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In the Earth's general circulation, polar jets act as baroclinic pumps of angular momentum and heat. Reanalysis datasets indicate that shear changes near jets induce upward displacements of the jet cores, suggesting a weakening thermodynamic pumping over the last 50 years. From secondary flow theory, a well‐established principle in fluid dynamics, an increased frequency of heatwaves and persistent winter storms is expected. The ageostrophic wind shear between 700 and 50 hPa indicates the strength of this secondary circulation. The weakening tendency during the reanalysis period also exists in multimodel simulations under the RCP8.5 emissions scenario for the 21st century. The reduction between the periods 2005–2025 and 2081–2100 reaches 18%, 5.3% and 19%, respectively, for the North America, Mid‐Europe and East Asia sectors of the Northern Hemisphere polar jet. Within this background, cold‐surge events are the result of synergic co‐working of several factors. The occurrence trend for transitional season winter extreme events also is examined. The winter extremes seemingly have larger temperature drops. However, in a warming climate, they emerge more rapidly from extreme cold states. The storm tracks, especially over North America, have equatorward extensions, indicating that winter storms can reach lower latitudes. Due to the temperature‐dependence of air viscosity, secondary flows decrease more slowly than the main zonal flow. This imposes an important adjustment to the traditional polar amplification effects on midlatitude winter extremes. During a warmer winter (the primary manifestation of a warmer climate), spatially uniform positive trends in cold extreme events are not expected. There are, however, regions experiencing more winter extremes. These regions show consistent patterns in both the reanalysis period and the remainder of the 21st century. [ABSTRACT FROM AUTHOR]

In the early 21st century, there was an increase in precipitation during the retreat period of the East Asian summer monsoon (EASM). This study aims to explore the precipitation changes and their possible causes in the context of climate change. The findings indicate that the increased precipitation primarily occurred in the Yellow—Huai River valley during the early autumn. This corresponds to a delayed retreat of the EASM with a northward shift of 0.9°N after 2002. Notably, this anomalous changes in the EASM are associated with the significant warming in two Asian dryland regions. The warming of the central Asian dryland strengthens the midlatitude high‐pressure belt and the anomaly anticyclone over northeast Asia, which restrains the development of wave troughs and westerly cold air activity. Similarly, the warming China‐Mongolia dryland enhances the anomaly anticyclone over northeast Asia through the dry soil moisture feedback and reduced latitudinal temperature gradient. These two Asian drylands thereby hold the northward shift of the westerly jet stream and the northwest extension of the Japan Sea high and the western Pacific subtropical high. These changes result in maintaining the northward EASM circulation and driving the northwest water vapor flux from the northwest Pacific, leading to moisture convergence in northern China. The China‐Mongolia dryland warming also increases the land‐sea thermal contrast, which induces a dipole pattern over East Asia and drives the northward water vapor flux from the South Sea. As a result, the rapidly warming drylands restrict westerly activity and EASM retreat, ultimately leading to increased precipitation. Plain Language Summary: Northern China is experiencing a climate that resembles summer and delayed autumn in September and October, accompanied by increased precipitation, precipitation extremes, and high temperature. We have investigated this phenomenon and found that it is related to delayed retreat of East Asian summer monsoon (EASM) with the northernmost margin of the EASM (EASM‐NM) shifted northward by 0.9° latitude after 2002, which has contributed to a rise in precipitation of 80 mm in the Yellow—Huai River valley during the early autumn. These changes are associated with the rapid warming of the middle‐latitude Asian drylands. The warming of both central Asian dryland and China‐Mongolia dryland strengthens northward westerly jet stream and creates a high‐pressure anomaly from northeast China to Japan Sea. This is achieved through westerly waves and reduced meridional temperature gradient. These changes further maintain the northward circulations of EASM and weaken westerly activity. Consequently, more northwest and southerly water vapor is transported to northern China, resulting in increased precipitation. Key Points: Increased autumn precipitation corresponds to delayed retreat of the East Asian summer monsoon (EASM) with a northward shift of 0.9°NThe warming Asian dryland delays the retreat of EASM that enhances precipitation in early autumn in northern ChinaThe warming Asian drylands drive northward westerly jet and westward Japan Sea high through westerly waves and zonal thermal contrast [ABSTRACT FROM AUTHOR]

The Hadley circulation (HC) has been expanding poleward in recent decades. The Coupled Model Intercomparison Project Phase 6 (CMIP6) models predict that the expansion will accelerate in the future, more so in the Southern Hemisphere (SH). However, the extent of the expansion varies widely among the models. We investigate the mechanisms driving the intermodel spread in SH HC expansion predictions. The intermodel spread is obtained by an empirical orthogonal function analysis on the SH HC trend patterns of 16 CMIP6 model simulations using the historical and shared socioeconomic pathway 5–8.5 scenarios. The leading mode, showing a mean meridional stream function anomaly at the poleward SH HC extent, explains 49.73% of the variance and significantly correlates (r = 0.94) with the SH HC expansion. By analyzing the extended Kuo‐Eliassen equation, we find that the intermodel difference in the representation of diabatic heating is responsible for about 14% of the intermodel spread. The meridional eddy momentum and heat fluxes contribute to about 21% and 18% of the intermodel spread, respectively. The models simulating a relatively large SH HC expansion tend to show increased precipitation in the Southern Pacific Convergence Zone, reduced baroclinic instability in the subtropics, and an enhanced poleward shift of jet stream in the midlatitudes. This suggests that the uncertainty in the HC projection may be constrained by reducing the bias in the trend of the mean fields. Plain Language Summary: The Hadley circulation (HC), which affects global climate patterns, is expanding toward the poles. However, different climate models predict varying extents of this expansion, especially in the Southern Hemisphere (SH). We aim to understand why. By analyzing climate model data, we identified key factors influencing the spread in SH HC expansion. Our findings suggest that factors like diabatic heating and meridional eddy momentum play significant roles. Models showing greater SH HC expansion tend to simulate increased precipitation in specific regions and shifts in atmospheric jet streams. Addressing these model biases could help reduce uncertainty in future HC projections. Key Points: The first intermodel stream function trend mode effectively captures the spread of the Southern Hemisphere Hadley Circulation expansionThe intermodel spread in expansion is explained by the spread of the meridional eddy momentum, heat fluxes, and diabatic heatingThe spread of the eddy fluxes and diabatic heating are linked with the changes in precipitation, jet, and static stability [ABSTRACT FROM AUTHOR]

This study investigates the impact of a shock generator on the fuel distribution of downward step fuel injection in a scramjet engine's combustor. Computational simulations of CFD are employed to analyze the three-dimensional structure of multi-jets released in a downward-step configuration. The inlet is subjected to a free stream supersonic flow with Mach = 4 , while hydrogen gas is injected at Mach = 1 through injectors situated on the vertical planes of the downward step. A shock generator with a 3 0 ∘ angle is positioned at the top of the domain to induce a bow shock, which significantly affects fuel distribution. The research assesses the influence of free stream and jet pressure on the fuel distribution behind the jets. The results indicate that increasing the free stream flow strengthens the induced bow shock, leading to enhanced fuel jet interactions and improved fuel mixing behind the jets. [ABSTRACT FROM AUTHOR]

This study examines the effectiveness of adaptive observation experiments using the ensemble transformation sensitivity (ETS) method to improve precipitation forecasts during heavy rainfall events in South China and the Sichuan Basin. High-resolution numerical models are employed to simulate adaptive observations. By identifying the sensitive areas of key weather system positions 42 hours before heavy rainfall events, the adaptive observations improve the prediction of jet streams, strong winds, and shear lines, which are essential for accurate heavy rainfall forecasting. This improvement is reflected in both the precipitation structure and location accuracy within the verification region. In South China, targeted observations enhance rainfall predictions by improving water vapor transport. In the Sichuan Basin, adaptive observations refine water vapor transport and adjust vortex dynamics. This research highlights the importance of accurately predicting shear lines and jet streams for forecasting heavy rainfall in these areas. Overall, this study found that adaptive observation enhances the precipitation forecast skills of the structure and location for heavy rainfall in South China and the Sichuan Basin, emphasizing their potential utility in operational numerical weather prediction. [ABSTRACT FROM AUTHOR]

In order to investigate the premixed combustion characteristics of CH4/H2/air in a micro-mixing combustor, the effects of different micro-mixing head-ends (HE1, HE2, HE3) and hydrogen mixing ratios on the temperature distribution, heat transfer process, emission characteristic, flames shape are analyzed. The results show that compared with swirl head-end combustion, the micro-mixing combustion performance is better. Among the three head-ends, HE3 has the best combustion characteristics and stable flames. The temperature distribution in the high-temperature zone is uniform, and low-temperature zone is concentrated near the jet, which can suppress the flashback. The velocity and temperature gradient near the central axis of jet streams show a strong synergistic effect. The flames are plume shaped and flames stability is mainly influenced by the H2 combustion process. Increasing the jet diameter, decreasing the jet spacing and increasing the hydrogen mixing ratio all contribute to the flames stability, but these three methods can stabilize the flames by affecting fluid Reynolds number, interaction between small flames and combustion rate, respectively. Moreover, small jet diameter and high hydrogen mixing ratio can reduce OTDF, which contributes to improve outlet temperature uniformity. [ABSTRACT FROM AUTHOR]

Persistent fluctuations in the latitudinal position of the North Atlantic (NATL) jet stream are closely linked to extreme weather anomalies, particularly over Europe. Accurately representing jet stream persistence in climate models is essential for enhancing the reliability of future regional extreme weather projections. This study investigates the summer persistence of the NATL jet stream latitudinal fluctuations and the Summer North Atlantic Oscillation (SNAO) using data from CMIP6 models and ERA5 reanalysis. Our findings reveal that CMIP6 models generally overestimate persistence compared to ERA5 during the historical period. Utilizing the relative vorticity tendency equation, we assess the strength of the eddy‐mean flow feedback and identify a positive relationship between persistence and feedback strength across models. We also found that a large fraction of the intermodel spread in precipitation persistence is associated with differences in the persistence of jet fluctuations. Plain Language Summary: The NATL jet stream plays a crucial role in shaping the weather and climate over the NATL and European regions. The jet continuously fluctuates in latitude and strength and guides the storms along its path. When these fluctuations become unusually persistent, they can lock certain weather patterns in place for extended periods, leading to extreme events such as droughts or floods. In this study, we investigate the persistence of the summer NATL jet stream latitudinal fluctuations and that of the SNAO using models and the ERA5 reanalysis. We found that the strength of the eddy‐mean flow feedback explains a large fraction of the intermodel differences in jet persistence. Moreover, we found that differences in jet persistence are closely associated with differences in precipitation persistence over Europe. Key Points: CMIP6 models tend to overestimate the persistence of summer jet latitudinal fluctuations and the Summer North Atlantic OscillationThere is a robust positive relationship across models between the SNAO vorticity anomalies and the strength of the eddy‐mean flow feedbackA significant portion of the intermodel spread in precipitation persistence is linked to jet fluctuations persistence differences [ABSTRACT FROM AUTHOR]

Chang (2024, https://doi.org/10.1029/2024gl110011) challenged the methodology proposed recently by Okajima et al. for evaluating cyclonic and anticyclonic contributions to Eulerian eddy statistics and atmospheric energetics based on the local flow curvature. He argued that using the local wind curvature to separate energetic contributions from cyclones and anticyclones is not physically meaningful. Here we argue that his claims are based on an unrealistic assumption of monopolar relative vorticity in an entire storm‐track domain and a meridionally uniform zonal background flow atypical to midlatitudes. We also demonstrate that the error in attributing eddy statistics to cyclones and anticyclones is significantly smaller than his estimation. Rather, we further demonstrate that the curvature‐based methodology effectively eliminates the shear influence to identify cyclonic and anticyclonic regions, which is dismissed in his argument. We conclude that the curvature‐based methodology is beneficial in evaluating distinct cyclonic and anticyclonic contributions to atmospheric energetics in realistic conditions. Plain Language Summary: Chang (2024, https://doi.org/10.1029/2024gl110011) raised a question about the effectiveness of the recent method by Okajima et al. for evaluating cyclonic and anticyclonic contributions separately to eddy statistics based on the local flow curvature. He brings idealized cases to claim that the method by Okajima et al. is not physically meaningful. Here we show that Chang's arguments rely on an unrealistic assumption that an entire storm‐track region contains only one or two isolated cyclonic vorticity patches placed within horizontally uniform westerlies, which are atypical of midlatitudes. We also reveal that the error in attributing eddy statistics to cyclones and anticyclones is much smaller than Chang's result. The curvature‐based method effectively removes the influence of shear vorticity to determine cyclonic and anticyclonic regions, which is overlooked in his argument. We conclude that our curvature‐based method is useful for evaluating the separate contributions of cyclones and anticyclones to atmospheric energetics in realistic conditions in midlatitudes. Key Points: The error in attributing eddy statistics to cyclones and anticyclones is significantly smaller than that presented by Chang (2024, https://doi.org/10.1029/2024gl110011)The arguments of Chang (2024, https://doi.org/10.1029/2024gl110011) are based on the unrealistic assumptions of monopolar vorticity and an atypical background flowThe curvature‐based evaluation of cyclonic and anticyclonic contributions is a practical, beneficial method for realistic flow fields [ABSTRACT FROM AUTHOR]

In this study, we propose a method to predict the classification of the eruption plume types: "strong plumes" and "weak plumes". Our method reproduces the general features that the eruption plume types depend on, both the meteorological conditions and eruption conditions, and successfully reproduces the classification of the plume types for historical eruption events. Application of the method to Monte Carlo simulations revealed that eruption plumes at a low latitude tend to form strong plumes due to weak ambient winds, whereas eruption plumes at a mid latitude tend to form weak plumes due to the strong ambient winds associated with the jet stream. The simulation also revealed that mid‐latitude eruption plumes tend to be strong plumes in the summer when ambient winds are lowered by a weakening and poleward movement of the jet stream. Plain Language Summary: When an explosive volcanic eruption occurs, an eruption plume rises up to a few kilometers or tens of kilometers while incorporating ambient air. After the plume density equals to that of the atmosphere, the plume intrudes horizontally into the stratified atmosphere. This intrusion is classified into mainly two types: a radial spreading type (i.e., strong plume, often called an "umbrella cloud") and a downwind spreading type (i.e., weak plume). Estimation of the intrusion type is vital to forecast ash dispersal, because an input parameter for ash dispersal calculation depends on the intrusion type. In this study, we introduce a method to predict the plume intrusion type, and we validated the method for 14 historical eruptions. In addition, to investigate the relationship between the intrusion type and meteorological conditions at the volcano's location, we conducted Monte Carlo simulations with random eruption intensities and actual long‐term meteorological data. The simulation results revealed the following trends: (a) eruption plumes at a low latitude tend to be a radial spreading type, and (b) eruption plumes at mid latitudes tend to be a downwind spreading type except during the summer. These features can be understood from the meteorological conditions at the volcano's location. Key Points: A method to predict an eruption plume type ("strong plume" or "weak plume") is introducedEruption plumes at low‐latitudes tend to be strong plumes, because ambient winds at high altitude are weakStrong wind associated with jet stream at mid latitude affects eruption plumes, favoring weak plumes in all seasons except summer [ABSTRACT FROM AUTHOR]

The atmospheric circulation in the northern hemisphere interacts with that in the southern hemisphere mainly via lateral coupling and atmospheric mass exchanges across equator. Using the NCEP‐NCAR daily reanalysis over period 1979–2019, we examine the Inter‐Hemispheric Oscillation (IHO) on the synoptic timescale during boreal winter and its associations with Rossby waves by employing a 9‐day high‐pass filter and the time‐lag regression method. Our results demonstrate that the IHO exist in the surface air pressure anomalies on synoptic timescale which links to both the quasi‐stationary and migratory Rossby waves with large amplitudes in the mid‐ and high‐ latitudes from the troposphere up to the stratosphere. The synoptic‐IHO related quasi‐stationary waves appear mainly in zonal, roughly dominated by waves with wave‐number 1–7 at different latitudes. These waves disperse the wave energy eastward along westerly jet streams. On the other hand, the synoptic‐IHO related migratory Rossby waves are mainly observed in mid latitudes between 30°N–65°N and 30°S–60°S with stronger intensity in northern than in southern hemisphere. The phase speed of these waves is roughly estimated at about 9.5lon/d. The energy of intrinsic Rossby waves also propagates mainly eastward. The synoptic‐IHO has important impacts on surface air temperature via both the quasi‐stationary and migratory Rossby waves, inducing significant temperature changes over both Eurasian continent and North America as estimated from 40 winters. All the results are beneficial for us to better understand the IHO along with its interaction with Rossby waves and the formation mechanisms of cold weather and climate extremes during boreal winter. Plain Language Summary: The weather/climate extremes occur frequently in northern hemisphere as a result of the circulation variations on multitimescales. The circulation changes are partly related to atmospheric mass exchange between the northern hemisphere and southern hemisphere, which results in a phenomenon named as the Inter‐hemispheric Oscillations (IHO). Using the NCEP‐NCAR daily reanalysis from 1979 to 2019, we examined the IHO on synoptic timescale during boreal winter and its association with Rossby waves by employing the time‐lag regression method. Our results demonstrate that the IHO appears in zonal mean surface air pressure anomalies on synoptic timescale, which links to both the quasi‐stationary and migratory Rossby waves. The quasi‐stationary waves with larger amplitudes appear in middle latitudes, dispersing the wave energy eastward along westerly jet streams. On the other hand, the synoptic‐IHO‐related migratory Rossby waves move eastward with phase speed at about 9.5lon/d, also dispersing energy mainly eastward. The synoptic‐IHO significantly affects the wintertime surface air temperature via both the quasi‐stationary and migratory Rossby waves over Eurasian continent and North America. All these results are beneficial for us to better understand the IHO along with its interactions with Rossby waves and the formation mechanisms of weather and climate extremes during boreal winter. Key Points: Inter‐hemispheric Oscillations (IHO) of atmospheric mass is found to exist on synoptic timescale during boreal winterSynoptic‐IHO links to both the quasi‐stationary and migratory Rossby waves in the mid and high latitudesSurface weather and climate in mid to high latitudes are possibly influenced by synoptic IHO via both stationary and migratory Rossby waves [ABSTRACT FROM AUTHOR]

India has observed increasingly persistent heat extremes in recent decades. North‐Central India, a highly populated region prone to heatwaves, has experienced record maximum temperatures (> ${ >} $48°C) during the pre‐monsoon season. While studies have shown positive trends in heatwaves due to rising air temperature, we identify a shift in pre‐monsoon mean daily maximum temperature over North‐Central India, resulting in an increase in temperature by 0.7°C post‐1998. The jump in temperature is associated with a northward migration of the subtropical westerly jet since 1998. We find that the meridional shift in the subtropical westerly jet explains more than 25% of the variability in heatwave characteristics over North‐Central India, implying that the increase in heatwaves post‐1998 is associated with a northward shift of the jet. These findings highlight that the exacerbation of heatwaves in North‐Central India is driven by atmospheric dynamical changes triggered by a regime shift, further compounded by global warming. Plain Language Summary: In recent years, India has been experiencing more frequent and intense heatwaves, especially over the North‐Central region. We found that since 1998, the temperature over North‐Central India has increased by about 0.7°C during the pre‐monsoon season. This increase appears to be due to the northward displacement of the band of strong upper tropospheric winds, known as the subtropical westerly jet. The shifting in the jet stream is making the heatwaves occur more frequently and last longer, exacerbating heatwave risks in this densely populated region. Overall, our findings emphasize the importance of understanding how winds have changed in the atmosphere due to natural variability or climate change and their contribution to the intensification of heatwaves in the region. Such understanding has potential applications in forecast of heatwaves in India. Key Points: The pre‐monsoon season witnessed a regime shift in 1998, resulting in a rise in temperature over North‐Central IndiaThe subtropical westerly jet index positively correlates with the heatwave characteristics, explaining more than 25% of heatwave variabilityThe increase in heatwaves post‐1998 can be partially attributed to the northward shifting of the subtropical westerly jet [ABSTRACT FROM AUTHOR]

Atmospheric blocking is characterized by persistent anticyclones that "block" the midlatitude jet stream, causing temperature and precipitation extremes. The traffic jam theory posits that blocking events occur when the Local Wave Activity flux, a measure of storm activity, exceeds the carrying capacity of the jet stream, leading to a pile up. The theory's efficacy for prediction is tested with atmospheric reanalysis by defining "exceedance events", the time and location where wave activity first exceeds flow capacity. The theory captures the Northern Hemisphere winter blocking climatology, with strong spatial correlation between exceedance and blocking events. Both events are favored not only by low carrying capacity (narrow roads), but also a downstream reduction in capacity (lane closures causing a bottleneck). The theory fails, however, to accurately predict blocking events in time. Exceedance events are not a useful predictor of an imminent block, suggesting that confounding factors explain their shared climatological structure. Plain Language Summary: An atmospheric block is a large, high pressure weather pattern that blocks the jet stream, affecting many regions in the midlatitudes including North America and Europe. Blocks are notable for their persistence, driving extreme weather conditions for up to a week or longer. Despite their significant societal impact, we don't fully understand the mechanism(s) that generate blocks. A traffic jam theory was proposed, which suggested that the onset of a block is caused by having too much "storm activity flux", which leads to a pile up of storm activity, just as a traffic jam is precipitated by conditions where the vehicular flux exceeding the road capacity, blocking traffic. This analogy is useful for understanding the preferred locations of atmospheric blocks in the time‐mean sense, but is not predictive in terms of individual blocking events. We further propose to incorporate additional regional constraints on flux capacity, analogous to "traffic bottlenecks", to improve our understanding of preferred blocking locations. Key Points: Flow capacity exceedance events, predictors of blocking onset in the traffic jam theory, are defined and evaluated in climate reanalysesA downstream reduction in flow capacity is ubiquitous for both exceedance and blocking events: lane closures favor traffic jamsBlocks are co‐located with exceedance events in space but not in time, limiting the utility of the traffic jam theory for prediction [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, the characteristics of tropical cyclones (TCs) over the Bay of Bengal (BoB) that affect snowfall on the Tibetan Plateau (TP) and spatiotemporal distribution of snowfall related to BoB TCs are statistically analysed by using multi‐sources data from 1981 to 2020, with partitioning TC‐influenced snowfall by tracking cloud clusters. The results show that 141 TCs formed during the 40‐year period of 1981–2020, of which about 35% (50 TCs) impacted snowfall at 83% of meteorological stations on the TP during their northward or westward movement, and the average distance between the TC centre and the snowfall stations is 1277 km. The proportion of snowfall‐related TC frequency shows a significantly decreasing trend with a predominant cycle of 10a. The TC‐influenced snowfall frequency (SF), precipitation amount (PA) on a snowfall day and snow depth (SD) during 1981–2020 all show a non‐significant weak decreasing trend, while TC‐influenced snowfall is significantly increased in the eastern and southern edges of Xizang, western Sichuan and the southern margin of Qinghai. PA and SD in December account for more than 75% and 55% of the monthly total, respectively. The spatial pattern of PA could be objectively categorised into west‐type (24%) and southeast‐type (76%). The moisture transported by the BoB TC and a southerly jet stream formed between the trough and the western Pacific subtropical high (WPSH), the convergence of cold air and warm–moist airstream over the TP and the change in position of the south Asian high in the upper troposphere are significant factors causing the different spatial distribution. The results can provide reference for TC‐related snowfall, SD prediction and disaster assessment on the TP. [ABSTRACT FROM AUTHOR]

A 38‐year aerosol optical thickness (AOT) satellite product from the Advanced Very High‐Resolution Radiometer (AVHRR) is used to investigate the aerosol variabilities in the North Indian Ocean (NIO). A dipolar mode with a notable meridional contrast between the equatorial and northern NIO is revealed by the second mode of Empirical Orthogonal Function (EOF) analysis with a sharp rise over the Arabian Sea (AS) and Bay of Bengal (BoB) since 2002. Our results show that the aerosol dipolar variation is primarily modulated by an El Niño–Southern Oscillation (ENSO) during 1983–2001 (ID1) and by a warm phase of Atlantic Multi‐decadal Oscillation (AMO) during 2002–2020 (ID2), leading to a dry‐and‐warming condition spanning the coasts of East Africa, the Arabian Peninsula (AP), and South Asia (SA) that favors increasing aerosol emission and a longer life cycle in the upstream regions. A warm phase of the AMO tends to excite an anomalous cyclone in western Siberia and reinforces anticyclonic circulation in the Tibetan Plateau. Correlation analyses between the second EOF mode time series and other parameters in ID1 and ID2 show that an interannual dry‐and‐warming condition over the AP–SA is associated with a noticeable north‐south contrast of convection between the equatorial NIO and AP–SA in ID1. But in ID2, a zonal contrast of anomalous convective activities between the AP–SA and the BoB‐South China Sea is dominant, partially due to the warming AMO, which aids the development of anticyclonic cells over the TP and the strengthened subtropical westerly jet streams. Plain Language Summary: Using satellite aerosol observations during 1983–2020 and a multivariate statistical technique for temporal‐spatial analysis, we explored the aerosol long‐term and year‐by‐year variations in the North Indian Ocean (NIO). Our results show a significant rise in springtime aerosol over the Arabian Sea and Bay of Bengal since the early 2000s, which is attributed to intensification of a dry‐and‐warming condition in the coasts of East Africa, the Arabian Peninsula, and South Asia. A relatively low aerosol concentration before the early 2000s is mainly modulated by the anomalies of sea surface temperature (SST) in both the West and East Pacific associated with an El Niño or La Niña event, but an increasing aerosol concentration after the early 2000s is linked to warmer SST associated with a phenomenon called Atlantic Multi‐decadal Oscillation (AMO). A warm phase of the AMO may aid the development of the remote teleconnection pattern in the midlatitude regions of the Northern Hemisphere, and thus reinforce the anomalies of atmospheric circulation in the subtropical regions. Key Points: A notable springtime aerosol rise over the Arabian Sea and Bay of Bengal (ASBoB) since the early 2000s is revealed by using satellite dataThe El Niño events may cause a relatively low aerosol loading over the ASBoB during 1983–2001A warm‐phase Atlantic Multi‐decadal Oscillation may favor a notable high aerosol loading over the ASBoB during 2002–2020 [ABSTRACT FROM AUTHOR]

The evolution of Storm Ciarán was different from what is commonly attributed to windstorms affecting northwest Europe. Ciarán initially developed as a diabatic Rossby wave, driven by low‐level latent heat release and with negligible upper tropospheric forcing. It then crossed the jet stream, intensifying explosively while developing a low‐level warm seclusion and producing extreme near‐surface winds, including sting jets. Ciarán is an example of an extreme windstorm following this pathway and highlights the need of a systematic assessment of its relevance in our warming climate. [ABSTRACT FROM AUTHOR]

Climate models exhibit biases in the mean state and in variability across different regions of the Earth. For example, atmosphere‐only models have a poleward bias in summertime jet streams across the Northern Hemisphere (NH). This can result from many processes, including misrepresentation of Rossby waves that can propagate in different directions and thereby interact with jet streams. However, Rossby‐wave biases can result from biased background state of the climate system as well. The propagation speed of Rossby waves depends on jet stream strength, thus a poleward displacement of the jet stream can hinder westward propagation of Rossby waves at higher latitudes and displace eastward propagating Rossby waves (downstream development). These biases then impact other regions resulting in biased atmospheric circulation across the NH. Indeed, in this study we confirm this via regional nudging experiments within the Norwegian Earth System Model. Namely, nudged horizontal winds over the North Pacific can improve Rossby wave statistics and thereby atmospheric circulation over Eurasia (i.e., upstream). However, nudging over the North Atlantic has little effect on boreal summer atmospheric circulation. This implies that improving biases over the North Pacific is crucial for a better representation of modelled boreal summer circulation over Eurasia. [ABSTRACT FROM AUTHOR]

In this study, the medium‐range predictability of heatwave (HW) onsets in four midlatitude European regions is investigated statistically with the help of ensemble reforecasts for the period 2001–2018. The concept of Euro‐Atlantic weather regimes is adopted to characterise HWs (about 50 in each region) and to study whether forecast skill may depend on the large‐scale dynamical setup. HW onsets over the British Isles and Scandinavia are mainly associated with Scandinavian and European blocking regimes, whereas the "no regime" case is observed more frequently for Central Europe. Stratified by weather regime, the predictability of heatwave onsets is then studied by means of a multiple metric‐based analysis of European Centre for Medium‐Range Weather Forecasts (ECMWF) and Global Ensemble Forecast System Version 12 (GEFSv12) ensemble reforecasts. For two of the regions considered, Central Europe and the British Isles, a conclusive picture is obtained: medium‐range predictive skill is significantly higher for HW onsets associated with Scandinavian or European blocking compared with cases with no pronounced regime. This skill advantage mostly concerns the large‐scale flow and, to some extent, 850‐hPa temperatures, but is generally not reflected in the correct prediction of near‐surface temperatures. Finally, we investigate for two regions how exceptionally good or poor forecasts are related to the atmospheric state during or shortly after forecast initialisation. At 10 days lead time, poor large‐scale flow predictive skill for Central European HW onsets is linked to anomalously high baroclinicity further upstream and an intensified North Atlantic jet stream, whereas good forecasts on average feature an initial state close to climatology. Forecast skill for near‐surface temperatures is not affected by such dynamical precursors, but rather by pre‐existing soil‐moisture anomalies. For the British region, exceptionally good forecasts of both large‐scale flow and near‐surface temperatures are associated with an already established continental blocking. In contrast to Central Europe, pre‐existing soil‐moisture anomalies play less of a role there. [ABSTRACT FROM AUTHOR]

Based on the properties of Rossby waves at the tropopause resonantly forced by solar declination in harmonic modes, which was the subject of a first article, case studies of heatwaves and extreme precipitation events are presented. They clearly demonstrate that extreme events only form under specific patterns of the amplitude of the speed of modulated airflows of Rossby waves at the tropopause, in particular period ranges. This remains true even if extreme events appear as compound events where chaos and timing are crucial. Extreme events are favored when modulated cold and warm airflows result in a dual cyclone-anticyclone system, i.e., the association of two joint vortices of opposite signs. They reverse over a period of the dominant harmonic mode in spatial and temporal coherence with the modulated airflow speed pattern. This key role could result from a transfer of humid/dry air between the two vortices during the inversion of the dual system. Finally, focusing on the two period ranges 17.1–34.2 and 8.56–17.1 days corresponding to 1/16- and 1/32-year period harmonic modes, projections of the amplitude of wind speed at 250 mb, geopotential height at 500 mb, ground air temperature, and precipitation rate are performed by extrapolating their amplitude observed from January 1979 to March 2024. Projected amplitudes are regionalized on a global scale for warmest and coldest half-years, referring to extratropical latitudes. Causal relationships are established between the projected amplitudes of modulated airflow speed and those of ground air temperature and precipitation rate, whether they increase or decrease. The increase in the amplitude of modulated airflow speed of polar vortices induces their latitudinal extension. This produces a tightening of Rossby waves embedded in the polar and subtropical jet streams. In the context of climate change, this has the effect of increasing the efficiency of the resonant forcing of Rossby waves from the solar declination, the optimum of which is located at mid-latitudes. Hence the increased or decreased vulnerability to heatwaves or extreme precipitation events of some regions. Europe and western Asia are particularly affected, which is due to increased activity of the Arctic polar vortex between longitudes 20° W and 40° E. This is likely a consequence of melting ice and changing albedo, which appears to amplify the amplitude of variation in the period range 17.1–34.2 days of poleward circulation at the tropopause of the Arctic polar cell. [ABSTRACT FROM AUTHOR]

Subseasonal variability of atmospheric circulation in mid-high latitudes is highly correlated to the upper-level jet streams, acting as a major contributor to the global or regional persistent climate anomalies. In this study, the features of subseasonal concurrent variations (SCVs) between East Asian Polar Front Jet (EAPJ) and Subtropical Jet (EASJ), and corresponding atmospheric circulation and temperature anomalies in China are investigated by using the observational data and NCEP/NCAR reanalysis datasets. Results show that the main variability modes of the winter upper-level wind fields on subseasonal time scales are characterized by the meridional shift in opposite directions and out-of-phase variations in the intensity of the EASJ and EAPJ. These concurrent variations between the two jets have a significant oscillation cycle with a period of 10–25 days. Accompanied by the location (intensity) SCV of the two jets, the mid-latitude circulation systems show significant zonal shifts (local intensity changes), resulting in alternations of the atmospheric circulation between zonal and meridional types. The typical meridional circulation is characterized by a strong East Asian trough and its upstream strong high-pressure ridges. Meanwhile, gradient force between Siberian high and the Aleutian low is anomalously strong. As a result, the SCVs between two jets can cause widespread and persistent low temperature anomalies over China, usually lasting more than three days. In the case of the location SCV of the two jets, the low temperature anomalies concentrate in northern China, particularly in north of 40°N. In the case of the intensity SCV of the two jets, the low temperature anomalies occur in most parts of China, notably in south of 40°N. These results are helpful to improve the prediction of atmospheric circulation and temperature anomalies in East Asia during winter. [ABSTRACT FROM AUTHOR]

The storms over the Bay of Bengal (BoB) often combine with the weather systems such as the South Branch Trough (SBT) and the West Pacific Subtropical High (WPSH) to transport plenty of moisture inducing extreme precipitation on the Tibetan Plateau (TP). Determining the fine moisture structures of storms helps understand mechanism of this kind of extreme precipitation. An extreme precipitation occurred on the TP influenced by storm Rashmi (2008). A Lagrangian approach is scrutinized the forward and backward moisture transport trajectories of Rashmi and the TP, respectively. The moisture source of this extreme precipitation is relatively clear, which comes from the collaborative influence of Rashmi with the southwest jet generated by the SBT and the WPSH. Utilizing a three‐dimensional K‐means clustering method devised in this study, the Rashmi's forward trajectories are classified into three categories, the particles ascending with the northward movement of Rashmi (45%), consistently below 1 km (37.5%), and rapidly ascending into the southwest jet stream (17.5%). Notably, 97.5%, 1.2%, and 91% of these categories impact the TP, respectively. The moisture transport structure of storm is verified by backward tracking of moisture over the TP. In addition, the three‐dimensional moisture trajectories classification method is recommended when trajectories suffer rapid altitude changes. [ABSTRACT FROM AUTHOR]

As the only remaining ice sheet in the Northern Hemisphere, the Greenland ice sheet (GrIS) plays a crucial role in influencing atmospheric circulations, particularly with its rapid melting under global warming. In this paper, the influences of GrIS topography and surface thermal conditions are investigated by a series of aquaplanet experiments. The results show that the GrIS topography induces stationary waves and favors more blocking events through the generation of negative potential vorticity (PV) anomalies, while it tends to suppress local storm activities through the induced stationary waves. The surface cooling center of the GrIS is found to strengthen the jet streams by enhancing the meridional temperature gradient and thermal wind, while it causes the PV and static stability to increase during near-Greenland blocking days, thereby disfavoring blocking onset. Altogether, the topography and surface thermal effects of GrIS appear to compete with each other so that the net effect would determine the final response. Nevertheless, nonlinearity is found in both GrIS-topography alone and GrIS-surface temperature alone experiments, where nonlinear responses of atmospheric circulation are detected when the GrIS topography height or surface temperature exceeds their critical values, respectively. Hence, through this study, the response of the blocking in the vicinity of Greenland to the combined effects of topography and surface thermal conditions may shed light on comprehending the underlying mechanism of blocking alteration in a changing climate. Significance Statement: Although there have been numerous observation-based studies showing that the blocking around Greenland has increased over the past few decades, which is a predominant driver in accelerating the Greenland ice sheet (GrIS) melting, the reasons for this change are still unclear. In addition, the impact of GrIS on blocking still needs investigation. Through idealized aquaplanet simulations, this study examines the effect of GrIS's topography and surface thermal conditions upon blocking onset. It suggests the nonlinearity of the blocking response in the vicinity of Greenland to the combined effects in the variation of the height and surface temperature of GrIS. This study will enhance our understanding of possible changes in blocking due to global warming. [ABSTRACT FROM AUTHOR]

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The meteorology of cutoff lows over South Africa is characterized by statistical analysis of daily field data in the period 1970–2023. An index is formulated by subtracting 500 hPa geopotential height in the mid-latitudes from the subtropics. Cutoff lows (COL) are identified by positive values, mostly in autumn and spring. Statistics indicate that climate forcing is seasonal: La Nina/El Nino favors COL in March–May/September–November. Hemispheric regressions reveal anomalous highs across the southern mid-latitudes when COL are frequent over South Africa. A 14-case composite was formed from the most intense daily COL events in autumn and spring. The composite shows a NW- tilted Rossby wave and jet stream loop around the COL. Maritime easterlies induce a warm east—cool west SST pattern, but composite moist inflows are shallow, so stormy weather hugs the coastal plains. Overturning circulations meet in an upper-level "saddle" over South Africa. 500 hPa sinking motions to the Southwest are of similar strength to rising motions to the Northeast. A COL case study exhibited hourly rain rates >10 mm at Port Alfred 18–20 October 2012 fed by tropical inflow. New insights emerged from this study via composite height sections over South Africa. [ABSTRACT FROM AUTHOR]

Latent heating modifies the jet stream by modifying the vertical geostrophic wind shear, thereby altering the potential for baroclinic development. Hence, correctly representing diabatic effects is important for modelling the mid-latitude atmospheric circulation and variability. However, the direct effects of diabatic heating remain poorly understood. For example, there is no consensus on the effect of latent heating on the cross-jet temperature contrast. We show that this disagreement is attributable to the choice of spatio-temporal averaging. Jet representations relying on averaged wind tend to have the strongest latent heating on the cold flank of the jet, thus weakening the cross-jet temperature contrast. In contrast, jet representations reflecting the two-dimensional instantaneous wind field have the strongest latent heating on the warm flank of the jet. Furthermore, we show that latent heating primarily occurs on the warm flank of poleward directed instantaneous jets, which is the case for all storm tracks and seasons. [ABSTRACT FROM AUTHOR]

Atmospheric blocking is a synoptic-scale phenomenon that consists in an obstruction of the normal easterly progression of weather patterns in the midlatitudes, leading to persistent atmospheric conditions sometimes associated with extreme weather. State-of-the-art climate models systematically underestimate winter atmospheric-blocking frequency, especially over Europe. This is often attributed to a poor representation of small-scale processes that are fundamental for the onset and maintenance of blocking events. Here, we explore how the implementation of two stochastic parameterizations, namely the stochastically perturbed parameterization tendencies (SPPTs) and the stochastic kinetic energy backscatter (SKEB) schemes, influences the representation of Northern Hemisphere winter blocking in EC-Earth3. We show that the activation of the two stochastic schemes has moderate detrimental effects on blocking representation, when assessed through a gradient reversal index. Using a zonal–blocked flow linear decomposition, we attribute such modification to changes in the mean winter atmospheric circulation, primarily manifested in a strengthening of the midlatitude jet stream and an intensification of the Hadley cell. Ultimately, an analysis of the meridional transport of zonal momentum by stationary and transient eddies reveals that these circulation differences arise from changes in tropical stationary-eddy activity. Our findings reconnect with earlier literature on similar experiments and suggest that the activation of stochastic parameterizations may require a retuning of the model to account for the resulting significant changes in the mean atmospheric circulation. [ABSTRACT FROM AUTHOR]

In order to explore the drag reduction characteristics of different jet parameters in the bulb through pump, the bionic design was carried out for the shark gills, and the model of the bulb through the pump with a jet port at the front end of the bulb body was established, and the influence mechanism of the placement position, jet velocity and number of jet ports on the flow field of the bulb through the pump was studied by CFX. The results show that the near-wall main stream collides with the jet, and the axial momentum transmitted by the main stream to the jet weakens the axial velocity of the near-wall main stream. The radial momentum of the jet to the main stream increases the radial velocity of the near-wall mainstream, and lifts the near-wall high-velocity zone away from the bulb body, so that the near-wall medium and low velocity bands are widened, the high-velocity zone is contracted, and the wall shear stress is reduced. The calculation predicts that the drag reduction effect is up to 23.069%, the head is increased by 0.083 m, and the efficiency is increased by 1.729%, which can provide guidance for the optimal design and efficient operation of the bulb through-flow pump. [ABSTRACT FROM AUTHOR]

The effect of modified equator‐to‐pole temperature gradients on the jet stream by low‐level polar warming and upper‐level tropical warming is not fully understood. We perform aquaplanet simulations to quantify the impact of different sea surface temperature distributions on jet stream strength, large wave amplitudes and extreme waviness. The responses to warming in the waviness metrics Sinuosity Index and Local Wave Activity are sensitive to the latitude range over which they are calculated. Therefore, we use a latitude range that accurately represents the position of the jet. The uniform warming scenario strengthens the jet and reduces large wave amplitudes. Reductions in meridional temperature gradients lead to weakened mid‐latitudinal jet strength and show significant decreases in large wave amplitudes and jet stream waviness. These findings contradict the mechanism that weakened jet streams increase wave amplitudes and extreme jet stream waviness. We conclude that weakened jet streams do not necessarily become wavier. Plain Language Summary: This research letter considers how different patterns of atmospheric warming, like low‐level warming at the poles and at high altitude in the tropics, impact the jet stream, which is a strong "river" of high‐altitude wind. We use numerical model simulations to mimic different scenarios of warming that maintain or reduce the temperature gradient between equator and poles. We find that when an Earth‐like planet completely covered by water warms in specific ways, it strengthens the jet stream when it is warmed uniformly or weakens the jet stream when only mid‐ and high latitudes are warmed. For the simulations with a reduced mid‐latitudinal temperature gradient, the size of largest jet stream waves reduces and makes the extreme waviness episodes less wavy. This research letter conclude that weakened jet streams do not necessarily become wavier, which opposes the popular paradigm that weakened jet streams become wavier due to warming in polar regions. Key Points: On an aquaplanet, weakened jet streams do not become wavier with reduced temperature gradients due to warming in mid‐ and high latitudesIn reduced temperature gradient simulations, the magnitude of large wave amplitudes and jet stream extreme waviness decrease robustlyThe Sinuosity Index and the Local Wave Activity waviness metrics are sensitive to the latitude range over which they are calculated [ABSTRACT FROM AUTHOR]

On 18 December 2022, Hawaiian Airlines flight HA35 encountered severe turbulence without warning in a cloud‐free height. We reproduced this incident using the Weather Research and Forecasting Model (WRF) at a convection‐permitting resolution. We found that this case of tropical upper‐level turbulence occurred primarily due to the fast‐growing convective tower in the unstable layer created by gravity wave breaking. At lower altitudes, a mesoscale convective system (MCS) caused a decrease in wind speed in both upstream and downstream regions. At upper levels, a large‐scale jet descended and accelerated after flowing over the top of the MCS, which acted like a barrier and produced a situation similar to a downslope windstorm due to mountain terrain. Upper‐level turbulence is 2–3 km higher than the top of the MCS. The critical level above the jet and the locally self‐induced critical level created the locally enhanced descending jet stream, which destabilized the flow through Kelvin–Helmholtz instabilities. The convective tower existed near the flight route and played an important role in triggering turbulence in the unstable environment through its convective gravity waves. Plain Language Summary: On 18 December 2022, Hawaiian Airlines flight HA35 encountered severe turbulence without warning, resulting in injuries to some passengers and damage to the equipment. This incident requires careful investigation to broaden our understanding of aviation turbulence. A numerical model at kilometer‐scale resolution was used to simulate this case and reveal the dynamics, and we successfully captured the occurrence of turbulence with the simulation. The cause of the turbulence was a previously non‐highlighted mechanism. Severe turbulence was generated by a fast‐growing convective tower at an unstable level. This level was caused by the interaction between the convective system that acted as a barrier and an upper‐level jet. Understanding the causes and spatial distribution of turbulence can help aircraft avoid potential areas with turbulence. Key Points: A regionally convection‐permitting model was used to reproduce a severe turbulence encounterDeep convection acted like terrain and interacted with a jet stream, causing an upper‐level windstorm downstream of the convection topFast‐growing convective tower and downslope windstorm induced turbulence at different locations of the jet stream [ABSTRACT FROM AUTHOR]

The mid‐latitude extreme weather disasters are often associated with explosively developing cyclones (ECs). Based on different vertical development characteristics, 4,608 ECs identified over the North Pacific in the cold season of 44 years of NCEP‐CFSR reanalyzes are divided into four types of upward development and four types of downward development categories. ECs with vertical upward (downward) development follow a northeastward (nearly eastward) path, mainly explosively developing over the Northwest Pacific (Asia continent and Pacific). Furthermore, utilizing the piecewise potential vorticity inversion method reveals the synergetic forcing of the turbulent heat transport and baroclinicity in the lower troposphere, the latent heat release in the middle levels, the upper‐level jet stream, and the downward intrusion of stratospheric potential vorticity on the ECs. Different configurations of these influences from the troposphere to the stratosphere result in the occurrences of eight types of ECs in the cold season over the North Pacific. Plain Language Summary: Since the late twentieth century, there has been a significant variation in the frequency of explosively developing cyclones (ECs), which bring extreme weather disasters to the mid‐latitudes. This study classifies winter North Pacific ECs into four upward and four downward developing categories. The upward developing ECs include those enhanced only in the lower troposphere (UL‐EC), those developing upward to the middle troposphere (UM‐EC), those developing upward to the upper troposphere (UU‐EC), and those developing upward into the stratosphere (US‐EC). In contrast, the downward developing ECs can be classified as those intensifying only in the upper troposphere (DU‐EC), those developing downward to the middle troposphere (DM‐EC), those developing downward to the lower troposphere (DL‐EC), and those developing downward from the stratosphere (DS‐EC) to the lower troposphere. Furthermore, the piecewise potential vorticity inversion results show that the explosive development of UL‐EC and UM‐EC are dominated by the thermodynamical processes in the middle‐to‐lower troposphere, exhibiting a baroclinic structure with opposite anomalous geopotential height between the troposphere and stratosphere. However, the other types have a consistent stratospheric‐to‐tropospheric negative anomalies, which is determined by the upper‐layer dynamical processes. The downward intrusion of stratospheric potential vorticity dominates the explosive development for the US‐EC and DS‐EC. Key Points: The North Pacific explosively developing cyclones during cold season are divided into eight up‐ and down‐ward vertically developing typesThe explosively developing cyclones are proved to be an important link between the stratosphere and troposphere in the mid‐high latitudesUsing the piecewise potential vorticity inversion method, the quantitative forcings on the explosively developing cyclones are explored [ABSTRACT FROM AUTHOR]

Circulation patterns linked to the East Asian winter monsoon (EAWM) affect precipitation, surface temperature, and air quality extremes over East Asia. These circulation patterns can in turn be influenced by aerosol radiative and microphysical effects through diabatic heating and its impacts on atmospheric vorticity. Using global model simulations, we investigate the effects of anthropogenic aerosol emissions and concentration changes on the intensity and variability of the EAWM. Comparison with reanalysis products indicates that the model captures the mean state of the EAWM well. The experiments indicate that anthropogenic aerosol emissions strengthen the Siberian High but weaken the East Asian jet stream, making the land areas of East Asia colder, drier, and snowier. Aerosols reduce mean surface air temperatures by approximately 1.5° $1.5{}^{\circ}$C, comparable to about half of the difference between strong and weak EAWM episodes in the control simulation. The mechanisms behind these changes are evaluated by analyzing differences in the potential vorticity budget. Anthropogenic aerosol effects on diabatic heating strengthen anomalous subsidence over southern East Asia, establishing an anticyclonic circulation anomaly that suppresses deep convection and precipitation. Aerosol effects on cloud cover and cloud longwave radiative heating weaken stability over the eastern flank of the Tibetan Plateau, intensifying upslope flow along the western side of the anticyclone. Both circulation anomalies contribute to reducing surface air temperatures through regional impacts on thermal advection and the atmospheric radiative balance. Plain Language Summary: The East Asian winter monsoon (EAWM) is a large‐scale circulation system that controls the occurrence of cold air outbreaks and severe winter storms throughout the densely populated land areas of East Asia. By reducing the amount of sunlight reaching the surface, warming the atmosphere, and changing the properties and lifetimes of clouds, aerosols can alter the atmospheric circulation and regional weather conditions. Here, we examine the characteristics of the EAWM in global model simulations with and without aerosol emissions from industry, energy generation, transportation, and other human activities. Our results show that the aerosols produced by these activities make East Asian land areas colder, drier, and snowier during winter. We explain the reasons for these changes by diagnosing how various physical effects of aerosols impact temperature and winds within the East Asian monsoon region. Key Points: Anthropogenic aerosols make East Asian land areas roughly 1.5° $1.5{}^{\circ}$C colder during winter while reducing precipitation and increasing snowfallAerosol effects are diagnosed by analyzing how changes in diabatic heating alter the winter monsoon potential vorticity intrusionColder surface temperatures can be attributed to both aerosol direct effects and circulation responses to cloud‐aerosol interactions [ABSTRACT FROM AUTHOR]

Climate change is associated with the modification of the polar jet stream, cyclone tracks and corresponding circulation patterns over midlatitudes. To apply these synoptic changes on regional climate, the changing sensitivity of a specific location to different pressure centre regions must be studied. An automated, objective circulation pattern detection method was developed to investigate the evolution of cyclonic and anticyclonic influence at any specific point within the European domain. The algorithm was used to assign each location to influencing low‐ or high‐pressure centres within the domain. Pressure centre displacements and the frequency redistribution among different centres were studied for each location. The 180‐year (January 1836–December 2015) mean sea level pressure dataset on the European domain with a 0.703° × 0.702° spatial and daily temporal resolution was obtained from NOAA 20th Century Reanalysis project. The presented method can apply the continental‐scale changes to specific locations. A significant increase of anticyclonic influence was found in southern and central Europe, in line with the northward displacement of Atlantic cyclones. Hundred and eighty year (180‐year) change of the number of days with anticyclonic versus cyclonic influence was found to be between +10% and 15% in the Mediterranean and +2% and 10% in most of Europe. The increasing anticyclonic influence in central Europe was strongest in the spring and the winter and was most attributable to the eastern European anticyclone. Results highlight the importance of research on the dynamical climate response of anticyclonic pressure systems. [ABSTRACT FROM AUTHOR]

The jet stream is a primary factor contributing to turbulence, especially for upper-level aircraft. This study utilized pilot reports and ERA5 data from 2023 to investigate the relationship between upper-level turbulence and the East Asian westerly jet (EAJ). The results indicate that approximately 45.9% of upper-level aircraft turbulence occurs within the jet stream, with the lowest proportion in August and the highest in January. Additionally, the strongest vertical wind shear (VMS) is found concentrated in the lower part of the jet stream core, particularly in the South–Down part of the jet stream, where upper-level aircraft turbulence occurs most frequently (27.1%). The most turbulent area is located between 30–40° N and 110–120° E, with the main air routes experiencing turbulence being the Henan sections of G212 and B208. From a seasonal perspective, there is less frequent occurrence of upper-level aircraft turbulence in summer and autumn but more in winter and spring. The EAJ volume increases with the strengthening of the jet core wind speed, with the jet core regions being most distinct at altitudes of 200~300 hPa. Meanwhile, the jet stream intensity index peaks at 70.6 m/s in January and reaches its lowest value of 7.1 m/s in August. The jet stream axis shifts southward in winter and northward in summer, reaching the southernmost position in December at 32.2° N and the northernmost position in August at 43.5° N. Furthermore, the VMS at turbulence points within the jet stream is higher than that at the turbulence points outside the jet stream, and the Richardson number (RI) is lower. Moreover, the temporal distribution of upper-level aircraft turbulence is primarily determined by the location and intensity of the jet stream, of which the jet stream intensity index provides guidance and thus serves as a reliable indicator. [ABSTRACT FROM AUTHOR]

In light of the rapid Arctic warming and continuous reduction in Arctic Sea ice, the complex two-way Arctic–midlatitudes connection has become a focal point in recent climate research. In this paper, we review the current understanding of the interactive influence between midlatitude atmospheric variability and Arctic Sea ice or thermal conditions on interannual timescales. As sea ice diminishes, in contrast to the Arctic warming (cooling) in boreal winter (summer), Eurasia and North America have experienced anomalously cold (warm) conditions and record snowfall (rainfall), forming an opposite oscillation between the Arctic and midlatitudes. Both statistical analyses and modeling studies have demonstrated the significant impacts of autumn–winter Arctic variations on winter midlatitude cooling, cold surges, and snowfall, as well as the potential contributions of spring–summer Arctic variations to midlatitude warming, heatwaves and rainfall, particularly focusing on the role of distinct regional sea ice. The possible physical processes can be categorized into tropospheric and stratospheric pathways, with the former encompassing the swirling jet stream, horizontally propagated Rossby waves, and transient eddy–mean flow interaction, and the latter manifested as anomalous vertical propagation of quasi-stationary planetary waves and associated downward control of stratospheric anomalies. In turn, atmospheric prevailing patterns in the midlatitudes also contribute to Arctic Sea ice or thermal condition anomalies by meridional energy transport. The Arctic–midlatitudes connection fluctuates over time and is influenced by multiple factors (e.g., continuous melting of climatological sea ice, different locations and magnitudes of sea ice anomalies, internal variability, and other external forcings), undoubtedly increasing the difficulty of mechanism studies and the uncertainty surrounding predictions of midlatitude weather and climate. In conclusion, we provide a succinct summary and offer suggestions for future research. [ABSTRACT FROM AUTHOR]

Changes in the jet stream not only affect the persistence of climate change and the frequency of extreme weather but are also closely related to climate change phenomena such as global warming. The manual way of drawing the jet stream axes in meteorological operations suffers from low efficiency and subjectivity issues. Automatic identification algorithms based on wind field analysis have some shortcomings, such as poor generalization ability, and it is difficult to handle merging and splitting. A semi-supervised learning jet stream axis identification method is proposed combining consistency learning and self-training. First, a segmentation model is trained via semi-supervised learning. In semi-supervised learning, two neural networks with the same structure are initialized with different methods, based on which pseudo-labels are obtained. The high-confidence pseudo-labels are selected by adding perturbation into the feature layer, and the selected pseudo-labels are incorporated into the training set for further self-training. Then, the jet stream narrow regions are segmented via the trained segmentation model. Finally, the jet stream axes are obtained with the skeleton extraction method. This paper uses the semi-supervised jet stream axis identification method to learn features from unlabeled data to achieve a small amount of labeled data to effectively train the model and improve the method's generalization ability in a small number of labeled cases. Experiments on the jet stream axis dataset show that the identification precision of the presented method on the test set exceeds about 78% for SOTA baselines, and the improved method exhibits better performance compared to the correlation network model and the semi-supervised method. [ABSTRACT FROM AUTHOR]

This study utilized high-resolution numerical simulation data from the WRF model to conduct a thermodynamic diagnosis of the process by which Typhoon "Maysak" transformed and merged with the Northeast Cold Vortex. The results indicated that the continuous intrusion of cold vortex air and the relative cold advection formed by the typhoon's movement led to the demise of the typhoon's warm core structure. The low-level low-pressure convergence and upper-level high-pressure divergence structure disappeared. After the transformation and merging with the Northeast Cold Vortex, the cyclone became cold throughout the entire layer, with a cold center appearing at low levels. During the process of the typhoon's transformation and merging with the Northeast Cold Vortex, cold air accumulated near the low levels of the cyclone, causing the pseudo-adiabatic potential temperature lines to tilt and resulting in the slanted development of vertical vorticity in the mid-levels of the cyclone. After the typhoon transformed and merged with the Northeast Cold Vortex, the positive vertical vorticity advection at the bottom of the upper-level cold vortex trough promoted the cyclone's development directly from the mid-levels to the upper levels, while the jet stream at the bottom of the cold vortex trough facilitated the maintenance of the positive vertical vorticity advection. Concurrently, the thermodynamic shear vorticity parameter could describe the typical vertical structure characteristics of the dynamic and thermodynamic fields above the rain area during the typhoon transformation process. In terms of temporal evolution trends, there was a certain correspondence with the development and movement of the ground rain area, and the perturbation thermodynamic divergence parameter had a good indicative effect on the area of heavy rainfall. [ABSTRACT FROM AUTHOR]

Recent research has revealed links between a quasi-decadal mode of climate variability over the North Pacific – the Pacific Decadal Precession (PDP) – and the North Pacific's western boundary current's extension – the Kuroshio Extension (KE). It is suggested that on decadal time scales the PDP both responds to and influences the KE variability. A question yet to be answered is whether it is the large-scale or the mesoscale variations of the KE region that link with the PDP evolution. Using high-resolution sea surface temperature data (1981–2018) from the global ocean Operational Sea Surface Temperature (SST) and Sea Ice Analysis, low-resolution Extended Reconstructed SST (ERSST) version 3b data (1949–2018), geopotential height reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) we find that it is the large-scale variations in the KE region that correlate best with the PDP-like response in the overlying and downstream atmosphere as compared to the mesoscale variations. In particular, the second mode of the large-scale KE region, which is characterized by the warming (cooling) of the ocean south (north) of the KE, sets up a PDP-like north-south atmospheric pressure dipole over the North Pacific Ocean by altering the large-scale baroclinicity of the atmosphere and zonal intensification of the subtropical jet stream. In turn, there is a reduction in the zonal propagation of stationary wave energy and an enhancement of the climatological zonal wave heights over North America, which results in a downstream response over the North American continent and the formation of a subsequent east-west pressure dipole over the North Pacific and North American continent. As a result, there is a strong correlation between large-scale SST variations in the KE region and the evolution of the PDP over the next three years. [ABSTRACT FROM AUTHOR]

There is an ongoing debate on the relationship between accelerated warming in the Arctic and extreme weather patterns in mid-latitudes. As extreme weather events have dramatic socioeconomic costs, it is important to investigate the possibility of the increased risk of such events in mid-latitudes. We investigated changes in the frequency of extreme weather events in Eastern Europe and Western to Central Asia (30–60 ∘ N, 20–75 ∘ E) in the post-Arctic amplification (2002–2022) compared to the pre-Arctic amplification (1979–1999) period. We analyzed the daily detrended near-surface temperature and precipitation of the ERA5 data. There is no robust evidence for the contribution of Arctic amplification to changes in extreme precipitation in Eastern Europe and Western to Central Asia. Most regions of the study area have experienced a decrease in annual precipitation in the post-Arctic amplification period. We also identified an increase in consecutive dry days in most parts of Central Asia by approximately 16 days per year, which could be attributed to a warmer climate because dry areas generally become drier in a warmer climate. Greater warming of the Earth in the more recent period has been associated with a significant increase in both warm days and nights and a significant decrease in cold days and nights over Eastern Europe and Western to Central Asia. Depending on the season, we identified both intensification and weakening of the upper tropospheric jet stream in the post-Arctic amplification period. The jet stream is intensified from the eastern Black Sea toward northern Kazakhstan and southeastern Russia in spring. In contrast, it has significantly weakened in the northern Mediterranean Sea and western Kazakhstan in summer and the Caspian Sea and Caucasus in autumn. Our results have important implications for a better understanding of the potential impact of rapid Arctic warming on extreme weather events in mid-latitudes. [ABSTRACT FROM AUTHOR]

Precipitation in Europe significantly influences local agriculture and water resources. Numerous research endeavours have explored the influence of tropospheric variables on precipitation variability in Europe. Recent investigations have identified a connection between alterations in high-latitude stratospheric ozone levels and precipitation patterns across Eurasia. This study investigates the correlation between the high-latitude stratosphere and precipitation trends in Western Europe, utilizing observational data, reanalysis, and atmospheric models, yielding new insights that upper tropospheric and lower stratospheric (UTLS) ozone in the northern high latitudes notably influence spring and autumn precipitation in Western Europe, and explained the mechanism. Increased UTLS ozone tends to enhance the precipitation in Western Europe, and vice versa. Further analysis reveals that elevated UTLS ozone radiatively warms the local region, shifting the polar jet stream over Europe equatorward in spring and autumn, which results in a cyclonic anomaly over Western Europe. This enhances the convection over Western Europe and transports more water vapor from the Atlantic Ocean. The influence of variations in UTLS ozone on precipitation patterns in Western Europe has been further corroborated through the sensitivity experiments with climate model. An increase of 20% in UTLS ozone across Europe, relative to the multi-year average, raises precipitation in Western Europe by 2.6–8 mm/month and decrease it by 10 mm/month to the north. Overall, our findings highlight the significant influence of the UTLS ozone on modulating spring and autumn precipitation in Western Europe, offering important implications into the prediction of future precipitation changes. [ABSTRACT FROM AUTHOR]

The Turkana low-level jet stream (TJ) is important to climatic conditions over northern Kenya and East Africa. The representation of the TJ in climate models varies due to the TJ interaction with Turkana channel that is influenced by model resolution and influences the model representation of the regional climate. This study compares features of the TJ in CMIP6 AMIP model simulations with ERA5. Models reveal climatological wind speeds that match those of the reanalysis from the ERA5 at the jet entrance (13 m/s) but lower magnitudes of wind speed and vertical shears compared to ERA5 within the Turkana channel. The models with slowest wind speeds, have a flattened Turkana channel and fail to exhibit the terrain constriction at 37° E which otherwise aids in accelerating winds to form a jet core. Furthermore, they fail to represent the narrowing of the channel as in ERA5, thereby forming blocking walls in the channel, forcing vertical ascent and mixing, and weakening shear. This boosting of ascent motion promotes rainfall formation and enhances wet anomalies at the exit of the TJ when the jet stream is weaker. By applying a new narrowing index, we demonstrate the need to improve topography details in the CMIP6 models, particularly those with resolution coarser than 1.5°, in order to properly simulate the TJ and the observed rainfall over the northwestern areas of eastern Africa. [ABSTRACT FROM AUTHOR]

It has been difficult to establish trends in the observed jet streams, despite modeling studies suggesting they will move polewards in a warming world. While this is partly due to biases between the models and observations, we propose that another uncertainty is rooted in the choice of statistic used to determine the 'jet latitude' — one measure used to quantify the jet position. We use seven different jet latitude statistics, four climate reanalysis products, and CMIP6 simulations to assess the relative importance of different uncertainties associated with lower‐tropospheric North Pacific Jet (NPJ) trends. Our results show a statistically significant poleward trend in the observed winter NPJ across all reanalyzes and using all jet latitude statistics. The magnitude of this trend is most sensitive to the choice of statistic. Furthermore, we find that the NPJ shifts poleward in Autumn under high emission scenarios, which is robust to the choice of jet statistic. Plain Language Summary: Jet streams are ribbons of fast‐flowing air that flow from west to east in both hemispheres high up in the atmosphere. Their speed and position affect how moisture and heat are transported across the planet, making them an important control on surface weather patterns. In a warming world, the atmosphere does not warm uniformly, creating an imbalance in the processes determining where jet streams form. While climate models have generally suggested that these processes will shift the jet streams toward the poles, this has been difficult to establish in observations. Here, we argue that a major part of the uncertainty of determining this poleward trend comes from precisely which statistic is used to define a jet's location. Our analysis measures the differences in the North Pacific Jet position trend using different jet statistics and data sets. We show that the choice of statistic used to define the jet stream produces more uncertainty than the choice of data set. We find a statistically significant poleward trend in the wintertime North Pacific Jet position in the observational record and a significant end‐of‐century autumn poleward shift projected under very high emission scenarios. Key Points: We find a significant poleward trend in the winter North Pacific Jet position that is robust to reanalysis and metric uncertaintyThe choice of jet metric creates more uncertainty than the choice of reanalysis in estimating the winter North Pacific Jet trendWe find an end‐of‐century poleward shift during autumn under very high emissions that is robust to metric and model uncertainty [ABSTRACT FROM AUTHOR]

The mechanisms by which clouds impact the variability of the mid‐latitude atmosphere are poorly understood. We use an idealized, dry atmospheric model to investigate the relationship between Atmospheric Cloud Radiative Effects (ACRE) and annular mode persistence. We force the model with time‐varying diabatic heating that mimics the observed ACRE response to the Southern Annular Mode (SAM). Realistic ACRE forcing reduces annular mode persistence by 5 days (−16%), which we attribute to a weakening of low‐frequency eddy forcing via modified low‐level temperature gradients, though this effect is partly compensated by reduced frictional damping due to zonal wind anomalies becoming more top‐heavy. The persistence changes are nonlinear with respect to the amplitude of ACRE forcing, reflecting nonlinearities in the response of the eddy forcing. These results highlight the ACRE's impact on low‐frequency eddy forcing as the dominant cause of changes in annular mode persistence. Plain Language Summary: In this study, we conduct novel modeling experiments aimed at understanding how clouds affect the unforced variability of jet streams—concentrated bands of eastward wind in the mid‐latitude atmosphere. We focus on direct heating and cooling of the atmosphere by clouds, called Atmospheric Cloud Radiative Effects (ACRE), and how these impact the annular mode—the most significant type of jet stream variability, which consists of north‐south shifts of the jet's position. Using a simplified atmospheric model, we investigate how the persistence and structure of the annular mode are impacted by heatings and coolings that mimic the observed ACRE response to the Southern Hemisphere Annular Mode (SAM). We find that the ACRE reduces the persistence of the annular mode (i.e., deviations of the jet stream from its mean position tend to persist for less time), which we attribute to a weakening of the feedback mechanism by which atmospheric waves strengthen the jet where it is already strongest. We identify some compensating effects, but, overall, our results highlight that ACRE impacts jet streams via low‐altitude temperature gradients and atmospheric wave generation. Key Points: The response of Atmospheric Cloud Radiative Effects (ACRE) to the Southern Annular Mode (SAM) reduces SAM's persistenceThe decrease in SAM's persistence is attributed to weakened low‐frequency eddy forcing via disruption of low‐level temperature gradientsThe change in persistence is nonlinear with respect to the amplitude of ACRE forcing [ABSTRACT FROM AUTHOR]

Using fifth-generation ECMWF Atmospheric Reanalysis (ERA-5) we investigate the topical changes in global teleconnection with seasonal precipitation dynamics in the backdrop of climate change particularly for the recent 20 years (L20:2002 to 2021) with respect to the previous 20 years (F20: 1982–2001). It unveils a considerable decrease in both monsoon and pre-monsoon precipitation over the Northwest Himalaya (NWH) in the Indian region and a decline in winter precipitation over the Tibetan Plateau (TP). Statistically robust K-S test shows notable changes (5% significance level) in the regional distribution of the atmospheric parameters in parts of the High Mountain Asia (HMA) region in recent decades (L20). Further, a weakening of the North Atlantic Oscillation (NAO) and El-Nino Southern Oscillation (ENSO) influence on precipitation pattern is noted in the L20 (2002–2021) compared to the F20 (1982–2001) years over parts of the HMA region. A regional heterogeneity is perceptible among different parts of HMA i.e. NWH in the Indian region and the TP. The NWH precipitation shows a diametrically opposite relation with the NAO index for winter precipitation in L20 (correlation coefficient R= -0.18, sample size: 240) compared to a positive correlation in F20 (correlation coefficient R = 0.15, sample size: 240) which is significant at 5% significance level. The plausible reasons for such a diametrical change in the relationship during winter are addressed by examining the changing signature of the subtropical jet stream and its relationship with NAO and ENSO. Sea surface temperature shows cooling of the tropical Pacific and the resulting weakening in El-Nino signatures in recent years. Our results point to the noteworthy changes in the atmospheric circulation pattern over the HMA region in recent decades which is attributed to the alteration in the global teleconnection patterns and subsequent their association with the precipitation distribution over the HMA region. This study would be useful for an improved understanding of the precipitation dynamics over the HMA region and global teleconnections in the future. [ABSTRACT FROM AUTHOR]

Central Asian vortex (CAV) is a kind of typical regional cyclonic vortex in the North Hemisphere and its important component, Central Asian moist vortex (CAMV), is one of the key systems causing Central Asian heavy precipitation. This study focused on the main characteristics and interannual forcing sources of January CAMV, which has led to rapid increase of Central Asian heavy snowfall during several decades, mainly discussing about the individual and cooperative effect of boreal surface thermal anomalies on CAMVs. The results indicate that the January CAMV demonstrates significantly interannual variability in its annual activity days, moreover, this interannual variability has been experiencing decadal enhancement since 2004, which explains the decadal increase in contemporaneous Central Asian heavy snowfall amount. The "northern moist vortex" (NMV) featuring the low vortex in the northern Central Asia accompanying the European and Tibetan Plateau Ridge, represents the typical circulation configuration of January CAMV. The interannual frequency of NMV is highly related to preceding North Pacific Victoria mode (VM), mid-latitude North Atlantic sea surface temperature (MNA) as well as the land-sea temperature contrast (LSTC) between Africa and eastern North Atlantic. Among them, VM and MNA could respectively stimulate circumglobal and Atlantic-Eurasian wave train structures into northern Central Asia. While, LSTC could excite anomalous lower-level cyclonic circulation over the Mediterranean entrance, accelerating and lifting the entrance of African-Asian jet stream via the way of "eddy-driven jet stream", thereby strengthening the docking of the African-Asian jet stream with the North Atlantic jet stream and enhancing the waveguide effect of two jet streams. When these three surface thermal factors coexist on the interannual scale, VM tends to play a leading role for contributing more than 50% to NMVs in three factors. While, MNA and LSTC play unneglected roles of "relay" to further modulate NMVs, and the cooperative effect of multiple surface thermal anomalies is the key to impacting January CAMVs. [ABSTRACT FROM AUTHOR]

Jiangsu is a province in China and has the highest frequency of tornado occurrences. Studying the meteorological background and mechanisms of tornado formation is crucial for predicting tornado events and preventing the resulting disasters. This paper analyzed the meteorological background, instability mechanisms, and lifting conditions of the two Enhanced Fujita Scale level 2 (EF2) and above tornadoes that occurred in southern Jiangsu on 14 May 2021 ("5.14") and 6 July 2020 ("7.06") using ERA5 reanalysis data. Detailed analyses of the internal structure of tornado storms were conducted using Changzhou and Qingpu radar data. The results showed that (1) both tornadoes occurred in warm and moist areas ahead of upper-level troughs with significant dry air transport following the cold troughs. The continuous strengthening of low-level warm and moist advection was crucial in maintaining potential instability and triggering tornado vortices. The 14 May tornado formed within a low-level shear line and a warm area of a surface trough, while the 6 July tornado occurred at the end of a low-level jet stream, north of the eastern section of a quasi-stationary front. (2) The convective available potential energy (CAPE) and K indices for both tornado processes were very close (391 for "5.14" and 378 for "7.06"), with the lifting condensation level (LCL) near the ground. The "5.14" showed greater instability and more favorable thermodynamic conditions, with deep southwesterly jets at the mid-level shear line producing rotation under strong convergent action (convergence center value exceeding −1 ×   10 − 4 s − 1 ). In contrast, the "7.06" was driven by super-low-level jet stream pulsations and wind direction convergence under the influence of the Meiyu Front (convergence center value exceeding −1.5 ×   10 − 4   s − 1 ), resulting in intense lifting and vertical vorticity triggered by a surface convergence line. (3) The "5.14" tornado process involved a supercell storm over a surface dry line experiencing tilting due to strong vertical wind shear, which led to the formation of smaller cyclonic vortices near a hook echo that developed into a tornado. The "7.06" developed on a bow echo structure within a mesoscale convective system formed over the Meiyu Front, where dry air subsidence, entrainment, and convergence of the southeast jet stream triggered a "miniature" supercell. The relevant research results provide a reference for the prediction and early warning of tornadoes. [ABSTRACT FROM AUTHOR]

Jupiter and Saturn exhibit alternating east‐west jet streams. The origin of these zonal flows has been debated for decades. The high‐precision gravity measurements by the Juno mission and the grand finale of the Cassini mission have revealed that the observed zonal flows may extend several thousand kilometres deep and stop around the transition region from molecular to metallic hydrogen, suggesting the magnetic braking effect on zonal flows. In this study, we perform a set of magnetohydrodynamic simulations in a spherical shell with radially variable electrical conductivity to investigate the interaction between magnetic fields and zonal flows. A key feature of our numerical models is that we impose a background dipole magnetic field on the anelastic rotating convection. By varying the strength of the imposed magnetic field and the vigor of convection, we investigate how the magnetic field interacts with the convective motions and the convection‐driven zonal flows. Our simulations reveal that the magnetic field tends to destroy zonal flows in the metallic hydrogen and suppress zonal flows in the molecular envelope, while the magnetic field may enhance the radial convective motions. We extract a quantitative relation between the magnetic field strength and the amplitude of zonal flows at the surface through our simulations, which roughly matches the observed magnetic field and zonal wind speed of Jupiter and Saturn. This discovery provides support from a new perspective for the scenario of deep convection‐driven zonal winds which are confined to the molecular hydrogen layers in giant planets. Plain Language Summary: This study explores the mysterious east‐west jet streams, known as zonal flows, observed on Jupiter and Saturn. Data from the Juno and Cassini missions suggested that these flows extend deep into the planets, stopping where hydrogen transitions from a molecular to a metallic state. Here we conduct computer simulations in a spherical shell geometry to understand ho'w the magnetic fields and zonal flows interact in the deep interior of giant planets. We show that magnetic fields disrupt or weaken jet streams, but may enhance radial convective fluid motions. Crucially, our simulations provided a relation to link magnetic field strength with jet stream intensity at the surface, aligning well with real observations from Jupiter and Saturn. This supports the theory that the jet streams on these giant planets are driven by deep convection and are confined to the outer molecular hydrogen layers. Key Points: We show that the magnetic field suppresses zonal flows in the molecular layer and enhances convective motionsWe extract a scaling relation between the magnetic field and the zonal flow at the surface that matches observations of Jupiter and SaturnThis study provides alternative evidence that the zonal winds on gas planets are deep‐seated and convection driven [ABSTRACT FROM AUTHOR]

Copyright of Plateau Meteorology is the property of Plateau Meteorology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Plateau Meteorology is the property of Plateau Meteorology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

During the first half of the Holocene (11 000 to 5000 years ago), the Northern Hemisphere experienced a strengthening of the monsoonal regime, with climate reconstructions robustly suggesting a greening of the Sahara region. Palaeoclimate archives also show that this so-called African humid period (AHP) was accompanied by changes in climate conditions at middle to high latitudes. However, inconsistencies still exist in reconstructions of the mid-Holocene (MH) climate at mid-latitudes, and model simulations provide limited support in reducing these discrepancies. In this paper, a set of simulations performed using a climate model are used to investigate the hitherto unexplored impact of Saharan greening on mid-latitude atmospheric circulation during the MH. Numerical simulations show Saharan greening has a year-round impact on the main circulation features in the Northern Hemisphere, especially during boreal summer (when the African monsoon develops). Key findings include a westward shift in the global Walker Circulation, leading to modifications in the North Atlantic jet stream in summer and the North Pacific jet stream in winter. Furthermore, Saharan greening modifies atmospheric synoptic circulation over the North Atlantic, enhancing the effect of orbital forcing on the transition of the North Atlantic Oscillation phase from predominantly positive to negative in winter and summer. Although the prescription of vegetation in the Sahara does not improve the proxy–model agreement, this study provides the first constraint on the influence of Saharan greening on northern mid-latitudes, opening new opportunities for understanding MH climate anomalies in regions such as North America and Eurasia. [ABSTRACT FROM AUTHOR]

Regional weather variability and extremes over Europe are strongly linked to variations in the North Atlantic jet stream, especially during the winter season. Projections of the evolution of the North Atlantic jet are essential for estimating the regional impacts of climate change. Therefore, separating forced trends in the North Atlantic jet from its natural variability is an extremely relevant task. Here, a deep learning based method, the Latent Linear Adjustment Autoencoder (LLAE), is used for this purpose on an ensemble of fully‐coupled climate simulations. The LLAE is based on a variational autoencoder and an additional linear component. The model uses detrended temperature and geopotential to predict the component of the zonal wind associated with natural variability. The residual between this prediction and the original wind field is interpreted as the forced component of the jet. The method is first tested for the geostrophic wind for which the forced trend can be obtained analytically from the difference between geostrophic wind computed from detrended and full geopotential. Despite the large variability of the total trends, the LLAE is shown to be effective in extracting the forced component of the trend for each individual ensemble member in both geostrophic and full wind fields. The LLAE‐derived forced trend shows an increase in the upper‐level zonal wind speed along a southwest–northeast oriented band over the ocean and a jet extension toward Europe. These are common characteristics over different periods and show some similarities to the upper‐level zonal wind speed trend obtained from the ERA5 reanalysis. Plain Language Summary: The North Atlantic jet stream, which is a narrow and strong air current flowing from west to east, has a substantial influence on daily weather variability and temperature and precipitation extremes in Europe. It is therefore essential to investigate how the North Atlantic jet is being modified under climate change. Trends in the North Atlantic jet are the combination of an external forcing produced by the increase in greenhouse gases and natural variability across multiple time scales. In this study, we use a data driven model called Latent Linear Adjustment Autoencoder (LLAE) to separate the jet trends related to the effect of the increase of greenhouse gases from the natural variability. The output of a set of climate simulations, namely zonal wind, temperature and geopotential, is used to adapt the adjustable parameters of the LLAE. After this step, the difference between the wind provided by the LLAE and the original field contains the effect of the external forcing. The method is successful in removing most effects associated with natural variability and reveals an intensification increase of the North Atlantic jet along a southwest–northeast oriented band and an extension over Europe. Key Points: A deep learning‐based method is able to extract forced trends in the winter North Atlantic jet stream from climate simulationsThe characteristics of the forced trends are consistent across different periodsForced trends show an intensification of the jet along a southwest–northeast oriented band and a jet extension over Europe [ABSTRACT FROM AUTHOR]