisture in East‐Central Europe point to the combined influence of many meteorological factors resulting from the atmospheric circulation, and are an indicator of the comprehensive relationships among those factors. The main factors affecting soil water content are precipitation and evapotranspiration, which in turn depend on air humidity, cloudiness, intensity of solar radiation and air temperature. The increase in the frequency and duration of sequences of days with blocking events in East‐Central Europe has contributed to an increased probability of longer periods with soil moisture negative anomalies. [ABSTRACT FROM AUTHOR]

From the beginning of the 21st century, the heat waves in Europe have become more frequent, severe and longer lasting, especially in the southern region. The longer duration of heat waves may be partly attributed to wave‐amplification, high‐amplitude waves in the mid‐latitudes, favouring the longer‐lasting blocking systems. The primary objective of the paper was to examine the long‐term changes of hot days and heat wave events and to identify the atmospheric circulation condition that prevails its occurrence. The other object was to investigate the correlations between hot days' occurrence in Poland and sea‐ice cover and sea surface temperature changes in the Arctic and its surroundings. Over the past few decades, Central and Southern Poland have experienced a considerable rise in maximum temperatures, which has led to an increase in the number of hot days. This increase is particularly prominent during the summer months (June, July and August) but also in May and September, which extends the hot season. Although heat waves have become more frequent and persistent since 2006, their mean intensity has remained unchanged. During all heat waves in Poland, the high‐pressure centre was located over the country or its close neighbourhood, mainly in the eastern sector but also in the south, southwest or over Scandinavia. The blocking systems were mainly related to the extended ridge of the Azores High, but also to strong blocking over the North Pole and Greenland reaching Scandinavia or anomalous high pressure in the East Arctic. The longest‐lasting heat waves were usually preceded by air masses advection from the Atlantic, but the highest potential temperature along the trajectories was observed for the southern direction of inflow. The growing number of hot days is statistically significantly related to the decreasing sea‐ice cover and increasing sea surface temperature in Arctic and Subarctic regions. [ABSTRACT FROM AUTHOR]

The Beaufort Sea High is a high‐pressure system located in the Beaufort Sea that influences ocean circulation in the western Arctic known as the Beaufort Gyre. Wrangel Island, located in the western Chukchi Sea, typically experiences easterly sea ice motion due to the Beaufort Gyre. We find that under these climatological conditions, moving ice is blocked by the island and accumulates on its eastern side, while ice on its western side continues to drift leaving an area of open water and reduced ice thickness along the western side of the island. This results in an ice thickness dipole across the island. A reversal in the atmospheric circulation across the western Arctic results in a dipole with the opposing sign. We find the dipole is present throughout the year and is strongest in January when the difference in ice thickness between the eastern and western sides of the island is approximately 1 m. During the spring, it is associated with the transient opening of a polynya to the west of the island. The dipole is the result of opposing ice divergence and convergence across the western Arctic and may impact ocean circulation and ecosystems within the Chukchi Sea. Plain Language Summary: The Arctic climate is rapidly changing with many important environmental consequences prompting increased scientific study. In this paper, we examine the interaction of drifting sea ice and Wrangel Island, which is located in the western Arctic Ocean's Chukchi Sea. We find that ice typically drifts from the east and is blocked by the island causing the ice to accumulate on its eastern side. Ice continues to drift away from the western side of the island, thus a contrast or "dipole" in ice thickness is created. This dipole is present throughout the year and is strongest in January when the ice thickness difference is around 1 m. We find that the formation of the dipole is associated with strong westward winds at the island. These winds are associated with a strong high‐pressure system called the Beaufort Sea High that results in anti‐cyclonic winds, ice motion, and surface currents throughout the region. A reversal from easterly to westerly winds results in a dipole of opposing sign. The dipole is associated with ice thickness anomalies across the western Arctic and may impact ocean circulation and ecosystems across the region. Key Points: Ice motion in the Beaufort and Chukchi Seas can result in a sea ice thickness dipole in the vicinity of Wrangel IslandA reversal in wind direction and sea‐level pressure anomalies across the western Arctic Ocean can cause the signal of the dipole to reverseThe dipole is the result of ice motion divergence and convergence across the region and may impact ocean circulation and ecosystems [ABSTRACT FROM AUTHOR]

The frequency of flash floods resulting from heavy rainfall over West Africa has increased in recent years with serious socio‐economic consequences. Therefore, the need to utilize numerical weather prediction models to forecast heavy rainfall events reliably is also rising at many operational meteorological centres in West Africa. This paper evaluates the performance of the Consortium for Small‐scale Modelling (COSMO) model of the German Meteorological Services (DWD) in predicting rainfall over West Africa for high‐impact rainfall events that occurred between 19 and 26 August 2017. The paper aims to investigate the synoptic forcings modulating daily rainfall variability during that period. Results show that COSMO simulates adequately the spatio‐temporal variability of rainfall distribution over West Africa, though with inherent biases. COSMO displays a decreasing skill in producing spatial rainfall distribution as rainfall amounts tend to 30 mm and above. Additionally, areas of heavy rainfall, mostly about 100–300 km southwest of the core of the Africa Easterly Jet (AEJ), often coincide with areas of decreasing mean sea level pressure of at least 0.6 hPa and areas of increasing convective available potential energy of at least 500 J/kg. Although not in all cases, the trough of the Africa Easterly Wave (AEW) is always located to the east of these areas. We show that not every storm, especially east of the prime meridian, is associated with an AEW trough. COSMO is able to reproduce the atmospheric dynamics modulating the daily rainfall variability, in addition to capturing the daily propagation of the AEW trough, and the core of the AEJ. However, the reproducibility skill of the model in predicting atmospheric dynamics may not transform into the predictive skill of the model in producing rainfall. Nevertheless, operational forecasters may be able to determine likely areas of heavy rainfall by estimating the position of the AEJ core based on the position of areas of the least falling pressure from COSMO. Finally, the incorporation of the fractions skill score metric based on the neighbourhood approach could also assist operational forecasters to decide at which scale a severe weather alert can be issued. [ABSTRACT FROM AUTHOR]

In this study, we obtained the first leading mode and principal component 1 (PC1) of North Pacific sea surface temperature (SST). The PC1‐related SST anomalies, located relatively north/south, are referred to as North/South PC1 events. Model outputs, observations, reanalysis datasets and sensitivity experiments show that during the North PC1 events, an enhanced Aleutian low occurs and is located relatively north, favoring anomalous southerlies over western North America. The anomalous southerlies induce a strong warming anomaly over North America via warm advection of the anomalous southerlies and temperature advection of the climatological westerlies over North America. However, the Aleutian low anomaly and corresponding southerly anomaly associated with South PC1 events shift southward, favoring weakened effects of South PC1 events on atmospheric circulations and surface temperatures over North America. The meridional position of PC1 events deserves to be considered in the studies of the PC1 and its related climate changes. Plain Language Summary: Understanding of surface temperature variations is important since it largely affects human and society. This paper categorizes the North Pacific sea surface temperature anomaly events, which are associated with the first leading mode of North Pacific sea surface temperature, into North and South events according to their meridional positions. Using model outputs, observations, reanalysis datasets and sensitivity experiments, we found that the surface temperature anomalies over North America during the North events are quite different from those during the South events. During the North events, an enhanced Aleutian low occurs over high‐latitude North Pacific. The enhanced Aleutian low tends to induce anomalous southerlies over western North America. The anomalous southerlies favor warming anomalies over North America via the warm advection of anomalous southerlies and temperature advection of climatological westerly. However, the Aleutian low anomaly and meridional wind anomaly during the South events shift southward, leading to weakened effects of the South events on atmospheric circulations and surface temperatures over North America. Key Points: North American warming anomaly occurs during North Pacific sea surface temperature anomaly events located northThe warming anomaly is related to an enhanced Aleutian low anomaly located north and climatological westerlyDuring the events located south, North American surface temperature responses are weak due to a southward‐shift Aleutian low anomaly [ABSTRACT FROM AUTHOR]

We investigated statistically the orientation of about 10,000 auroral arcs observed during 2016–2021 in Abisko (68.36°N, 18.81°E, Sweden) in the equatorward part of the nightside auroral oval. The observations were made between 19 and 06 magnetic local time (MLT). On average, the orientation of the arcs, that is, the angle between an arc and the West‐East direction, linearly changed with MLT at the rate 2.2°/hr. In most cases the mean orientation of the auroral arcs near midnight was parallel to the geomagnetic latitude of Abisko, except a few late winter or spring months. These anomalies cannot be explained by geomagnetic disturbances or interplanetary conditions. They do, however, coincide with spring transitions in the atmospheric circulations from winter‐to summer‐type. The spring transitions were manifested in the data of a meteor radar in the region of observations as reversals of the zonal wind at altitudes 90–100 km from eastward to westward. We propose that these spring transition changes in the global atmospheric circulation couple also the overlying thermosphere at 100–150 km where neutral winds may affect the ionosphere and subsequently the whole ionosphere‐magnetosphere system. Then, changes in the coupled ionosphere‐magnetosphere system are manifested in changes of the orientation of auroral arcs. Plain Language Summary: Aurora is a manifestation of the precipitation of charged particle of the magnetospheric origin into the Earth's ionosphere. The most prominent auroral forms populating in the auroral oval are relatively narrow multiple auroral arcs extended for many hundred kilometers within the oval. In the past a number of studies were made to understand how auroral arcs are oriented with respect to the East‐West direction in different local time and what may affect this orientation. It is commonly believed that the orientation of the arcs primary depends on the solar wind conditions and geomagnetic activity. However, precise analysis of about 10,000 auroral arcs observed by the optical camera located in the Northern Europe during a 6‐year period of 2016–2021 has revealed anomalous orientations of the arcs which cannot be explained by interplanetary or solar conditions. Unexpectedly we have found that the global atmospheric circulation apparently affects orientation of auroral arcs. This effect, which has been never reported before, is the main subject of the present paper. Key Points: Orientation of nightside auroral arcs may be affected by the neutral windThe effect is noticeable during the spring transition in the global atmospheric circulationThe neutral wind can influence the ionosphere‐magnetosphere system through the ionosphere dynamo processes [ABSTRACT FROM AUTHOR]

The international Atmospheric Circulation Reconstructions over the Earth (ACRE) initiative works to recover global climate history and build an accessible databank, with data from the past 250 years of terrestrial and marine surface. Argentina is part of that effort with ACRE Argentina recovering data from various sources that have been recorded throughout the country since the 19th century. In this paper, weather observations transcribed from the original records into digital form and taken every day during the years 1860–1879 at Bahía Blanca and from 1873–1886 at Corrientes are presented. The digitization was carried out through the project Meteorologum ad Extremum Terrae (MET) launched in the Zooniverse interface, which is currently working with near 900 citizen scientist volunteers per day. The present analysis corresponds to data retrieved from the collection 'Anales de la Oficina Meteorológica Argentina' and contains information on atmospheric pressure, air temperature, relative humidity, cloud cover, cloud types, wind direction, wind speed, rainfall and weather remarks. For the present analysis, only temperature and pressure values, measured in °C and mm of Hg, respectively, were considered, after a quality control of the digitization process was applied. Data values were tested and used to rebuild the time series of both places and correlation with SOI index and monthly pressure values for both places was tested using Spearman correlation. Results show that the influence of El Niño episode during 1877–1878 can be found in the pressure values at Corrientes. [ABSTRACT FROM AUTHOR]

Equatorial Vortex Experiment (EVEX), the EVEX mission of May 2013, carried out from Roi‐Namur, Kwajalein Atoll, explored the initiation physics of equatorial spread‐F with a pair of rockets launched simultaneously during local sunset to apogees of 182 and 331 km, respectively. EVEX measurements presented in this paper establish the existence of a differential zonal flow between plasma and neutrals at bottom‐side F‐region heights during the sunset time in support of a post‐sunset spread‐F initiation mechanism described in Kudeki et al. (2007) that is characterized by large growth rates for oblique‐propagating 10's‐of‐km‐scale electron density perturbations. F‐region plasma drifts were measured insitu using an electric field double‐probe instrument while neutral winds were measured by triangulating drifting vapor clouds. The state of the ionosphere and spread‐F irregularities were monitored by ground based ARPA Long‐Range Tracking and Instrumentation Radar and Illinois Radar Interferometer System radars using incoherent and coherent scatter techniques, respectively, as well as an ionosonde. Plain Language Summary: Understanding how the upper atmosphere and ionosphere vary with altitude at low latitudes represent a long standing goal of the space physics community. Importantly, how the motions of atmospheric and ionospheric gases change with altitude and local time at sunset at low latitudes is believed to be the key to understanding why the subsequent night‐time low‐latitude ionosphere becomes unstable. Using two simultaneous sounding rockets, a satellite pass, ground based radars, and an ionosonde, we show evidence for a vortex‐like pattern of the ionospheric motion at sunset, as well as shears in upper atmosphere motions or winds. We also provide key evidence from the measurements for the generation of post‐sunset disturbances and plasma depletions in the equatorial ionosphere caused by differential flows of the neutral and ionized gases during the sunset period. Key Points: Radar backscatter observations monitor the ionospheric density irregularities developing in pre‐ and post‐sunset equatorial E and F regionsTwo rockets launched into the equatorial ionosphere at sunset collect measurements of E‐ and F‐region plasma drifts and neutral windsLinks between the generation of post‐sunset F‐region turbulence and sunset time F‐region winds within the plasma drift vortex are examined [ABSTRACT FROM AUTHOR]

In this paper, the authors quantitatively investigated the joint impact of the Arctic Oscillation (AO) and El Niño‐Southern Oscillation (ENSO) on vegetation net primary productivity (NPP) over Indo‐Myanmar in boreal winter from 1981 to 2018, and found that there is a significant in‐phase variation between them. When the warm ENSO co‐occurs with the positive AO, the NPP in more than 80% of the grids in the study region significantly increases by approximately 24 gC m−2. For the cold ENSO plus the negative AO, the regionally averaged NPP anomalies are approximately −10 gC m−2, and the local minimum is as low as −60 gC m−2. The combination of AO and ENSO can explain approximately 36% of the total variance of NPP in Indo‐Myanmar. The AO/ENSO linkages to the NPP are very likely due to regional precipitation anomalies through atmospheric circulation. In association with the positive (negative) AO, the Rossby wave, propagating eastward along the subtropical jet stream, brings an anomalous cyclone (anticyclone) over Indo‐Myanmar. This enhances (weakens) Indo‐Myanmar Trough, and resulting in more (less) regional precipitation. During the warm (cold) ENSO, through the Gill‐type response, an anomalous high (low)‐pressure appears in the lower and middle troposphere over Philippine‐South China Sea. The Walker circulation is also weaker (stronger) than normal. These are conducive to anomalous southerly (northerly) winds in Indochina. As a result, in the warm ENSO plus positive AO winters, the mean precipitation anomaly in Indo‐Myanmar increased by 27% as averaged for five data sets. In contrast, when the cold ENSO co‐occurs with negative AO, the precipitation is significantly reduced where the largest anomaly exceeds 100 mm in the ERA5. The precipitation changes are consistent with the local NPP anomalies, whereas the temperature does not seem to be a dominant limiting factor. Plain Language Summary: Vegetation is closely dependent on regional climatic conditions such as temperature and precipitation, whereas these meteorological elements are sensitive to large‐scale climate factors. Therefore, it is necessary to explore the vegetation drivers and underlying physical mechanisms. In this research, we focused on investigating the joint impact of the Arctic Oscillation (AO) and El Niño‐Southern Oscillation (ENSO) on vegetation net primary productivity (NPP) over Indo‐Myanmar in boreal winter from 1981 to 2018, the results show that the combination of AO and ENSO can explain approximately 36% of the total variance in regional NPP. The AO/ENSO linkages to the NPP are very likely due to regional precipitation anomalies through atmospheric circulation. In association with the positive (negative) AO, an eastward Rossby wave propagating along the subtropical jet stream brings an anomalous cyclone (anticyclone) over Indo‐Myanmar. During the warm (cold) ENSO, a southerly (northerly) wind anomaly prevails over Indochina via the Gill‐type response and weaker (stronger) Walker circulation than normal. As a result, when the warm (cold) ENSO co‐occurs with the positive (negative) AO winters, Indo‐Myanmar could experience more (less) precipitation than normal, which can lead to more NPP here. Key Points: There is a significant and robust in‐phase variation between the Arctic Oscillation (AO)/El Niño‐Southern Oscillation (ENSO) and net primary productivity (NPP) over Indo‐Myanmar in boreal winterThe combination of AO and ENSO can explain 36% of the total variance in NPPPrecipitation plays a critical bridge in teleconnection between AO/ENSO and local NPP and explains nearly 40% of the total variance in NPP [ABSTRACT FROM AUTHOR]

This paper examines the changes of El Niño–Southern Oscillation (ENSO) associated winter rainfall relationship in Southeast Asia (SEA) in the historical period (1981–2010) and future period (2071–2100) based on the Coupled Model Intercomparison Project Phase 6 (CMIP6) models simulations. Twenty general circulation models (GCMs) are chosen from the Scenario Model Intercomparison Project (ScenarioMIP) highest emission scenario—Shared Socioeconomic Pathway (SSP) 5‐8.5. This study focuses only on the GCMs that have been able to replicate the consistent associated patterns observed, referred to as "teleconnection‐consistent models" (TCMs). During historical period, ENSO has significant negative association with SEA winter rainfall, particularly over Indochina mainland, Maritime Continent (MC) and northern Borneo. The significant negative‐associated teleconnection affected area has increased under future climate, particularly over southern part of South China Sea (SCS). The weakening of south westerlies at Western North Pacific (WNP) centred near 20°N reduces the moisture transport from Indochina into south of China, increase the negative correlated affected area in the SEA regions above equator. The northeasterlies/easterlies that prevailed from the WNP anticyclone (AC) brings significant moisture flux into southern Borneo, causing increment of significant positive‐associated moisture flux convergence to that region. The weakening of upward motion in TCM results in less warm and moist air rising over SEA MC, reducing the rainfall availability. The 1° eastward expansion of WC downward branch eastern edge leads to suppression of convective activities, increasing the dry affected area. The results suggest that the ENSO has a larger influence on the negative‐associated SEA rainfall and atmospheric circulation in the December–January–February (DJF) late‐21st century when compared to the 1980–2010. [ABSTRACT FROM AUTHOR]

Recent climate and environment over the Tibetan Plateau (TP) have undergone significant changes, dominated by variations in the Westerlies and the Indian summer monsoon. However, long‐term shifts in atmospheric circulation during the transitional seasons are still lacking. Here we investigate the modern distribution of Tsuga pollen over the central‐western TP and confirm it as an indicator of variable atmospheric circulation in spring. By combining our Tsuga record from Taro Co with existing records in the Tsuga pollen source area, we suggest that a potential particle transport pathway from the southern slope of the Himalayas to the interior of the plateau appeared in the spring of the late Holocene. Our results show that the springtime atmospheric circulation over the southwestern TP during the early and late Holocene is closely related to the substantial remnants of ice sheets at northern high latitudes and the frequency of El Niño events, respectively. Plain Language Summary: The spring climate over the Tibetan Plateau (TP) is undergoing significant changes that yield profound impacts on environmental changes such as variations in vegetation phenology and alpine treeline. Knowledge of long‐term variations in atmospheric circulation during spring can improve the understanding of current climatic and environmental change and the projection of future variability. In this paper, we use an exotic pollen grain, which can be transported long distances in the air, as an indicator to trace the variability of spring atmospheric circulation over the TP. The results indicate that the spring meridional atmospheric circulation from the southern slope of the TP to its interior has been enhanced in the last four thousand years, which is mainly influenced by the frequency of El Niño events. Key Points: Spring atmospheric circulation change over the southwestern Tibetan Plateau (TP) during Holocene is reconstructedMeridional atmospheric circulation in spring over the southwestern TP is strengthened in the late HoloceneHigh‐(low‐) latitude forcing mainly influence the variations in spring atmospheric circulation during the early (late) Holocene [ABSTRACT FROM AUTHOR]

Black carbon (BC) absorbs solar radiation, increasing the buoyancy and vertical ascent of absorbing aerosol in the atmosphere. This self‐lofting process has been observed for individual plumes in the troposphere and lower stratosphere but here we show it occurring at broader scales through enhanced large‐scale ascent over BC‐rich regions. This is demonstrated in a pair of simulation using the UKESM1 Earth‐System model where BC aerosols were modeled either with or without the ability to absorb radiation. With absorption included the annual global mean concentration of BC in the upper troposphere and lower stratosphere (8–22 km) rose by up to 50% and the column loading over some remote oceanic regions more than doubled. The increase in aerosol height was particularly notable over the southeast Atlantic where biomass burning aerosol from Africa was elevated up to 1 km higher when their absorption was included. Similar effects were seen over the Arctic where the absorbing haze was transported in at higher levels and surface concentrations were halved. The absorption by BC also increased ascent over southern Asia, which tended to thicken the Asian brown cloud during the dry season but in the wet season enhancing ascent promoted deep convection and had the tendency to deplete the aerosol through wash‐out. We conclude that representing aerosol absorption accurately is important in simulating the vertical distribution, transport and abundance of aerosol in the Earth‐system that will affect their interactions with climate. Plain Language Summary: Black carbon is an important component of atmospheric aerosols as it absorbs solar radiation thereby heating the atmosphere and potentially warming climate. The localized heating can also affect clouds and atmospheric motions making the regional and climate effects more complex and uncertain. The vertical distribution and geographic spread of the aerosol is also key to how it interacts with the climate system. In this paper we highlight the fact that the absorption taking place in BC aerosols can affect its vertical ascent in the atmosphere and therefore how it becomes distributed across the globe. This self‐lofting process was demonstrated in a climate model and was particularly important in aiding the elevation and downwind transport of absorbing smoke layers from regions such as central Africa, as well as increasing the amount of BC reaching the upper troposphere and lower stratosphere. Key Points: Absorption of solar radiation helps to raise the altitude of black carbon aerosol by increasing buoyancy and vertical ascentThis self‐lofting mechanism has been captured in a climate model where radiative heating affects the large‐scale ascent and circulationSelf‐lofting increased the altitude and long‐range transport of black carbon to remote regions and the upper troposphere [ABSTRACT FROM AUTHOR]

Sea ice is an important and complex component of the Earth system and is considered a sensitive indicator of climate change. The seasonal sea ice cycle regulates the exchange of heat and salinity, altering the energy balance between high and low latitudes as well as the ocean and atmospheric circulation. The accurate representation of Antarctic sea ice has been considered a hot topic in the climate modelling community and lacks conclusive answers. In this paper, we evaluated the ability of 11 climate models from Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6) to simulate the sea ice seasonal cycle in Antarctica in terms of area (SIA) and concentration (SIC), as well as the improvements in the most recent models' version, submitted to CMIP6. The results indicated that all models are able to accurately capture the seasonal cycle of the Antarctic SIA, with the minimum (maximum) occurring in February (September). In the Weddell Sea, Amundsen Sea, Bellingshausen Sea, and the Ross Sea, the simulated sea ice concentration revealed a large and systematic bias in February when compared to observations. In September, a large and systematic bias was found nearby the Southern Ocean's northern limit in the Polar Front. Several CMIP6 models exhibited slight improvements on the SIA and SIC estimate over the previous version (CMIP5). All models indicated a significant sea ice loss in the coming years as a response to CO2 forcing. Despite the advancements in the sea ice representation, our findings show that the models are still unable to accurately represent the regional sea ice changes [ABSTRACT FROM AUTHOR]

This paper describes the first implementation of the Δx = 3.25 km version of the Energy Exascale Earth System Model (E3SM) global atmosphere model and its behavior in a 40‐day prescribed‐sea‐surface‐temperature simulation (January 20 through February 28, 2020). This simulation was performed as part of the DYnamics of the Atmospheric general circulation Modeled On Non‐hydrostatic Domains (DYAMOND) Phase 2 model intercomparison. Effective resolution is found to be ∼6× the horizontal dynamics grid resolution despite using a coarser grid for physical parameterizations. Despite this new model being in an immature and untuned state, moving to 3.25 km grid spacing solves several long‐standing problems with the E3SM model. In particular, Amazon precipitation is much more realistic, the frequency of light and heavy precipitation is improved, agreement between the simulated and observed diurnal cycle of tropical precipitation is excellent, and the vertical structure of tropical convection and coastal stratocumulus look good. In addition, the new model is able to capture the frequency and structure of important weather events (e.g., tropical cyclones, extratropical cyclones including atmospheric rivers, and cold air outbreaks). Interestingly, this model does not get rid of the erroneous southern branch of the intertropical convergence zone nor the tendency for strongest convection to occur over the Maritime Continent rather than the West Pacific, both of which are classic climate model biases. Several other problems with the simulation are identified, underscoring the fact that this model is a work in progress. Plain Language Summary: This paper describes the new global 3.25 km version of the Energy Exascale Earth System Model (E3SM) atmosphere model and its behavior in a 40‐day northern‐hemisphere wintertime simulation. In exchange for huge computational expense, this high‐resolution model avoids many but not all biases common in lower‐resolution models. It also captures several types of extreme weather that would simply not be resolved in lower‐resolution models. Several opportunities for further development are identified. Key Points: Describes the Simple Cloud‐Resolving E3SM Atmosphere Model (SCREAM)SCREAM performs well in a 40‐day boreal winter simulation at 3.25 km ΔxResolving deep convection solves many long‐standing climate model biases [ABSTRACT FROM AUTHOR]

This study further evaluates the modeling approach of Jia et al. (2019), https://doi.org/10.1029/2019gl081960 (JEA19) to investigate the potential effects of mesoscale sea surface temperature (SST) variability on the atmospheric circulation. The approach employs a global atmospheric circulation model coupled to a slab ocean model to produce two ensembles of simulations: one in which the SST exhibits realistic mesoscale variability, and another in which the mesoscale SST variability is suppressed. The results of the present study suggest that the modeling approach yields the desired SST differences between the two ensembles at the mesoscales. They also show, however, that the approach can lead to undesirable SST differences at the large scales, if the prescribed pair of oceanic heat flux fields have large scale differences. In the experiments of JEA19, such forced, large scale SST differences dominate over the large scale differences that may develop in response to changes in the large scale atmospheric circulation by nonlinear interactions with the SST. This result suggests that finding a proper pair of estimates of the oceanic heat flux fields is necessary for the investigation of the potential upscale impact of mesoscale SST variability on the atmosphere by the approach of JEA19. The paper concludes with proposing a potential improved strategy to obtain such a pair of estimates. Plain Language Summary: This study evaluates a modeling approach to investigate the potential effects of mesoscale (scales smaller than 100 km) ocean sea surface temperature (SST) variability on the atmospheric circulation. The approach employs a global atmospheric circulation model coupled to a thermodynamical model of the ocean. Two ensembles of model simulations are prepared: one in which the ocean exhibits realistic mesoscale SST variability, and another in which that variability is suppressed. The results suggest that the modeling approach yields the desired mesoscale SST differences. They also show, however, that the approach can lead to undesirable large scale SST differences, if there are large scale differences between the prescribed oceanic heat transport fields of the two ensembles. In the experiments of JEA19, such forced, large scale SST differences dominate over the large scale differences that may develop in response to changes in the large scale atmospheric circulation by two‐way interactions with the SST. This result suggests that finding a proper pair of estimates of the oceanic heat transport fields is necessary for the investigation of the potential upscale impact of mesoscale SST variability on the atmosphere by the approach of JEA19. The paper concludes with proposing a potential improved strategy to obtain such a pair of estimates. Key Points: The large scale response in an uncoupled atmospheric simulation is the response of the model rather than that of the atmosphereA coupled modeling approach is necessary to investigate the large scale atmospheric response to mesoscale sea surface temperature variabilityA pair of oceanic heat fluxes that do not have large scale differences is critical to produce a realistic large scale atmospheric response [ABSTRACT FROM AUTHOR]

The PM2.5 (fine particulate matter with diameter ≤2.5 μm) concentration anomalies over the North China Plain (NCP) in early and late winter sometimes show a subseasonal reversal, which brings a great challenge for precise control of air pollution, and mechanisms are not well understood. This paper reveals the key role of the Barents Sea sea‐ice in this reversal. In early winter, a negative Scandinavian‐like pattern causes an anticyclonic anomaly over Northeast Asia and thus leads to the positive PM2.5 concentration anomaly over the NCP. In addition, anomalous warm advection associated with the positive North Atlantic Oscillation‐like pattern accelerates winter sea‐ice loss in the Barents Sea, especially in late winter, which increases the surface turbulent heating flux. These heating causes a negative Polar/Eurasian‐like pattern that induces a cyclonic anomaly over Northeast Asia and eventually leads to a negative PM2.5 concentration anomaly over the NCP in late winter. Vice versa. Plain Language Summary: In this study, we find that there is a significant seesaw pattern of PM2.5 concentration anomalies over the North China Plain (NCP) in early and late winter. Further results reveal that variations in Barents Sea sea‐ice play a crucial role. More specifically, a negative (positive) Scandinavian‐like pattern in early winter tends to induce an anticyclonic (a cyclonic) anomaly over Northeast Asia. This anticyclonic (cyclonic) anomaly leads to a higher (lower) PM2.5 concentration anomaly over the NCP in early winter. In addition, a North Atlantic Oscillation‐related warm (cold) temperature advection promotes (inhibits) the sea‐ice loss in the Barents Sea, especially in late winter. The Barents Sea‐ice loss (increment) in late winter tends to heat (cool) the lower‐level atmosphere and thus triggers an anticyclonic (a cyclonic) response in the upper troposphere there. The anticyclonic (cyclonic) response in the Barents Sea region as a part of the negative (positive) Polar/Eurasian‐like pattern induces a cyclonic (an anticyclonic) anomaly over Northeast Asia, which leads to a lower (higher) PM2.5 concentration anomaly over the NCP in late winter. These findings could be of great value for the subseasonal predictions of PM2.5 concentrations over the NCP in winter. Key Points: PM2.5 concentration anomaly over the North China Plain shows an visibly subseasonal reversal in early and late winter on interannual scaleThe Scandinavia‐like pattern and Polar/Eurasian‐like pattern are responsible for the seesaw pattern of PM2.5 concentration anomaliesThe sea‐ice variations in the Barents Sea play a key role in modulating such atmospheric circulations [ABSTRACT FROM AUTHOR]

Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary: This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points: A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening [ABSTRACT FROM AUTHOR]

As a crucial component of the climate system, snow cover plays an important role on surface energy budgets, hydrological cycles and socioeconomic development. This paper investigated the spatiotemporal patterns of snow depth across the Qinghai Plateau (QP) during 1980–2018 based on passive microwave (PMW) satellite observation and reanalysis products. The study identified the relationships between the changes in PMW snow depth and topographic features and revealed the impact of climatic variables (air temperature, precipitation and wind speed) and large‐scale atmospheric circulations on observed snow depth variability. The results show that the spatial pattern of snow depth climatology is similar in all datasets, except in terms of magnitude, with high values in the southern and southeastern parts of the QP and low values in the eastern and northwestern parts. Average snow depth correlates positively with precipitation, elevation and slope, and negatively with air temperature and wind speed. The long‐term trends in snow depth vary with the season and the datasets. The PMW snow depth across the QP shows a significant annual (−0.125 cm·decades−1, p <.10) and spring (−0.184 cm·decades−1, p <.05) negative trend, while snow depth for ERA5 and MERRA2 does not present significant trends. Air temperature dominates total snow depth variation over the QP, explaining 43.85, 24.88 and 47.28% of annual, winter and spring PMW snow depth variations, which significantly affects snow depth variations in most parts of the QP, yet the effects of precipitation and wind speed on snow depth variation exhibit significant regional differences, and atmospheric circulations (e.g., AMO) also have a remarkable controlling effect on some localized areas. [ABSTRACT FROM AUTHOR]

Monitoring and quantifying high‐latitude atmospheric circulation variability and trends are important towards understanding the anatomy of extreme events and constraining their probabilities under continued Arctic change. The greater Alaska region stands out as one region of enhanced warming and environmental changes over the Arctic amplification era, which has shown proclivity to extreme events and speaks to the need to consistently monitor overlying atmospheric variability. In this paper, we describe the creation and analysis of an updated, daily mid‐tropospheric Alaska Blocking Index (ABI) time series from 1948 to 2020. Sensitivity testing and index modifications from the initial dataset are discussed, and the new ABI is evaluated over the full period and subperiods. Additionally, we assess the role of the ABI as a regional climate indicator by analysing its variability against surface air temperature and precipitation from 13 climate divisions across Alaska as well as broader‐scale hemispheric temperatures. Months of the highest ABI means mainly transpired in the most recent climatological period, 1991–2020, and most notably during the spring and autumn transition seasons, reflecting the nonstationary nature of the jet stream magnitude through time atop Alaska. ABI trends are positive across all months, seasons, and annually only in the 1991–2020 period. Contemporaneous ABI values are strongly and positively correlated with air temperature across all Alaskan climate divisions during the warm season, but negatively correlated with winter temperatures over southeastern Alaska. Meanwhile, the ABI is significantly anticorrelated with summer precipitation over Interior and southern Alaska, but only over southeastern Alaska in other seasons. The ABI is statistically differentiated from the primary modes of atmospheric variability as shown by its generally weak correlations with the Arctic Dipole and the Arctic Oscillation (r ≤ 0.40). [ABSTRACT FROM AUTHOR]

Clouds' susceptibility to aerosols is considered to be a leading source of uncertainty in climate research. An inability to account simultaneously for the large range of scales involved in cloud‐aerosol‐climate interactions has hindered the progress of research. In this study, using a novel system of idealized large‐eddy‐simulations that explicitly resolves clouds but also accounts, in an idealized manner, for large‐scale changes in the thermodynamic and dynamic conditions and for inter‐cloud regime coupling, it is shown that aerosol perturbation in the sub‐tropics increases cloudiness in the tropics. Specifically, aerosol‐driven sub‐tropical rain suppression leads to increased advection of cold and moist air from the sub‐tropics to the tropics, thus enhancing tropical cloudiness. The increased tropical cloudiness has a strong cooling effect by reflecting more of the incoming solar radiation. The classical "aerosol‐cloud lifetime effect" is shown here to have a small local effect (in the sub‐tropics) but a strong remote effect (sub‐tropical aerosols increase cloudiness in the tropics), thus widening the concept of cloud adjustments to aerosol perturbation. Plain Language Summary: Challenges in understanding the link between clouds, aerosols and the atmospheric circulation hindered the progress in climate‐change research. In the tropics, different cloud regimes are formed at slightly different locations and the different regimes are connected by the tropical circulation. Thus, aerosol (which serve as cloud condensation nuclei) effects on one cloud regime may also affect the properties of different regimes. In this paper a novel setup of idealized simulations is developed to study the interactions between different cloud regimes and the aerosol effect on them. Based on these idealized simulations we demonstrate that by suppressing rain in the sub‐tropics, aerosols could enhance tropical cloudiness. The enhanced tropical cloudiness results in a net local cooling effect due to a larger shortwave reflection at the top of the atmosphere. These results suggest that aerosol‐cloud‐interactions should be examined under broad spatial‐temporal scales to account for the fact that aerosol perturbation in one location could affect the cloud properties in different, remote locations due to variations in the large‐scale circulation. Key Points: A new large‐eddy simulation setup is developed to investigate tropical‐sub‐tropical couplingSub‐tropical aerosols could enhance tropical cloudiness by suppressing rain and affecting the water and energy fluxesAerosol‐cloud‐interactions should be examined under broad spatial‐temporal scales to account for inter‐cloud regime connections [ABSTRACT FROM AUTHOR]

To assess the effect of ocean‐atmosphere coupling in the climate response to forced sea ice loss, the Polar Amplification Model Intercomparison Project protocol includes centennial coupled atmosphere‐ocean general circulation model simulations with imposed sea ice loss. The protocol, which specifies sea ice concentration and thickness distribution targets, does not prescribe a method for achieving them. Although different methods for imposing sea ice loss (or growth) in models have been documented, testing of the method‐dependence of the resulting climate responses has been limited. Achieving the targeted sea ice state has proven to be challenging using the so‐called ghost‐flux nudging method, which induces ice melt from below, as this method does not constrain the partitioning between thickness and concentration. We propose, describe and test a simple method that combines the advantages of direct sea ice nudging and ghost‐flux nudging. The hybrid nudging method better captures the partitioning between thickness and concentration while conserving total water content. We document how this novel sea ice constraining method reaches specific targets, enhances surface turbulent heat flux responses to sea ice loss, and induces tropospheric warming for both polar regions. Plain Language Summary: The Arctic is warming faster than the global average due to several processes that, once combined, lead to so‐called Arctic Amplification. Part of this anomalous polar warming comes from an intense reduction in ice cover allowing heat into the ocean, warming the Arctic ocean near the surface, hence melting more ice. A joint effort by several climate modeling groups called the Polar Amplification Model Intercomparison Project (PAMIP) aims at better understanding Arctic Amplification through a coordinated set of climate simulations. Among this ensemble of simulations is a set of centennial simulations performed with fully coupled state‐of‐the‐art climate models. In these experiments, Arctic (and Antarctic) sea ice are forced to reach specific states in order to better isolate Arctic Amplification and sea ice loss from the rest of anthropogenic global warming. In this paper, we propose a simple technique to nudge sea ice models to specific states such as prescribed by PAMIP. This new method combines advantages from existing techniques to improve the control over the extent and the thickness of the ice. We document how our novel method produces atmospheric warming and circulation responses that are more consistent with previously published results than the existing techniques. Key Points: Already established sea ice nudging method leads to issues in the partitioning between sea ice concentration (SIC) and sea ice thickness (SIT)New hybrid nudging method significantly improves capture of both SIC and SIT targets for Polar Amplification Model Intercomparison Project, in particular for Arctic sea iceHybrid scheme increases Arctic turbulent heat flux and warming [ABSTRACT FROM AUTHOR]

As a result of their important role in weather and the global hydrological cycle, understanding atmospheric rivers' (ARs) connection to synoptic‐scale climate patterns and atmospheric dynamics has become increasingly important. In addition to case studies of two extreme AR events, we produce a December climatology of the three‐dimensional structure of water vapor and O3 (ozone) distributions associated with ARs in the northeastern Pacific from 2004 to 2014 using MERRA‐2 reanalysis products. Results show that positive O3 anomalies reside in dry intrusions of stratospheric air due to stratosphere‐to‐troposphere transport (STT) behind the intense water vapor transport of the AR. In composites, we find increased excesses of O3 concentration, as well as in the total O3 flux within the dry intrusions, with increased AR strength. We find that STT O3 flux associated with ARs over the NE Pacific accounts for up to 13% of total Northern Hemisphere STT O3 flux in December, and extrapolation indicates that AR‐associated dry intrusions may account for as much as 32% of total NH STT O3 flux. This study quantifies STT of O3 in connection with ARs for the first time and improves estimates of tropospheric ozone concentration due to STT in the identification of this correlation. In light of predictions that ARs will become more intense and/or frequent with climate change, quantifying AR‐related STT O3 flux is especially valuable for future radiative forcing calculations. Plain Language Summary: Long filaments of rapidly moving water vapor in the atmosphere, known as atmospheric rivers (ARs), play a vital role in the Earth's water cycle. Because of this, research continues to expand into ARs' relationship with large‐scale climate patterns. In this paper, we use data from the Modern Era Retrospective analysis for Research Applications to examine several extreme ARs that made landfall on the U.S. West Coast and their relationship to the transport of ozone from the stratosphere to the troposphere. We then combine 11 years of December AR and ozone data in order to study the average trend of ozone transport in connection with ARs. We quantify the AR‐related ozone transport for the first time, and we find ARs with more intense water vapor transport result in the transport of higher concentrations of ozone. Quantifying ozone transport into the troposphere in connection with ARs is important as ARs may become more intense and/or more frequent with climate change, and ozone in the troposphere has consequences for the greenhouse effect. Key Points: Case studies and December climatology using MERRA‐2 reveal positive tropospheric ozone anomalies within dry intrusions associated with ARsAverage excess ozone concentrations are 10–13 ppbv at 400 hPa, and are even greater for increasing intensity of ARsSTT of ozone associated with ARs in the NE Pacific may account for (13 ± 2)% of the total December Northern Hemisphere STT ozone flux [ABSTRACT FROM AUTHOR]

The physicochemical properties and origins of atmospheric aerosols in the Tibetan Plateau (TP) region are a research topic of great interest, but an in‐depth understanding of this topic is challenging, partially due to a lack of intensive in situ observations. Thus, a field campaign was conducted over Yadong, a remote area on the south slope of the TP from June 11 to 31 August 2021. The aerosol loading was low, with a black carbon mass concentration of 147.4 ± 98.4 ng·m−3. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and increased from 450 to 700 nm wavelength. Organic matter (OM) accounting for 69.6% of the total aerosol mass and relatively high secondary organic carbon ratios, highlighting the importance of secondary formation. An interesting phenomenon observed was that the evolution of aerosols was mainly characterized by diurnal variation, which could not be explained by large‐scale atmospheric processes such as Indian summer monsoon. Instead, it was found that regional mountain‐valley winds between the Himalayas and South Asia transported polluted air masses toward the TP, especially in the afternoon when regional valley wind are expected to be the strongest and the boundary layer in South Asia is deepest. Additionally, daytime local valley wind further elevated these aerosols to higher altitudes on the TP. This paper provides insights into the transport mechanisms of aerosols from South Asia to the TP. These findings are of great importance since aerosols exhibit significant diurnal variations in the TP region. Plain Language Summary: Previous studies focused on the analysis of the physical or chemical properties of aerosols on the Tibetan Plateau, but this study provides a comprehensive examination of both. The findings reveal that aerosols on the southern slope of the Tibetan Plateau exhibit strong absorption efficiency. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm), which may be attributed to aerosol secondary generation and coating. Finally, the mechanism of pollutant transport from South Asia to the Tibetan Plateau was analyzed relies on site observations, satellite, and reanalysis data to highlight the link between diurnal variations of pollutants and transport mechanism. The specific transport mechanism be understood uniformly across different scales, including Indian summer monsoon, regional mountain‐valley winds between the Himalayas and South Asia, and local mountain‐valley winds circulation. Key Points: Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and secondary organic matter was the major aerosol componentThe evolution of aerosols was mainly characterized by diurnal variation that was related to transport mechanism over YadongThe Himalayas‐South Asia regional mountain‐valley winds combined with local mountain‐valley winds transport aerosols to the Tibetan Plateau [ABSTRACT FROM AUTHOR]

Atmospheric eddies serve as a crucial component of global circulation on Mars. It is of great scientific and engineering significance to study the spatio‐temporal characteristics of atmospheric eddies and eddy sequences. In this paper, based on the Open access to Mars Assimilated Remote Soundings (OpenMARS) dataset, we used an eddy detection and sequence tracking method to extract a total of 4,089 eddies in LS = 109° MY28 to LS = 109° MY29, which comprised 3,385 cyclones and 704 anticyclones. Notably, the northern hemisphere exhibited a markedly higher eddy number compared with the southern hemisphere, and more than 82% of eddies had an equivalent radius greater than 400 km. These eddies mainly gathered in the interior and vicinity of basins, mountains, and broad plains in the northern hemisphere. Martian atmospheric eddies can be classified into two categories: topographic eddies, which are related to the various landforms on Mars and non‐topographic eddies, which do not show obvious terrain dependence. Additionally, our analysis yielded 83 extracted eddy sequences, with nearly 90% of them exhibiting short life cycles of less than 40 Martian days (sols). Eddy sequences mainly arise in the spring and summer seasons of the northern hemisphere. Considering the instrument criterion of minimal atmospheric eddy activity for the Tianwen‐3 mission, Chryse Planitia was identified as the optimal landing area. Plain Language Summary: Atmospheric eddies, or "a movement of air, dust, or water in a circle" are a crucial component of Mars' global circulation. Studying eddy sequences is essential to understand the atmospheric circulation on Mars. During LS = 109° MY28 to LS = 109° MY29, a total of 4,089 eddies were identified using the OpenMARS dataset, with a higher occurrence of cyclones in both hemispheres during spring and summer compared to autumn and winter. Based on the distribution and density of the eddy centers, we classified the eddies into two categories: topographic eddies and non‐topographic eddies. Topographic eddies are influenced and restricted by the topography of the Martian surface, mainly concentrated in areas with positive and negative topographical features. Non‐topographic eddies are predominantly dispersed over high‐latitude regions. The moving distances of the topographic eddies are usually short, and they are more stable than the non‐topographic eddies. In addition, we extracted 83 eddy sequences, of which nearly 90% had short lifecycles less than 40 Martian days (sols). China's Tianwen‐3 mission is scheduled to return samples in 2030. We think Chryse Planitia is the optimal landing area for the Tianwen‐3 mission according to the spatio‐temporal distribution characteristics of Martian eddies. Key Points: A total of 4,089 eddies were extracted in MY28‐29 using the eddy detection and sequence tracking methodEddies can be classified into topographic eddies (related to landforms) and non‐topographic eddies (no obvious terrain dependence)Considering the features of Martian atmospheric eddies, Chryse Planitia was identified as the optimal landing site for Tianwen‐3 mission [ABSTRACT FROM AUTHOR]

This paper reports the climatology of medium‐scale traveling ionospheric disturbances (MSTIDs) observed between September 2018 and August 2019 in the nighttime redline airglow intensity measurements from an all‐sky imager located in Sutherland, South Africa (32.4°S, 20.8°E; magnetic latitude: 40.7°S). The nighttime MSTIDs appeared as either single or multiple dark bands, commenced mostly before midnight, and predominantly occurred during the southern winter solstice. They primarily propagated westward with speeds of 31–127 m/s. The periods and wavelengths of the multi‐band MSTIDs were in the ranges of 26–155 min and 134–578 km, respectively, while the temporal and spatial scales of the single band MSTIDs were 11–30 min and 43–143 km, respectively. Analysis of neutral wind measurements from a co‐located Fabry‐Perot interferometer showed that the thermospheric wind was mostly northeastward in the evenings of MSTID occurrence and that the favored westward propagation of the MSTIDs was the least restricted propagation direction by the background wind. Based on data from nearby ionosondes, the MSTIDs were mostly accompanied by sporadic E and spread‐F structures. From the seasonal occurrences of MSTIDs and sporadic E as well as the MSTID propagation and prevalent wind directions, it is concluded that Perkins instability is the most likely source of the majority of the observed nighttime MSTIDs, while the remainder may be seeded by gravity waves. Key Points: Properties of dark band structures representing nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed over South Africa presented for the first timeMSTIDs prevalent in southern hemisphere winter and primarily propagated westwardOccurrence of MSTIDs is mostly accompanied by sporadic and spread‐F, while propagation is supported by favorable neutral wind circulation [ABSTRACT FROM AUTHOR]

Coupled atmosphere‐fire modeling is recognized as a relevant approach for the representation of the interaction between a wildland fire and local meteorology at landscape scales. The atmospheric model component used in the coupled system is based on several approximations, which are adopted for computational efficiency or physical processes representation, including the widely used anelastic approximation. The validity domain of the anelastic approximation may be questioned in the context of high‐resolution wildland fire modeling due to the large fire‐induced heat releases near the surface. This study aims to study this question with the MesoNH anelastic model coupled with the Blaze fire model. A compressible version of the MesoNH‐Blaze coupled model has been developed for comparison with the anelastic system. The FireFlux I experimental fire is used for this comparative study conducted at a 10‐m and a 25‐m horizontal atmospheric resolution. Results show significant anelastic/compressible differences at a 10‐m resolution on the physical processes occurring near the fire with higher horizontal velocities and the presence of gravity waves downstream of the fire. This is in addition to the fire plume with realistic larger vertical velocities. Differences at a 25‐m resolution are much smaller in all evaluated processes. The compressible system only enriches the physics underlying fire‐atmosphere interactions at a very high resolution, which means that the anelastic approximation remains relevant for large‐scale coupled atmosphere‐fire simulations, considering the significant economy concerning numerical costs. Plain Language Summary: Wildfires burn large amounts of forests each year, destroying the living habitat of many species, endangering human settlements and provoking cross‐continent smoke events leading to air quality issues. Wildfires result from complex physical, chemical and biological processes. Understanding the fundamental processes driving wildfire behavior is a key point for the prediction of fire spread across the landscape and the induced atmospheric dynamics. Numerical models coupling the atmospheric dynamics (wind, temperature, air density and pressure, etc.) and fire front evolution are efficient tools in this scientific process. This paper evaluates how such model simulations are affected by the way in which the effects of high air density gradients induced by the very large amount of heat released into the atmosphere are modeled. When the spatial resolution of the atmospheric model is as high as 10 m, an exact – compressible – formulation leads to the formation of gravity waves downstream of the fire, and to stronger fire‐induced wind, fire propagation and larger vertical velocities than an approximate (and computationally faster) approach. At a resolution of 25 m, which is still very high, the approximate approach leads to results, which are as good as the exact one. This can then be used to simulate wildland fire behavior at landscape‐to‐meteorological scales. It also paves the way for future accurate wildfire forecast systems. Key Points: A compressible version of the MesoNH atmospheric model has been developed to evaluate equation system approximations in wildfire simulationHorizontally buoyancy‐driven effects induce turbulent movements, waves ahead of the fire, and a bending of the plumeThe anelastic approximation for the atmosphere model is suitable to represent wildland fire effects, except for resolutions of 10 m or finer [ABSTRACT FROM AUTHOR]

In this paper, we investigate the (2+1)‐dimensional compressible Navier–Stokes equation with density‐dependent viscosity coefficients. We introduce a novel power‐type elliptic vortex ansatz and thereby obtain a finite‐dimensional nonlinear dynamical system. The latter is shown to not only have an underlying integrable Ermakov structure of Hamiltonian type, but also admit a Lax pair formulation and associated stationary nonlinear Schrödinger connection. In addition, we construct a class of elliptical vortex solutions termed pulsrodons corresponding to pulsating elliptic warm core eddies and discuss their dynamical behaviors. These solutions have recently found applications in geography, and oceanic and atmospheric dynamics. [ABSTRACT FROM AUTHOR]

As wind‐borne migration of insects is strongly influenced by atmospheric circulation and weather systems, the relationships between insect population abundance and meteorological conditions are substantial. The brown planthopper (BPH), Nilaparvata lugens (Stål), is one of the major wind‐borne insects hampering rice production in Asia, and its migration is significantly influenced by the Asian monsoon, a main component of the Asian climate system. However, the degree and mechanisms of the effects of monsoon on BPH migration have been little studied. In this paper, the effect of the Asian monsoon on the northward migration of BPH from the Indochina Peninsula to northern South China (NSC) was investigated based on the BPH light‐trap data and meteorological data for 39 years. The results show that the level of BPH migration in May could indicate the degree of annual pest outbreaks in NSC and that the southeast monsoon was more critical in this northward migration than the previously recognized southwest monsoon. The significance of the southeast monsoon stemmed from the strong effect of South China Sea subtropical high on BPH migration under a unique weather system distribution, the Polar low–Continental low–West Pacific subtropical high pattern. Furthermore, based on the key areas of migration obtained from the composite analysis, the multivariable linear regression prediction model could effectively predict the occurrence of BPH in NSC. These results indicate the importance of interactions between monsoons during BPH northward migration, which reflects the complex relationship between BPH migration and atmospheric motion. [ABSTRACT FROM AUTHOR]

The latest version of the atmospheric general circulation model ARPEGE‐Climat was used to perform 5‐member ensemble simulations for both present and RCP8.5 scenario climates (mid‐21st century). The rotated/stretched configuration enables a local horizontal resolution of less than 15 km over the tropical North Atlantic basin. Moreover, a tracking algorithm was used to extract tropical cyclones (TCs) simulated by the model. Through an Eulerian approach, this paper focuses on the relationships between TCs and rainfall over three French islands in the West Indies. Although the model underestimates the occurrence of TCs over this latitude band, especially in September, precipitation rates during TC days are realistic. Indeed, the model shows a good capacity to reproduce different relationships between island rainfall and TC characteristics such as proximity and intensity. In addition to rainfall distribution, the TC contribution to annual cumulative rainfall is also well captured by the model. We used three different series characterizing precipitation at the island scale to underline that the model overestimates the area that is impacted by TC rainfall. According to the simulations, the number of minor TCs tends to decrease in the future (about −15%) over the study domain (50–70°W × 10–25°N) despite a +1.6°C over ocean warming. In contrast, no trend was detected in the number of major hurricanes. Except for annual precipitation that decreases significantly in the future (about −15%), no significant change was detected in the relationships between TC properties and island rainfall. [ABSTRACT FROM AUTHOR]

The key factor that affects the inflow of radiant energy to the Earth's surface is the circulation of the atmosphere (caused by uneven distribution of air pressure on the globe) and the related changes in the amount of aerosols and cloudiness. This paper identified the areas in the Euro‐Atlantic region where the air pressure change had a significant impact on global solar radiation (GSR) changes over Poland, during the period 1986–2015. In general, growing GSR sums over Poland are to some extent related to the positive phase of the NAO (simultaneous pressure growth in the area of the Azores High and the pressure decrease in the area of the Icelandic Low). Correlation coefficient between the NAO index and GSR over Poland equals 0.18 (statistically significant at α = 0.05). In turn, in summer and autumn the pressure growth in southern Scandinavia results in a significant increase in the amount of GSR over Poland. Days with extremely large GSR sums (above the 95th percentile) are also prompted by the Azores High ridge which covers Central and Southern Europe, and by air mass inflow from the Atlantic. [ABSTRACT FROM AUTHOR]

Recent observational and numerical studies have investigated the dynamics of fine‐scale gravity waves radiating horizontally outward from tropical cyclones. The waves are wrapped into spirals by the tangential wind of the cyclone and are described as spiral gravity waves. This study addresses how well numerical simulations of these waves compare to observations as the horizontal grid spacing is decreased from 2.0 to 1.0 to 0.5 km, and the number of vertical levels changes from 25 to 50 to 100. Spectral filtering is applied to separate the fine‐scale waves in vertical velocity (w) and the larger‐scale waves in pressure (p) from moist updrafts and downdrafts in the eyewall and rainbands. As the grid spacing decreases, the radial wavelengths of the w waves decrease from 20 to 7 km, approaching observed values. For grid spacing 1.0 km, the p waves become well‐resolved with wavelength 70 km. The outward phase speeds range from 15 to 30 ms−1 for the w waves and 50 to 70 ms−1 for p waves. Analysis of the upper‐level outflow region finds that the spiral w waves propagate 5–10 ms−1 faster due to radial advection, but also finds what appear to be different classes of larger‐amplitude, slow‐moving spiral waves. Similar waves can be seen in satellite images, which appear to be caused by dynamical instability of the strongly vertically sheared radial and tangential winds in the TC outflow. Plain Language Summary: Tropical cyclones—known also as hurricanes, typhoons, and cyclones—cause some of the most significant weather impacts around the world. Recent work has found that these storms causes oscillations in the atmosphere that radiate outward in expanding spirals. These are called spiral gravity waves. This paper investigates how well computer models reproduce these waves as the pixel sizes used by the model become smaller (requiring more computer power). As the pixels decrease to 1.0 km or less, the waves become increasingly realistic. The simulations show two types of spiral gravity waves, which can be seen either in vertical motion or in the atmospheric pressure field. The vertical motion waves are 5–10 km wide and move outward at 90 km hr−1, while the pressure waves are 60–80 km wide and move outward at 220 km hr−1. Focusing on the cloud shield at the top of the cyclone (what we see in satellites) reveals a new class of slow‐moving, spiral waves that are distinct from the gravity waves. Key Points: Computer simulations of tropical cyclones show the detailed structures of outward propagating spiral gravity wavesAs resolution increases, the wave properties come close to, but do not converge to observed valuesA new class of waves is identified in the upper‐level outflow, which are slow‐moving spiral waves due to dynamical instabilities [ABSTRACT FROM AUTHOR]

This study investigates the emergence of hurricane‐like vortices in idealized simulations of rotating moist convection. A Boussinesq atmosphere with simplified thermodynamics for phase transitions is forced by prescribing the temperature and humidity at the upper and lower boundaries. The governing equations are solved numerically using a variable‐density incompressible Navier‐Stokes solver with adaptive mesh refinement to explore the behavior of moist convection under a broad range of conditions. In the absence of rotation, convection aggregates into active patches separated by large unsaturated regions. Rotation modulates this statistical equilibrium state so that the self‐aggregated convection organizes hurricane‐like vortices. The warm and saturated air converges to the center of the vortices, and the latent heat released through the upwelling, forms the warm core structure. These hurricane‐like vortices share characteristics similar to tropical cyclones in the earth's atmosphere. The hurricane‐like vortices occur under conditionally unstable conditions where the potential energy given at the boundaries is large enough, corresponding to a moderate rate of rotation. This regime shares many similar characteristics to the tropical atmosphere indicating that the formation of intense meso‐scale vortices is a general characteristic of rotating moist convection. The model used here does not include any interactions with radiation, wind‐evaporation feedback, or cloud microphysics, indicating that, while these processes may be relevant for tropical cyclogenesis in the Earth atmosphere, they are not its primary cause. Instead, our results confirm that the formation and maintenance of hurricane‐like vortices involve a combination of atmospheric dynamics under the presence of rotation and of phase transitions. Plain Language Summary: This paper investigates the emergence of hurricane‐like vortices in a numerical simulation of an idealized atmosphere system. The complicated thermodynamics for water vapor is simplified. The remaining system can still produce a conditionally unstable atmosphere in which unsaturated air parcels experience a stable stratification and unsaturated parcels experience an unstable one. We find hurricane‐like vortices in the presence of rotation. The warm and saturated air converges to the center of the vortices, and the latent heat released through the upwelling forms the warm core structure and easily observed eyewall. The structure of hurricane‐like vortices is analogous to tropical cyclones in the earth's atmosphere. This idealized configuration produces characteristics similar to the tropical atmosphere so that the hurricane‐like vortices can be considered as the outcome of the tropical cyclogenesis, with reasonable simplifications. However, the tropical cyclogenesis here does not require any interactions with radiation, surface flux feedback, or precipitation. Our results confirm that the formation and maintenance of hurricane‐like vortices involve a combination of rotation and thermodynamic forcing. Key Points: The impact of rotation on simulations of idealized simulations of moist convection is studiedThe formation and maintenance of hurricane‐like vortices involve a combination of conditional instability and rotationRotating moist convection in a conditionally unstable environment should spontaneously generate hurricane‐like vortices [ABSTRACT FROM AUTHOR]

It is often desirable to simulate a single temperature series or a collection (network) of temperature series. Accurate simulations can enhance our understanding of temperature trends and variabilities. Simulation can also be used to generate data with known specifications, which are useful in assessing climate data processing routines such as quality control and homogenization algorithms. Possessing multiple realistic temperature series is often beneficial as only one natural record of our climate is available. The current popularity of general circulation models (GCMs) demonstrates how important simulation techniques are. However, even with sophisticated downscaling, it is difficult to replicate station temperature data that have realistic seasonal cycles as well as realistic temporal and spatial correlations. This paper reviews and studies statistical time series methods that can replicate a single series or a network of series from data. The methods are purely statistical—no atmospheric dynamics are involved—and attempt to produce replicates of the records under study. The work here develops (a) methods that simulate temperatures from a fixed season (say a particular day or month of the year) that match the distributional characteristics of the observed data; (b) methods for simulating an entire series that matches the series' temporal autocorrelations and seasonal cycle; and (c) methods for simulating a network of series that reproduce the data's observed seasonal cycles and spatial and temporal autocorrelations. Applications are given throughout, including one where a GCM series and local station data are used in tandem to describe long‐term trends and inject realistic station‐level short‐term fluctuations. This paper can be used as a tutorial for the simulation of a single climate observation, an entire climate series, or a network of multiple climate series simultaneously. Extensions of the ideas that involve GCMs are also examined. [ABSTRACT FROM AUTHOR]

New Zealand has a long, continuous record of annual measurements of the altitude of end‐of‐summer‐snowline for a set of 50 Southern Alps "index glaciers." The record begins for the 1976–1977 glacier year and continues to the present. These are equivalent to equilibrium line altitude (ELA) data. An earlier paper used these to estimate annual mass balance and ice volume for all glaciers up to 2008 and showed a steady decline in ice volume, but with considerable inter‐annual fluctuations. These results are updated to 2018 for the more than 3,000 small and medium‐size glaciers of the Southern Alps. The inclusion of the latter year enabled assessment of the impacts on ice volume of these glaciers from an unprecedented coupled ocean‐atmosphere heatwave in the New Zealand region during the austral summer (DJF) 2017/18. The main purpose of this paper is to examine how these changes in ice volume relate to variability and trends in atmospheric circulation and weather types over four decades. Years with ice volume gains are characterized by below average temperature anomalies, enhanced southwest flow and a tendency for trough weather types over New Zealand. Years with ice volume losses have above average temperature, reduced southwest flow, and increased blocking weather types, with more anticyclones east of New Zealand. Largest annual ice volume loss of 3.6 ± 0.6 km3 (−13.5%) occurred during the 2017/18 summer heatwave for which the main atmospheric anomalies are discussed. Total ice volume of the Southern Alps for the small and medium glaciers has decreased from 26.6 km3 in 1977 to 17.9 km3 in 2018 (a loss of 8.6 km3 or 33%) at a rate of 0.21 km3 a−1. From 1977 to 1997 there was an annual ice gain of +0.30 km3 a−1 but was followed by an accelerating ice loss of −0.67 km3 a−1 for the period 1998–2018. There are significant correlations between ice volume changes and the Trenberth regional circulation indices Z1, Z3 and MZ3. Strongest relationships are with the Southern Annular Mode: a trend towards more positive values coincides with a general loss of ice volume. The Inter‐decadal Pacific Oscillation and El Niño/Southern Oscillation relate to ice volume changes on decadal to inter‐annual time scales. Positive phases of the Inter‐decadal Pacific Oscillation and El Niño years favour ice volume growth as they are cooler, with more precipitation, troughs and southwest flow over New Zealand. Negative phases of the Inter‐decadal Pacific Oscillation and La Niña years favour ice volume loss as they are warmer with less precipitation, more anticyclones and east or northeast flow. The dominant driver for 2017/18 was the positive Southern Annular Mode, and a smaller contribution from La Niña on a background of regional warming. Climate scenarios suggest that such conditions will be rather frequent by the 2,080–2,100 decades, with consequent large reductions in ice volume. [ABSTRACT FROM AUTHOR]

Self‐organizing maps (SOMs) represent a popular tool for classifying atmospheric circulation patterns. One of their traditional applications has been to link typical synoptic‐scale patterns to large‐scale teleconnections, or modes of low‐frequency circulation variability. However, recently there have been attempts to interpret an array of SOM nodes directly as a continuum of teleconnections, grounded in SOMs' ability to combine two otherwise distinct approaches to data analysis, that is, exploratory projection (or, dimensionality reduction) and classification. This conceptual shift calls for methodological studies that would improve our understanding of how orthogonal modes of variability, typically used to describe teleconnections, relate to SOM outputs. Here, we define three idealized modes of variability and use their various combinations to generate synthetic data sets. Many variants of SOMs are generated for SOMs of various shapes and sizes. The results show that projection of modes on a SOM array is sensitive not only to data structure, but also to various SOM parameters. The leading mode of variability projects rather strongly on SOMs if its explained variance is markedly higher than that of the second‐order mode; the remaining modes project considerably more weakly, and all modes tend to blend when their explained variance is similar, which leads to underrepresentation of some phases of modes and/or combinations of modes among the SOM patterns. Furthermore, we show that some features of SOM topology that were previously considered a proof of data nonlinearity appear even if the underlying modes of variability are strictly linear. Plain Language Summary: One of the tasks of a climatologist is to group together days with similar weather over a region, and to find out how such types of regional weather are influenced by distant locations. These distant influences, also known as teleconnections, are intricate and studying how the methods we use to study them deal with these intricacies is one of the keys that helps us understand the climate. One way to start this task is to apply simple and well‐understood data sets to methods and examine how they interpret the data. In the paper, we studied the popular method of self‐organizing maps in such a way. We found that the method usually identifies the strongest teleconnection, but the images of weaker teleconnections tend to be unclear or even blend together. We use our results to revisit findings of some previous studies and to discuss how self‐organizing maps could be improved in the future to give us more accurate information on the atmosphere. Key Points: Synthetic fields are generated as linear combinations of three idealized functions of spatial co‐variability, or circulation modesSelf‐organizing maps have limited ability to represent the second and third modes, and modes tend to blend when their variance is similarPrincipal component analysis may detect data structures conducive to high sensitivity of self‐organizing maps to the choice of parameters [ABSTRACT FROM AUTHOR]

In ocean and atmospheric dynamics, the strong horizontal turbulent mixing tends to produce horizontal eddy viscosity, so the horizontal scale is much larger than the vertical scale. At the same time, as a fluid, the motion of the atmosphere or the ocean has a delayed property to some extent. In this paper, we consider 3D viscous primitive equations with only horizontal eddy diffusivity and delays in both convective and heat source terms. With some suitable assumptions on delay term, we prove the existence and uniqueness of strong solution with H2$$ {H}^2 $$ initial data and obtain the existence and uniqueness of the stationary solution. Moreover, we prove that the strong solution converges exponentially to the stationary solution under some suitable assumptions. [ABSTRACT FROM AUTHOR]

The most serious heavy precipitation in the past 43 years occurred over Fujian‐Jiangxi region from late May to early June of 2006, causing significant economic losses. Using the ERA5 reanalysis database and daily precipitation collected at 2,479 surface meteorological stations in China, the present study investigates the relationship between the heavy precipitation over Fujian‐Jiangxi region in late spring‐early summer of 2006 and baroclinic wave packets in the upper troposphere. Information flow between the two systems has been diagnosed. Results indicate that different from the disturbance source for Meiyu in the middle and lower reaches of the Yangzi River, the disturbance source for this heavy precipitation originated from areas near the Syrian Desert to the north of the Arabian Peninsula and propagated along the northwest‐southeast direction, reaching Fujian‐Jiangxi region 4 days later. This kind of baroclinic wave packets provide the necessary energy for the occurrence and persistence of heavy precipitation. Analysis of wave activity flux vectors indicates that during the heavy precipitation period, disturbance energy was transported from the upstream westerly belt to Fujian‐Jiangxi region almost every day. Obviously, there existed information transfer between the two regions, re‐confirming that the upstream Rossby wave packets affect the Fujian‐Jiangxi precipitation. The above results provide helpful hints for a better understanding of the mechanisms for heavy precipitation in this region and will be helpful for its effective prediction. Plain Language Summary: In the late spring and early summer of 2006, the strongest precipitation occurred in Fujian‐Jiangxi in China from 1979 to 2021 at the same time. It is of significance to study the causes of such extremely heavy precipitation events to predict future similar events in advance to reduce casualties and economic losses. Atmospheric motions could be regarded as wave motions, and Rossby wave is one of the major atmospheric waves. Multiple Rossby waves together could be simply regarded as Rossby wave packets (RWPs). In this paper, we have used as many methods as possible to find whether RWPs have influence on precipitation and how. A large amount of evidence shows that RWPs have significant influence on this extreme precipitation, especially by providing energy. Four days before the beginning of the heavy precipitation, RWPs began to spread from near the Syrian desert to Fujian‐Jiangxi region. Moreover, a new mathematical method is used to prove that there is a causal relationship between wave packet activities and this heavy precipitation. Nevertheless, the above research is only for such an extreme precipitation event. Whether each extreme precipitation event corresponds to obvious wave packet activities needs further study. Key Points: Rossby wave packets (RWPs) propagated from areas near the Syrian Desert to Fujian‐Jiangxi region before the heavy precipitation beganRWPs at 300 hPa provide disturbance energy daily to heavy precipitation over Fujian‐Jiangxi region during the precipitation periodThere is evident information transfer between RWPs activities from the upstream area with the heavy precipitation in Fujian‐Jiangxi region [ABSTRACT FROM AUTHOR]

Data‐driven parameterizations offer considerable potential for improving the fidelity of General Circulation Models. However, ensuring that these remain consistent with the governing equations while still producing stable simulations remains a challenge. In this paper, we propose a combined Variational‐Multiscale (VMS) Artificial Neural Network (ANN) discretization which makes no a priori assumptions on the model form, and is only restricted in its accuracy by the precision of the ANN. Using a simplified problem, we demonstrate that good predictions of the required closure terms can be obtained with relatively compact ANN architectures. We then turn our attention to the stability of the VMS‐ANN discretization in the context of a single implicit time step. It is demonstrated that the ANN parameterization introduces nonphysical solutions to the governing equations that can significantly affect or prevent convergence. We show that enriching the training data with nonphysical states from intra‐time step iterations is an effective remedy. This indicates that the lack of representative ANN‐induced errors in our original, exact training inputs underpin the observed instabilities. In turn, this suggests that data set enrichment might aid in resolving instabilities that develop over several time steps. Plain Language Summary: Computer models of weather and climate have coarse resolutions, which prevent them from accurately predicting the effect of small atmospheric motions, for instance in low‐lying clouds, on the global climate. Recent studies indicate that improvements might be had by using models for the small motions that are informed purely by data, for example Artificial Neural Network (ANN) models. However, capitalizing on this potential is challenging in practice, since ANNs can introduce instabilities to the numerical model for larger‐scale motions. In this study, we zero in on these instabilities by introducing a model framework in which prediction accuracy is only limited by an ANN's ability to predict the net influence of small atmospheric motions. We contextualize this framework by contrasting it to more commonly used approaches. Using a simple test case based on the motions underpinning the development of low clouds, we illustrate that standard training procedures do not prepare ANN models for their interaction with the rest of the numerical model, and that this can contribute to numerical instability. We demonstrate an effective remedy for a specific type of instability, and suggest how this technique might also be used to treat other types of instabilities prompted by ANN models. Key Points: Artificial Neural Networks (ANNs) can parameterize exact closure terms of a Variational Multiscale method without a priori model form assumptionsThe ANN parameterization outperforms state‐of‐the‐art schemes in offline testsInstabilities in the numerical model comprising the ANN parameterization are amendable through data set enrichment [ABSTRACT FROM AUTHOR]

Significant progress has been made in our understanding of extratropical impacts on the tropical climate via energetics framework. It is of question whether the impact of extratropical thermal forcing in one hemisphere would extend far into high‐latitudes of the other hemisphere. We examine the possibility of the pole‐to‐pole linkage via atmospheric teleconnections by imposing a cyclic surface thermal forcing in the northern extratropics of an aquaplanet slab ocean model. We reveal a synchronous temperature response between the two poles mediated by zonal‐mean atmospheric dynamics. A warming in one polar region leads to a strengthened Hadley circulation of the unforced hemisphere, fluxing more momentum toward the subtropics, thereby pulling the eddy‐driven jet equatorward. A consequent anomalous descent over the polar region causes warming. The polar surface warming in the unforced hemisphere reaches 30% of that in the forced hemisphere, inferring a significance of the pole‐to‐pole connection. Plain Language Summary: Regional climate change effects can be global. This paper reveals a pole‐to‐pole linkage in the absence of ocean dynamics. A high‐latitude surface thermal forcing confined to one hemisphere causes changes in the tropical circulation and mid‐latitude jet, thereby inducing a secondary circulation in the unforced hemisphere. The atmosphere‐driven pole‐to‐pole connection is characterized by synchronous temperature changes between the two poles. We speculate the factors not considered in this study such as ocean‐biogeochemistry feedback may shift temperature changes from being synchronous to asynchronous on longer than multidecadal timescales. Hence, future studies are warranted that gradually add complexities to a model to elaborate pole‐to‐pole teleconnection. Key Points: High‐latitude thermal forcing confined to one hemisphere can cause surface temperature changes globally including the opposite poleThe pole‐to‐pole linkage via atmospheric pathway results in synchronous polar temperature changesThe interhemispheric teleconnection leads to mini‐global warming in response to heating confined to one hemisphere [ABSTRACT FROM AUTHOR]