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Midlatitude weather extremes such as blocking events and Rossby wave breaking are often related to large meridional shifts in the westerly jet stream. Numerous diagnostic methods have been developed to characterize these weather events, each emphasizing different yet interrelated aspects of circulation waviness, including identifying large-amplitude ridges or persistent anomalies in geopotential height. In this study, we introduce a new metric to quantify the circulation waviness in terms of effective time scale. This is based on the Rossby wave packet from the one-point correlation map of anomalous meridional wind, applicable to jet waviness involving multiple wavenumbers. Specifically, we estimate the intrinsic frequency of Rossby waves and decay time scale of wave amplitude in the reference frame moving at the local time mean zonal wind. The resulting effective time scale, derived from linear theory, serves as a proxy for the eddy mixing time scale in jet meandering. Remarkably, its spatial distribution roughly resembles that of circulation waviness in the Northern Hemisphere winter as depicted by local wave activity (LWA). In the high-latitude regions characterized by weak zonal winds, the long time scale in waviness aligns with large values in LWA. By contrast, short waviness time scales in subtropical jet regions correspond to the suppressed amplitude in waviness despite large values in eddy kinetic energy (EKE). Furthermore, the effective time scale in waviness largely captures the interannual variability of LWA in observations and its projected future changes in climate model simulations. Thus, this relation between the waviness time scale and zonal wind provides a physical mechanism for understanding how zonal wind changes impact regional weather patterns in a changing climate. Significance Statement: The purpose of this study is to better understand what controls weather extremes in midlatitude regions such as blocking events and Rossby wave breaking. We introduce a novel concept, the effective time scale of jet stream meandering, which sheds light on these phenomena. Through analyzing Rossby waves in the reference frame moving at the local time mean zonal wind, we derive a scaling relation between circulation waviness and eddy mixing time scale. Our findings reveal that this time scale closely mirrors the spatial distribution of circulation waviness in the Northern Hemisphere winter. Importantly, it captures interannual variability and climate change responses. These insights provide a physical basis for understanding how changes in zonal wind impact regional weather patterns in observations and climate models. [ABSTRACT FROM AUTHOR]

The variability of the phase speed of the Madden–Julian oscillation (MJO) is poorly understood. The authors assess how the phase speed of the convective signal of the MJO associates with the background states over eastern Africa and the Indian Ocean. Relaxation of the coupling between tropical modes and their circulation has been previously linked to faster propagation; for example, the MJO speeds up over the eastern Pacific where its convective signal decouples from the circulation. In contrast, our results show that fast MJO events happen to exist during periods of wetter background states (>90 days) from East Africa across the Indian Ocean, whereas slow MJO is associated with dry background states. We found that fast MJO exhibits strong active and inactive phases with a structure suggesting more hierarchical convection. Results indicate that the association of the phase speed of the MJO as seen in the integrated filtered moist static energy with its tendency is stronger than the association of the phase speed as observed in the dry static energy with its tendency which is consistent with the acceleration of the MJO during wet background states. Also, our results indicate that the MJO may be faster during periods of enhanced low-level moisture because these periods have anomalously weak upper-tropospheric easterly background winds, which reduce the westward advection of the MJO by the background easterly wind, resulting in higher eastward phase speed of the MJO. The acceleration of the MJO by the background zonal wind overwhelms the deceleration associated with the moist-wave dynamics. Significance Statement: This study shows that the Madden–Julian oscillation (MJO), which is the dominant subseasonal weather signal in the tropics, moves eastward more quickly across eastern Africa and the Indian Ocean when the region is abnormally moist. The faster propagation does not appear to result from the higher moisture but instead from encountering weaker-than-normal upper-air winds from the east that tend to occur during moist periods. [ABSTRACT FROM AUTHOR]

Sporadic E (Es) layers are irregular structures that occur at the E-layer height of the ionosphere, significantly affecting the reliability and accuracy of wireless communications, navigation, and satellite remote sensing. This study utilized the S4max data collected from the Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) occultation observations from 2007 to 2016 to identify the Es layer and investigate its climatological variations. The Horizontal Wind Field model (HWM14), in conjunction with the International Geomagnetic Reference Field model (IGRF13), is used to calculate vertical ion convergence (VIC) and analyze its correlation to the Es layers. The results of this study showed that the occurrence of Es has apparent hemispheric asymmetry. In the mid- and low latitudes, Es layer activity is more intense in the summer hemispheres, with center peak altitudes of around 105 km. The summer hemisphere exhibits a semi-diurnal periodic pattern, whereas the winter hemisphere shows a weakened diurnal variation. Simulation studies indicate that VIC induced by neutral wind shear contributes to the asymmetry in Es layer activities observed between the Northern and Southern hemispheres, and the zonal wind shear plays a more critical role than the meridional wind one. [ABSTRACT FROM AUTHOR]

The impact of El Niño–Southern Oscillation (ENSO) on the equatorial Atlantic zonal mode (AZM) is investigated using two atmospheric reanalysis products and 44 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). While the impact of ENSO on equatorial Atlantic SST is inconsistent, as previously reported, there is a very robust influence of decaying ENSO events in boreal spring on the contemporaneous surface zonal winds over the equatorial Atlantic, with El Niño events associated with anomalous easterlies that cool the equatorial Atlantic. This dynamic impact is counteracted by a thermodynamic impact, in which El Niño events cause changes in surface heat fluxes that lead to warming over the tropical Atlantic. The thermodynamic influence is active throughout the life cycle of the ENSO event, but the dynamic influence is highly seasonal with a pronounced peak in boreal spring. Thus, a quickly decaying El Niño event will lead to warming, while a slowly decaying event will lead to cooling in the equatorial Atlantic. The ENSO–AZM relation is further complicated by partially independent Atlantic variability, in particular that associated with the South Atlantic subtropical dipole (SASD). Prediction experiments with a simple linear regression model support the role of the SASD as an AZM precursor. Many CMIP6 models show skillful predictions of the June–August AZM based on SST indices in the preceding December–February, with correlations above 0.5. When predictions are restricted to ENSO years, even more models exceed this threshold, but skill in the reanalyses remains relatively low, possibly due to insufficient training data. Significance Statement: El Niño is the strongest climate signal on interannual time scales. Its influence on the equatorial Atlantic, however, is surprisingly inconsistent. Here, we show, using an ensemble of climate models and observation-based data, that El Niño has a very robust influence on equatorial Atlantic surface winds, but only in boreal spring. This leads to a large difference in how El Niño events influence the equatorial Atlantic, based on how quickly they decay in boreal spring. We also find that variability in the South Atlantic, which is partly independent of El Niño, has a strong influence on the equatorial Atlantic, supporting the result of previous studies. These findings suggest that the predictability of equatorial Atlantic temperature anomalies may be higher than previously thought. [ABSTRACT FROM AUTHOR]

Climate change simulations generally indicate the strengthening of El Niño–Southern Oscillation (ENSO) sea surface temperature (SST) variability through the twenty-first century, yet a robust physical mechanism explaining this change across different models is still lacking, and the projections for ENSO amplitude exhibit a large spread. Most commonly, changes in the background state of the tropical Pacific are invoked to explain these changes of ENSO. Here, we show that changes in the structure of wind stress anomalies associated with ENSO are potentially as important as these background state changes. Specifically, changes in the magnitude, meridional width, and zonal structure of wind stress anomalies can explain approximately 53% of the intermodel variance in the projected change of ENSO magnitude through the twenty-first century as well as 43% in ENSO periodicity changes. Among these changes in the wind structure, the meridional widening of wind anomalies plays the most important role. To demonstrate that these changes are indeed critical, we develop a hybrid model of ENSO based on the Community Earth System Model, version 2, which incorporates a dynamical ocean coupled to a simplified statistical atmosphere within the tropical Pacific. In the absence of external forcing and corresponding mean-state changes, the imposed changes in wind stress anomalies in this hybrid model result in an increase of ENSO amplitudes of nearly 10% along the equator. Our results are also theoretically supported by a recharge-oscillator model that incorporates the meridional wind structure. Thus, changes in the structure of wind stress anomalies, together with changes in the mean state, likely play a critical role in the projected strengthening of ENSO. [ABSTRACT FROM AUTHOR]

ABSTRACT Exploring the impacts of climate variability on the marine fishery ecosystems in the Southwest Atlantic Ocean is conducive to establishing an ecosystem‐based approach for the protection and rational utilization of fishery resources. In this study, long‐term fisheries data, 23 environmental data from the entire Southwest Atlantic, and 25 global climate data have been used to explore the regime shift of the fishery ecosystem and the response of fishery resources to climate change from 1950 to 2018. The results indicated that changes in the Southwest Atlantic fishery ecosystem exhibited a significant nonstationary trend, and there were three noteworthy regime shifts in 1976/1977, the late 1980s, and the late 20th century. The temperature, sea surface height, water runoff, and cloudiness were the environmental variables with the greatest impact on fishery resources within the Southwest Atlantic Fishery Ecosystem, while zonal wind speed and air temperature yielded a more significant impact on low latitude areas. In terms of climate indices, fishery resources have the most obvious response to the Global Mean Land‐Ocean Temperature Index and Antarctic Sea Ice Extent, and the Atlantic Multidecadal Oscillation had an intense impact on low latitude areas concurrently. The study highlights the climate‐related nonstationary changes in the Southwest Atlantic fishery ecosystem. [ABSTRACT FROM AUTHOR]

In this study, we demonstrate the dynamical core and applicability of Aeolus 2.0, a moist-convective thermal rotating shallow water model of intermediate complexity, along with its novel bulk aerodynamic and moist-convective schemes, in capturing the effects of increased radiative forcing on zonal winds and heatwaves. Simulations reveal seasonal patterns in zonal wind, temperature, and energy anomalies under increased radiative forcing during the summer solstice, winter solstice, and equinoxes. Increased radiative forcing enhances mid-latitudinal temperatures during the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere, leading to increased zonal wind velocity in the affected hemisphere, especially in the subtropics, while decreasing it in the opposite hemisphere. This thermal forcing also reduces the zonal wind velocity of polar cyclones in the hemisphere experiencing increased radiative forcing. During the autumn equinox, zonal wind velocity diminishes in the Southern Hemisphere, while a similar reduction occurs in the Northern Hemisphere during the spring equinox. Heightened meridional gradients significantly influence the poleward displacement of atmospheric circulation, particularly during the summer (northward) and winter (southward) solstices. Poleward eddy heat fluxes persist across hemispheres, indicating a consistent response to external heating. Increased radiative forcing during the summer and winter solstices amplifies prolonged heatwaves across land and ocean, exceeding impacts observed during the spring and autumn equinoxes. [ABSTRACT FROM AUTHOR]

Analysis of ionosonde observations over Chumpon (10.7°N, 99.4°E, 3.3° Mag. Lat.) and Kototabang (0.2° S, 100.3° E, 10° S Mag. Lat.) during the low solar activity year 2020 reveals that the occurrence of post-sunset spread-F maximises during winter (November-February), whereas that of post-midnight spread-F maximises during summer (May-August). The thermospheric winds observed by the Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) on board the Ionospheric Connection Explorer (ICON) satellite over the Indonesian longitudes (95°E-115°E) are investigated to determine their impact on the generation of spread-F. The thermospheric winds at equatorial and low latitudes (10°N) are strongly eastward (> 50 m/s) around post-sunset hours (∼19:00 LST) in winter and equinox. The strong eastward wind is present only around midnight hours (after 22:00 LST) in summer. However, the zonal wind does not appear to contribute to the generation of plasma bubbles. The ICON-MIGHTI meridional wind at low latitude (10°N) is observed to be equatorward throughout the day during summer. Compared to the post-sunset hours, the intensity of trans -equatorial wind is stronger on the windward side (60–70 m/s) than on the leeward side (10–20 m/s) during midnight hours. The component of the equatorward meridional wind along the magnetic field lines can lift the F-layer to higher heights on the windward side leading to a net decrease in the field line-integrated Pedersen conductivity thereby increasing the growth rate of the Rayleigh Taylor instability that can result in the generation of post-midnight spread F. [ABSTRACT FROM AUTHOR]

The radiation coming from the Sun heats the Earth's atmosphere and generates atmospheric tides. In this paper, we model atmospheric tides in the presence of background wind and dissipation processes and investigate their effects. The equations for tidal oscillations of wind and temperature in the atmosphere encompass factors such as background wind, temperature profile, and background composition including ozone, carbon dioxide, hydro-magnetic interactions, Newtonian cooling, eddy, and molecular diffusion. These components interact to define the behavior of tidal phenomena comprehensively. Thermal forcing processes include the insolation absorption of H 2 O in the troposphere, O 3 in the stratosphere, and a contribution from O 2 absorption in the thermosphere. The method of solution for the equations is outlined for the solar diurnal tides during the March equinox by considering all these dissipation processes and the background wind. The obtained results are in good agreement with the Global Scale Wave Model (GSWM-00). It is found that the background wind plays significant role in affecting the horizontal wind oscillations below 100 km. At higher altitudes (100–200 km), the background wind, ion drag force, divergence of momentum, and heat fluxes due to molecular and eddy diffusion have a considerable role in affecting the zonal and meridional winds. [ABSTRACT FROM AUTHOR]

This study aims to understand the source of surface air temperature bias over the contiguous United States (CONUS) during boreal summer (June–September) in the Unified Forecast System (UFS) coupled model prototype 8 (P8), developed by the National Centers for Environmental Prediction (NCEP) and the National Oceanic and Atmospheric Administration (NOAA). The focus is on the subseasonal variability defined as a weekly average in weeks 2–5 of forecast leads (total 224 cases; 4 weeks × 2 initialization dates × 4 months × 7 years). The large-scale surface air temperature bias pattern is extracted using the empirical orthogonal function (EOF) analysis. The associated principal component describes the variability of bias for each reforecasting week throughout the 2011–17 reforecasting period. The leading EOF of surface air temperature bias exhibits an east–west dipole pattern over the CONUS, explaining 31.6% of the total variability of weekly temperature bias. This bias pattern is strongly related to the upper-level Rossby wave induced by a bias in convection over the central tropical Pacific. Furthermore, the mean bias of background flow in the extratropics degrades the representation of teleconnections from the tropics to the midlatitudes. UFS P8 has weaker zonal wind over the North Pacific with stronger vertical wind shear than the ERA5 reanalysis. The weak zonal wind hampers the Rossby wave's propagation, while strong vertical shear reduces its amplitude. [ABSTRACT FROM AUTHOR]

The impact of global warming on Central Asian precipitation suffers from huge uncertainties, yet their origins remain largely unidentified. This study investigates the inter‐model spread in the projection of winter Central Asian precipitation (WCAP) using models from the Coupled Model Inter‐comparison Project Phase 6. The results reveal a homogenous wetting trend in WCAP, with the primary source of uncertainty in its magnitude stemming from the dynamic component of vertical moisture advection. This dynamical uncertainty is further attributed to an out‐of‐phase variation between two Eurasian westerly jets, which can enhance the upward motion over Central Asia through positive relative vorticity advection and warm temperature advection. The opposing variation between two westerly jets can be traced to cooling in the North Pacific, which alters jet intensity through changes in the meridional temperature gradient. This study informs policy and adaptation strategies to better cope with future climate shifts in Central Asia. Plain Language Summary: Central Asia (CA) faces high water demand for agriculture and livelihoods, but struggles with scarce precipitation and dry soils. Therefore, accurate simulation and projection of precipitation in CA are crucial. Based on Coupled Model Inter‐comparison Project Phase 6 models, this study shows a consistent wetting trend in winter precipitation across CA under global warming, with an 8.84%/K increase projected under the SSP5–8.5 scenario. The moisture budget analysis reveals that the primary source of uncertainty in precipitation projections comes from the dynamic component of vertical moisture transport. The strengthened ascending motion over CA is further attributed to an enhanced Eurasian polar front jet (EAPJ) and a weakened Eurasian subtropical jet (EASJ) through positive relative vorticity advection and warm temperature advection. The intensity changes of these two jets can be traced back to cooling in the North Pacific, which results in dipole meridional temperature gradient (MTG) changes over CA. Negative (positive) MTG changes in northern (southern) CA enhance (reduce) the mean‐state MTG and increase (decrease) upper‐level zonal winds according to the thermal wind relation, thereby intensifying (weakening) the EAPJ (EASJ). This study advances the understanding of uncertainties in precipitation projection and informs policymakers on addressing water shortages in the climate‐sensitive CA. Key Points: Winter precipitation projections over Central Asia exhibit substantial inter‐model uncertainty in the magnitude of the wetting trendThis uncertainty in precipitation projection is primarily driven by the dynamic component of vertical moisture advectionThe dynamical uncertainty is attributed to the out‐of‐phase variation between two westerly jets, which is traced to North Pacific cooling [ABSTRACT FROM AUTHOR]

ABSTRACT The Brazil current (BC) is a westerly current that flows south along the Brazilian coast, being part of the Southern Brazilian Continental Shelf (SBCS). Recent studies have indicated a trend towards intensification and a shift to the south of the region where this current predominates. We analysed the seasonality of the relationship between atmospheric variables and sea surface temperature (SST) with potential trends and changes in the spatio‐temporal pattern of these variables from 1980 to 2020 over the BCs area in the South Atlantic Ocean. For this purpose, monthly data on SST, air temperature at 2 m above the surface (T2M), mean sea level pressure (MSLP), zonal wind (U10) and meridional wind (V10), obtained from ERA5 reanalysis, were used. Descriptive statistical analyses, trends using the Mann‐Kendall test, correlation matrices and Pettitt's test revealed a significant spatial correlation between the variables, with temporal trends of variation, especially over the BCs area. The meridional (zonal) wind predominantly exhibited a north–south (west–east) direction, supporting the hypothesis that the study region was displaced. Additionally, statistically significant positive trends were observed for SST (0.02°C ∙ dec−1 in austral autumn, winter and spring and 0.01°C ∙ dec−1 in austral summer), T2M (0.02°C ∙ dec−1 in austral winter and spring), MSLP (0.05°C ∙ dec−1 in austral autumn) and negative for U10 (−0.01°C ∙ dec−1 in austral spring). Pettitt's test results confirm significant changes in the behaviour of most analysed variables from the late 1990s to the early 2000s. The post‐breakpoint periods of the variables consistently showed above‐average values compared to the pre‐breakpoint periods, supporting positive upward trends consistent with literature findings. In particular, they highlight the notable upward slope in SST in the BC region. [ABSTRACT FROM AUTHOR]

Mixed Rossby‐gravity (MRG) waves play a significant role in tropical variability. Their kinetic energy spectra exhibit maximal amplitudes at synoptic scales in the upper troposphere and at planetary scales in the upper stratosphere. The mechanism for different scale selection in the two regions has remained elusive. Here, we use a spherical barotropic model with the background zonal wind profiles derived from ERA5 reanalysis to show that the recently introduced MRG wave excitation mechanism − ${-}$ wave‐mean flow interactions produces MRG waves with the observed scale properties in the two regions. Simulations with idealized zonal jets show that the jet position determines the MRG scale selection: the closer the jet to the equator, the smaller the scale of the excited MRG waves. Therefore, midlatitude jets, such as found in the upper stratosphere, support the excitation of planetary‐scale MRG waves. Plain Language Summary: Mixed Rossby‐gravity (MRG) waves, which are important for tropical weather and climate, obtain different spatial scales in two distinct atmospheric regions: synoptic scales in the upper troposphere and planetary scales in the upper stratosphere. So far, the mechanism responsible for this behavior is not identified. In this study, we propose interactions involving waves that originate from the tropics and zonal jets (i.e., wave‐mean flow interactions) as the mechanism behind the observed MRG wave scales. We provide evidence by conducting numerical simulations with an idealized model that resolves MRG waves with high accuracy. We identify MRG waves generated by wave‐mean flow interactions and show that their peak scale can be attributed to the position of the jet. The closer the jet to the equator, the smaller the scale of generated MRG waves. This not only offers an explanation for the observed spatial scales of the tropospheric and stratospheric MRG waves, but also points out the importance of the background flow to correctly represent MRG waves in weather and climate models. Key Points: Mixed Rossby‐gravity waves in the upper stratosphere and troposphere have peak scales at zonal wavenumbers 1–3, and 6, respectivelyObserved peak scales are generated by wave‐mean flow interactions in numerical simulations with the observed zonal‐mean zonal wind profilesThe peak MRG scale is associated with the jet position: the closer the jet to the equator, the smaller the scale of the excited waves [ABSTRACT FROM AUTHOR]

The predictability of the El Niño‐Southern Oscillation (ENSO) is significantly limited by the spring predictability barrier (SPB). Our observational analysis suggests that considering high‐frequency wind components during the spring season may mitigate the SPB impact on ENSO predictability. The intraseasonal zonal wind in March over the equatorial western Pacific initially perturbs the sea surface temperature (SST). It contributes nearly 40% of the east‐west SST gradient after 2–3 months, ranking first among other calendar months. This significant contribution causes March to become the earliest month to effectively indicate the following ENSO direction due to the active eastward‐propagating Madden‐Julian oscillation (MJO) activity. Through the obvious variation of the eastward‐propagating MJO speed, it also shows the possible close relationship between the mean SST state and the interdecadal variability in intraseasonal zonal wind. Additionally, the current strong variability of intraseasonal zonal wind suggests the important role of atmospheric information in recent ENSO development. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) is known to have a far‐reaching impact on global climate. Our present climate models, however, remain large challenges in predicting ENSO events when the predictions made cross the boreal spring. In this study, we find that utilizing the atmospheric information in spring could weaken the challenges mentioned above. The intraseasonal zonal wind with a main period of 30–90 days over the equatorial western Pacific in March provides strong support for the ENSO developments after 2–3 months, with a contribution as high as nearly 40%, ranking first among other calendar months. This significant contribution establishes a strong connection between the ENSO peak states and the high‐frequency winds, and it causes March to become the earliest month to effectively indicate the following ENSO developments. Changes in long‐term mean sea surface temperature modulate the eastward propagation speed of convective activity, and it possibly establishes significant relationship with the intraseasonal zonal wind in March. Recently, the strong intraseasonal zonal wind has alerted us to focus on the high‐frequency winds in spring and their controlled processes, which could improve our prediction skills across the spring season. This study highlights the potential impact of intraseasonal zonal winds in March on ENSO's predictability as a critical predictor. Key Points: The intraseasonal zonal wind over the equatorial western Pacific in March has a uniquely connection with the subsequent ENSO evolutionThe background SST zonal gradient modulates the contribution of intraseasonal zonal wind through the eastward propagation of MJO activityRecent spring's high‐frequency wind components possibly enhance ENSO predictability by exciting subsequent significant oceanic response [ABSTRACT FROM AUTHOR]

Impacts of El Niño−Southern Oscillation (ENSO) on the quasi‐biweekly oscillation over the western North Pacific (WNP‐QBWO) in boreal winter are investigated in the study. The WNP‐QBWO in boreal winter primarily propagates westward from the tropical western Pacific to WNP. During the La Niña winter, the QBWO over the WNP has stronger intensity and propagates westward at a faster speed, while it is weaker and propagates more slowly during the El Niño winter. The possible mechanisms may involve the ENSO‐related background moisture and zonal wind vertical shear changes that can significantly modulate the WNP‐QBWO's behaviours in boreal winter. A 2.5‐layer atmospheric model is applied in the study and confirms the results. It is further revealed that the moisture change is dominant in modulating the WNP‐QBWO's intensity, while both the moisture and vertical shear changes may together contribute to the zonally propagating speed of the WNP‐QBWO in boreal winter. These results can deepen our understanding of dynamic processes associated with the WNP‐QBWO in boreal winter and are conducive to the predictability study. [ABSTRACT FROM AUTHOR]

The meridional displacements of the East Asian westerly jet (EAJ) exhibit remarkable interannual variability characteristics, significantly modulating the weather and climate over East Asia. Our study aims to refine the thermal‐driven (angular momentum transport) and dynamically driven (atmospheric eddy activities) processes that dominate the interannual meridional motion of the wintertime EAJ. We employ the recently proposed three‐pattern decomposition of global atmospheric circulation (3P‐DGAC) theory to decompose the zonal momentum equation for the purpose of diagnosing the zonal momentum budget of the EAJ from three‐dimensional (3D) horizontal, meridional and zonal circulation perspectives. Results indicate that, in addition to the widely recognised driving mechanisms of angular momentum transported by thermal direct meridional Hadley circulation and horizontal eddy momentum flux convergence, the previously neglected role of local zonal circulation in transporting angular momentum significantly impacts the meridional shift of EAJ. The uneven accumulation of angular momentum transported by asymmetric zonal circulations emerges on the northern and southern sides of EAJ axis, causing an abnormal meridional displacement of EAJ. Notably, the local zonal circulation rising from the Tibetan Plateau and descending across the North Pacific Ocean transported angular momentum to the core and downstream regions of EAJ, thereby significantly accelerating zonal winds and pushing the EAJ northward. Further investigation into the potential mechanisms suggests that anomalous snow depth on the southern Tibetan Plateau is the main factor causing abnormal local zonal circulation. Our study addresses the deficiency in traditional two‐dimensional (2D) circulation decomposition methods that neglect the significant role of zonal circulation in the thermal‐driven process of EAJ, complements the understanding of 3D angular momentum transport mechanisms and reveals potential nonlinear synergies of 3D circulations related to the interannual meridional displacement of EAJ. [ABSTRACT FROM AUTHOR]

This study investigates the inter‐model spread of climatological extratropical westerly jets in boreal winter, using the historical simulation of 52 Coupled Model Intercomparison Project phase 6 (CMIP6) models from 1851 to 2014. The results show that there is a substantial spread in the latitude of the upper‐tropospheric westerly jet across models, characterised by large inter‐model standard deviations to both the poleward and equatorward sides of the jet axis, although the multi‐model ensemble mean (MME) performs well in simulating meridional position of westerly jets. Furthermore, we detect the consistency of inter‐model jet position spread between the Northern and Southern Hemispheres, based on the inter‐model empirical orthogonal function (EOF) decomposition and correlation of regional‐averaged zonal winds. Specifically, the models that simulate the westerly jets poleward/equatorward relative to the MME position in one hemisphere also tend to simulate the jets poleward/equatorward in the other hemisphere. Accordingly, we define a global jet spread index to depict the concurrence of jet shift in the two hemispheres. The results of inter‐model regression analyses based on this index indicate that the models positioning the jets poleward than the MME tend to simulate a wider Hadley Cell, a poleward‐shifted Ferrel Cell in the Southern Hemisphere, enhanced precipitation in the subtropics and suppressed precipitation in the tropics, and warmer sea surface temperatures in the subtropics and mid‐latitudes. The present results suggest that improving the simulation of jet positions in climate models requires a comprehensive consideration of thermal states in the tropics and subtropics/mid latitudes. [ABSTRACT FROM AUTHOR]

This study investigates the causes of shifts in the subsiding edge of the boreal winter Hadley cell (HC) in response to a comprehensive treatment of ocean surface albedo (OSA) in the fully coupled CESM2. The focus is on an in‐depth understanding of the atmospheric dynamical processes that influence the HC subsiding edge. Two sets of experiments were performed: one utilizing the default OSA, and the other employing the comprehensive OSA that accounts for realistic physical mechanisms. The results show that implementing the comprehensive OSA simulates an El Niño‐like warming pattern in reference to the default experiment, which leads to an HC contraction. Examination of zonal mean momentum dynamics in the upper troposphere reveals that variations in meridional winds, crucial for determining the HC extent, are primarily driven by the differences in the horizontal eddy momentum flux derivative. The findings indicate that the equatorward shift in meridional temperature gradients enhances subtropical zonal winds and baroclinicity along their equatorial flanks, amplifying equatorward‐propagating Rossby waves. This, in turn, alters the eddy momentum flux, reshaping the pattern of the derivatives of horizontal eddy momentum flux, constraining meridional winds, and resulting in the equatorward movement of the HC subsiding edge. A scaling theory further supports the results of the HC contraction, showing that the increased subtropical zonal winds and the equatorward shift of the Intertropical Convergence Zone (ITCZ) elevate the atmospheric angular momentum and eventually limit the expansion of the HC. Plain Language Summary: The Hadley cell (HC) is a large‐scale tropical circulation system that exerts a substantial influence on global and regional climates. However, it is necessary to further explain the mechanisms that control the HC edge. This study investigates how the HC descending edge shifts when a comprehensive ocean surface albedo (OSA), including more realistic physical processes, is used in a fully coupled climate model. Two sets of simulations were conducted: one using the default OSA, which neglects certain physical processes, and the other using the comprehensive OSA. The comprehensive OSA experiment simulated an El Niño‐like warming climate in contrast to the default simulation, with equatorward movement of the HC edge. The present study finds that the equatorward shift in the air meridional temperature gradient results in the intensification of the subtropical zonal winds and baroclinic instability along their equatorial flanks, enhancing equatorward Rossby wave activities and affecting the pattern of the horizontal eddy momentum flux. This constrains the meridional winds, causing the HC edge to move equatorward. A scaling theory supports these findings by showing that the stronger subtropical winds and the equatorward shift of the Intertropical Convergence Zone (ITCZ) increase the atmospheric angular momentum and subsequently prevent the HC expanding. Key Points: The El Niño‐like warming pattern due to considering ocean surface albedo physics comprehensively is demonstratedEquatorward shift in temperature gradients alters eddy momentum flux, limits meridional winds, and contracts the Hadley Cell (HC)A scaling theory supports HC contraction with stronger subtropical zonal winds, enhancing angular momentum and limiting its extent [ABSTRACT FROM AUTHOR]

The impact of the quasi-biennial oscillation (QBO) and Madden–Julian oscillation (MJO) on the dynamics of major sudden stratospheric warmings (SSWs) observed in the winters of 2018, 2019, and 2021 is investigated. Using data from the MERRA-2 reanalysis, we analyze the daily mean variability of critical atmospheric parameters at the 10 hPa level, including zonal mean polar cap temperature, zonal mean zonal wind, and the amplitudes of planetary waves 1 and 2. The results reveal dramatic increases in polar cap temperature and significant wind reversals during the SSW events, particularly in 2018. The analysis of planetary wave (PW) amplitudes demonstrates intensified wave activity coinciding with the onset of SSWs, underscoring the pivotal role of PWs in these stratospheric disruptions. Further examination of outgoing long-wave radiation (OLR) anomalies highlights the influence of QBO phases on tropical convection patterns. During westerly QBO (w-QBO) phases, enhanced convective activity is observed in the western Pacific, whereas the easterly QBO (e-QBO) phase shifts convection patterns to the maritime continent and central Pacific. This modulation by QBO phases influences the MJO's role during SSWs, affecting tropical and extra-tropical weather patterns. The day-altitude variability of upward heat flux reveals distinct spatiotemporal patterns, with pronounced warming in the polar regions and mixed heat flux patterns in low latitudes. The differences observed between the SSWs of 2017–2018 and 2018–2019 are likely related to the varying QBO phases, emphasizing the complexity of heat flux dynamics during these events. The northern annular mode (NAM) index analysis shows varied responses to SSWs, with stronger negative anomalies observed during the e-QBO phase compared to the w-QBO phases. This variability highlights the significant role of the QBO in shaping the stratospheric and tropospheric responses to SSWs, impacting surface weather patterns and the persistence of stratospheric anomalies. Overall, the study demonstrates the intricate interactions between stratospheric dynamics, QBO, and MJO during major SSW events, providing insights into the broader implications of these atmospheric phenomena on global weather patterns. [ABSTRACT FROM AUTHOR]

Equatorial Plasma Bubbles (EPBs) are zones characterized by fluctuations in plasma densities which form in the low-latitude ionosphere primarily during the post-sunset. They subject radio signals to amplitude and phase variabilities, affecting the functioning of technological systems that utilize the Global Navigation Satellite Systems (GNSS) signals for navigation. Thus, understanding EPB occurrence patterns and morphological features is vital for mitigating their effects. In this work, we employed two GNSS receivers and an All-Sky Imager (ASI) to conduct simultaneous observations on the morphology of EPBs over Brazil. The main objectives of the study were (1) to develop a Random Forest (RF) machine-learning model to estimate and predict the zonal drift velocities of EPBs, and (2) to compare the model predictions with actual EPB drifts inferred from the two instruments, as well as zonal neutral wind speeds obtained from the Horizontal Wind Model (HWM-14). In the model development, we utilized reliable EPB drift measurements made during geomagnetically quiet days between 2013 and 2017 in Brazil. The model predicted the velocities based on parameters including the day of the year, universal time, critical frequency of the F2 layer (foF2), solar and interplanetary indices. The correlation coefficients of 0.98 and 0.96 and RMSE values of 10.61 m/s and 10.06 m/s were obtained upon training and validation correspondingly. We evaluated the accuracy of the model in predicting EPB drifts on two geomagnetically quiet nights where an average correlation coefficient of 0.89 and an RMSE of 15.74 m/s were obtained. The predicted drifts, the zonal neutral wind velocities, and the GNSS and ASI velocity measurements were put into context for validation purposes. Overall, the velocities were comparable and ranged between ∼100 m/s and ∼30 m/s from the hours of 00 UT to 05 UT. The results confirmed the accuracy and applicability of the model, revealing the ionosphere-thermosphere coupling influence on the nocturnal propagation of EPBs under the full activation of the F region dynamo. [ABSTRACT FROM AUTHOR]

On 10 May 2024, a super geomagnetic storm occurred with a minimum Dst index of −412 nT. This study investigates the behavior of the ionospheric radial current (IRC) during the main phase of this storm using data from Swarm A and simulations from the Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIEGCM). At dawn, the IRC exhibits distinct temporal variations: a strong outward flow of 14 nA/m2 between 18:00 and 20:00 UT and an inward flow of −15 nA/m2 by 24:00 UT. At dusk, the IRC starts with an inward value of −6 nA/m2 and rapidly reverses to an outward flow peaking at 15 nA/m2 between 22:00 and 24:00 UT. The dawn‐dusk asymmetry of the IRC is primarily influenced by the merging electric field (Em). Increasing Em penetrates to low latitudes, establishing an eastward (dusk) or westward (dawn) electric field and enhancing the outward (dusk) or inward (dawn) IRC through meridional Hall currents. Model results indicate that the strong polarity changes in dawn IRC are linked to local disturbed zonal winds (Δuy ${{\Delta }u}_{y}$). Which are eastward at dawn and westward at other magnetic local times (MLT). The eastward/westward Δuy ${{\Delta }u}_{y}$ generate outward/inward F‐layer dynamo currents, driving the disturbed vertical electric field (ΔEz ${{\Delta }E}_{z}$) in opposite direction. The significant outward reversal of dusk ∆IRC could be attribute to the rapid enhancement of disturbed Pedersen conductivity (Δσp ${{\Delta }\sigma }_{p}$). Due to nonzero geomagnetic declination, eastward/westward Δuy ${{\Delta }u}_{y}$ corresponds to an increase/decrease in Δσp ${{\Delta }\sigma }_{p}$. Key Points: Ionospheric radial current (IRC) has an extreme response of approximately 15 nA/m2 to the super stormThe increase of Em can enhance the outward/inward IRC at dawn/duskΔuy ${{\Delta }u}_{y}$ (Δσp ${{\Delta }\sigma }_{p}$) is key in the polarity changes of IRC at dawn (dusk) [ABSTRACT FROM AUTHOR]

This study investigates the response of the equatorial ionosphere to the tropospheric Madden‐Jullian Oscillation (MJO) using the Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X). Main results indicate that the intraseasonal variability of ionospheric vertical plasma drifts (Viz) is generally strong around boreal winters and exhibits significant eastward‐propagating signals. Composite analysis showed that, during the December‐January‐February‐March (DJFM) season, Viz was generally negative when the MJO was active over the Maritime Continent (Phase 5) and positive when the MJO was active over the Indian Ocean (Phase 2). The magnitude of MJO impact on Viz achieves ∼1.2 m/s, representing roughly 10% of the seasonal mean. The eastward‐propagating wavenumber‐4 is apparently strong during MJO phases 5 and 1–2, which is examined to be associated with non‐migrating tides below. Term analysis revealed the importance of zonal winds on driving the Viz responses. This study emphasizes the importance of tropospheric sources that affect the geospace environment. Plain Language Summary: The Madden‐Jullian Oscillation (MJO) is the dominant intraseasonal variability (∼30–90 days) in the troposphere, bridging the gap between weather and climate. The MJO is characterized by the eastward‐propagating convection‐circulation system in the tropics. This study examined, for the first time, the impact of the MJO on the ionosphere, the uppermost atmospheric region embedded with ionized plasma. The study conducted whole atmosphere simulations under perpetual solar minimum and geomagnetic quiescent to reveal the intraseasonal variability of the equatorial ionosphere and responses to the active location of MJO convective activities (i.e., MJO phases). The study found that the intraseasonal variability of ionospheric vertical plasma drifts (Viz) was significant and generally became strong during boreal winters. Composite analysis showed that, during the December‐January‐February‐March season, the Viz responses to MJO phases are negative in MJO Phase 5 (P5) and positive around P2. The peak‐to‐peak differences can reach ∼10% of the seasonal mean. Additionally, there is an apparent eastward‐propagating wavenumber‐4 pattern responding to the MJO, which is examined to be associated with MJO‐modulated non‐migrating DE3 tide. Term analysis revealed the importance of zonal winds in generating the Viz responses. This work investigates an instance of troposphere‐geospace teleconnection and improves the understanding of atmosphere‐ionosphere coupling. Key Points: Responses of the equatorial ionosphere to tropospheric MJO are first studied based on whole atmosphere simulation by the WACCM‐X modelIonospheric plasma drifts depend on the location of MJO‐convection (MJO phase), with peak‐to‐peak anomalies of ∼10% of the seasonal meanProminent eastward‐propagating wavenumber‐4 is found in the ionospheric responses to the MJO, which is associated with the DE3 tide below [ABSTRACT FROM AUTHOR]

This paper investigates the role of coupled E‐ and F‐regions electrodynamics behind the rapid poleward extension of two dark fronts of an electrified medium‐scale traveling ionospheric disturbance (EMSTID). The fronts of EMSTID observed in O(1D) 630.0 nm airglow images extended poleward from the southern edge of the field‐of‐view to the location of the imager at Hanle (32.7°N, 78.9°E; Mlat. ∼ $\mathit{\sim }$ 24.1°N), India, on the geomagnetically quiet (Ap = 3) night of 09 June 2021. The existence of the fronts outside the southern edge of field‐of‐view before and during their extension was apparent from temporal fluctuations in the vertical total electron content. Additionally, favorable direction of thermospheric neutral wind to initiate Perkins instability existed beyond the southern edge of field‐of‐view prior to EMSTID's first appearance in the airglow images. Therefore, the observed EMSTID were likely generated outside the southern edge of field‐of‐view. Interestingly, the expected clockwise rotation of fronts with clockwise rotation of thermospheric neutral wind was not observed, although it was necessary to sustain the positive‐growth of Perkins instability. This indicates the involvement of E‐F coupling to sustain the positive‐growth of EMSTID since the presence of sporadic‐E layers was evident from the signal‐to‐noise ratio amplitude of L1 band and zonal wind profiles over the observation region. The enhanced polarization electric field caused due to E‐F coupling produced larger vertical E × B drift, which helped the EMSTID to sustain positive‐growth during the fronts' extension. Additionally, the enhanced northwestward E × B drift helped to maintain the northwest‐southeast orientation of EMSTID fronts, during the rotation of thermospheric neutral wind. Key Points: Rapid poleward extension of two dark fronts of EMSTID observed in O(1D) 630.0 nm airglow images over Hanle, IndiaSporadic‐E layers over the region was observed from the altitude profiles of zonal wind and signal‐to‐noise ratio amplitude of L1 bandModified electrodynamics due to E‐F coupling was plausible mechanism behind the poleward extension of EMSTID fronts [ABSTRACT FROM AUTHOR]

The polar vortices of Mars are characterized by strong zonal winds that isolate cold air above the pole, allowing CO2 to condense out of the atmosphere through snowfall and direct deposition. Due to their key role in seasonal variability of the atmosphere, it is important to understand the different factors that affect the strength, shape, and stability of the polar vortices and processes such as snowfall that occur within. We used atmospheric retrievals of temperature and CO2 ice cloud opacity from the Mars Climate Sounder (MCS) on board NASA's Mars Reconnaissance Orbiter (MRO) to characterize and analyze patterns in the polar vortices and CO2 ice clouds for Mars years (MY) 29–36. We couple the MCS data with a one‐dimensional snowfall model to determine CO2 snow precipitation rates and analyze patterns in the amounts and distribution of snowfall. We characterize the elliptical nature of both vortices and find that there is significant shrinking and warming of the polar vortex during regional dust storms in the summer hemisphere, which occur more frequently during northern winter. We also find that snowfall in the north pole exceeds that in the south and accounts for ∼1% of surface CO2 deposition, with a notable pause in snowfall during the solstice. We also find measurable variability in snowfall driven by both regional and global dust storms and persistent yearly patterns in the spatial distribution of snow clouds. Plain Language Summary: During the winter season when no sunlight reaches the pole of Mars, the atmosphere grows cold and strong winds trap a cold pocket of air above the poles. This region, called the polar vortex, reaches temperatures that are cold enough for CO2 ice (dry ice) snow clouds to form and snow out. Understanding this process and its effects will provide important insights into the overall physics of the Martian atmosphere and how it behaves over time. In this work, we use data from the Mars Climate Sounder to characterize the shape and temperature of the winter polar vortex in both poles and analyze changes with time. We find that most dust storms in the opposing hemisphere cause the polar vortex to shrink and warm. We also use the data to calculate the amount of snowfall occurring in the winter seasons and find that there is significant snowfall occurring in the north pole, accounting for ∼1% of seasonal CO2 exchange. We also find that there are notable patterns in the distribution of snowfall over both location and time. Key Points: The shape, size, and stability of the Martian polar vortices are controlled by topography, dust loading, and insolationMore snowfall occurs in the northern winter, with a notable reduction during solstice and following some dust activityThe south‐polar vortex has a more consistent size and shape than the north due to the less frequent dust activity during northern summer [ABSTRACT FROM AUTHOR]

The Scandinavian (SCA) pattern is an important climate signal in regulating the variability of summer extreme precipitation (SEP) over the eastern slopes of the Tibetan Plateau (ESTP). There is a strong negative correlation between the SCA pattern and SEP over the ESTP, with a correlation coefficient of −0.57. Moisture budget analysis showed that the dynamic component anomalies of both the zonal and vertical moisture advection induced by the SCA pattern play the dominant roles in extreme precipitation variability over the ESTP. These anomalies are solely linked to variations in zonal wind and vertical velocity. The anomalies of the SCA pattern propagate downstream through Rossby waves, leading to the formation of cyclonic circulations over northern East Asia and the Iranian Plateau. The westerlies on the southern side of these cyclonic circulations play a crucial role in influencing the development of significant anomalous westerlies over the ESTP. The anomalous westerlies bring climatologically moist air from the Tibetan Plateau (TP) to the ESTP, resulting in positive zonal moisture advection anomalies over the area. Furthermore, these anomalous westerlies transport climatologically warm air from the TP to the ESTP, leading to significant warm advection anomalies and vertical upward motion anomalies over the ESTP. The upward motion carries lower-tropospheric moisture upward, resulting in positive vertical moisture advection anomalies over the ESTP. The combination of these positive zonal and vertical moisture advection anomalies eventually leads to excessive SEP. [ABSTRACT FROM AUTHOR]

During the 2013 SSW, ionospheric disturbances were observed at eighteen stations on three meridional chains of 100°E, 110°E and 120°E ranging from 19.03°N to 49.21°N at latitudes over the East Asia region. The total electron content (TEC) response showed evident semidiurnal disturbances with TEC enhancement in the morning and depletion in the afternoon. The maximum TEC increased by more than 220 % along the 120°E longitude. In addition, the semidiurnal disturbance of the TEC was enhanced and indicated evident latitudinal and longitudinal dependence. The wavelet spectra of semidiurnal disturbance in TEC presented quasi-16-day planetary wave-like oscillations at middle latitudes and quasi-10-day planetary wave-like oscillations near Sanya at low latitudes. Moreover, the zonal winds and meridional winds at altitudes of 92 km and 96 km showed quasi-16-day planetary waves at the Mohe station located at middle latitudes and quasi-10-day planetary waves at the Sanya station located at low latitudes, which maybe relate to the semidiurnal disturbance of the TEC. In particular, the 12hrs oscillation of the TEC showed a quasi-16-day periodic component at middle latitudes and a quasi-10-day periodic component at low latitudes, indicating that the modulated semidiurnal tides may transmit these 10-day and 16-day planetary wave-like oscillations to the ionosphere. The coupling between the atmosphere and ionosphere may be strengthened by quasi-16-day waves at middle latitudes and quasi-10-day waves at low latitudes (near Sanya). This finding can further confirm the PW-tide interaction theory during the 2013 SSW event, and it is of significance in the middle and low latitude ionospheric response to SSW. [ABSTRACT FROM AUTHOR]

• Numerical simulation of atmospheric circulation during westerly, easterly and transitional QBO phases is performed. • QBO-induced changes in global circulation and interactions of planetary waves with the mean flow are investigated. • The westerly to easterly QBO transition phase has the strongest impact on the extratropical dynamics. • The signal from the QBO, including transition phases, can be traced up to the thermosphere. Three-dimensional numerical nonlinear model of general circulation of the middle and upper atmosphere (MUAM) is used to simulate changes in the atmospheric dynamical and thermal regimes related to changes in phases of equatorial stratospheric quasi-biennial oscillation (QBO). In addition to conventional (easterly and westerly) QBO phases, two transitional: westerly-shear (wsQBO, from easterly to westerly) and easterly-shear (esQBO, from westerly to easterly) QBO phases are added into consideration in order to research in details QBO-induced changes in global circulation and interactions of planetary waves (PWs) with the mean flow. To interpret the obtained results, the residual meridional circulation, Eliassen-Palm fluxes and meridional temperature gradients are calculated based on MUAM ensemble simulations for the four QBO phases. The simulation results showed different temperature and zonal wind structures in the extratropical winter stratosphere at different QBO phases. The esQBO transition phase is characterized by the strongest temperature and zonal wind changes. This is obtained both from the simulation and from the reanalysis data, and can be explained by PW influence directly through changes in Eliassen-Palm flux and indirectly through modifications in the meridional circulation. Abrupt changes in the subpolar stratosphere during esQBO are gradually compensated for the next 3 phases. Changes in the zonal wind in the thermosphere due to the QBO phases changes can reach 10 %. An increase in wave activity in the Northern Hemisphere is accompanied by a weakening of the zonal wind during the esQBO and eQBO phases above approximately 150 km. [ABSTRACT FROM AUTHOR]

The 12‐year continuous observation of gravity wave momentum fluxes (GWMFs) estimated by the Mohe meteor radar (53.5°N, 122.3°E) revealed prominent intraseasonal variability around the extratropical mesopause (82–94 km) during boreal winters. Composite analysis of the December‒January‒February (DJF) season according to the Madden‒Julian Oscillation (MJO) phases revealed that the zonal GWMFs notably increased in MJO Phase 4 (P4) by ∼2–4 m2/s2, and a Monte Carlo test was designed to examine the statistical significance. The response in zonal winds lags behind the GWMF response by two MJO phases (i.e., 1/2π), indicating a "force‒response" interaction between them. Additionally, time‐lagged composites revealed that strengthened westward GWMFs occurred ∼25–35 days after MJO P4, coincident with the MJO impact on the zonal winds in the stratosphere. The analysis results also suggested that the mechanism of MJO by which the MJO influences the stratospheric circulation might involve poleward propagating effects of stationary planetary waves with zonal wavenumber one. This work emphasizes the importance of GW intraseasonal variability, which impacts tropical sources from the troposphere to the extratropical mesopause. Plain Language Summary: Atmospheric gravity waves (GWs) are small‐scale high‐frequency perturbations that are important for the circulation in the middle atmosphere. GWs originate from the troposphere and propagate upward into the mesosphere, eventually breaking down and transporting momentum fluxes (GWMFs) to background winds. One of the tropospheric sources is convection, which has a dominant intraseasonal variation mode in the tropical region, named Madden‒Jullian Oscillation (MJO). This work investigates how tropical GW sources impact the extratropics, using 12‐year continuous observations by the Mohe meteor radar (53.5°N, 122.3°E). Obvious intraseasonal signals in the GWMF are found during northern winter. The GWMF data were then composited with respect to the MJO phases. Notable increases were observed when the MJO convection is enhanced over the Indian Ocean. The zonal wind response is obviously positive when the MJO is active over the Western Pacific Ocean, which occurs after the GWMF response. This result indicates an interaction between the GWMF and background winds by the MJO modulation. Furthermore, we examined GWMF responses to the MJO at different lag times, and the lagged response coincided with the background wind response in the stratosphere. The stratospheric wind variation is found to be modulated by poleward‐propagating planetary waves. Key Points: The 12‐year observations of the Mohe meteor radar revealed prominent intraseasonal variations in gravity wave momentum fluxes (GWMFs)The GWMF increased by ∼2–4 m2/s2 during Madden‒Julian oscillation (MJO) Phase 4 (P4), leading to the response of zonal winds to approximately 2 MJO phases (1/2π)Time‐lagged composites revealed enhanced westward GWMFs lagging MJO P4 ∼25–35 days, suggesting a possible mechanism via planetary waves [ABSTRACT FROM AUTHOR]

The tropical Angolan upwelling system (tAUS) is a highly productive ecosystem of great socio-economic importance. Productivity peaks in austral winter and is linked to the passage of remotely forced upwelling coastal trapped waves (CTWs), where the strength of the productivity peak is associated with the amplitude of the upwelling CTW. Here, we analyze the year-to-year variability in the timing and amplitude of the austral winter upwelling CTW by examining sea surface temperature, sea level anomaly, and wind fields. Our results show that the timing of the CTW is influenced by variability in the equatorial region and along the southern African coast. Weaker equatorial easterlies from April to July delay the generation of the upwelling Kelvin wave, leading to a later arrival of the upwelling CTW. In contrast, the amplitude of the CTW is primarily influenced by variability in the eastern equatorial Atlantic and the central South Atlantic, where the South Atlantic Anticyclone is located. A cooling in the eastern equatorial Atlantic three to four months before the arrival of the CTW causes stronger zonal winds, ultimately leading to a stronger austral winter upwelling CTW. Our results suggest that the timing and amplitude of the upwelling CTW in the tAUS during austral winter are predictable on seasonal time scales. [ABSTRACT FROM AUTHOR]

The Icosahedral Nonhydrostatic (ICON) general circulation model with upper atmosphere extension (UA-ICON) in the configuration with the physics package for numerical weather prediction (NWP) is presented with optimized parameter settings for the non-orographic and orographic gravity wave drag parameterizations (GWD). In this paper, we present UA-ICON(NWP) (version: ua-icon-2.1) in which we implemented optimized parameter settings for the GWD parameterizations to achieve more realistic MLT temperatures and zonal winds. The parameter optimization is based on perpetual January simulations targeting the thermal and dynamic state of the MLT and the Northern Hemisphere stratosphere. The climatology and variability of the Northern Hemisphere stratospheric winter circulation widely improve when applying UA-ICON with the NWP physics package compared to UA-ICON with ECHAM physics. Likewise improves the thermal and dynamic state of the MLT of the re-tuned UA-ICON(NWP) compared with the UA-ICON(NWP) using default settings. For UA-ICON(NWP), a statistical evaluation reveals a slight improvement in the stratosphere/mesosphere coupling compared to UA-ICON(ECHAM). The cold summer mesopause, the warm winter stratopause, and the related wind reversals are reasonably simulated. The GWD parameter optimization further significantly improves the frequency of major sudden stratospheric warmings (SSWs). However, the seasonal distribution needs improvement and the relative frequency of split vortex SSWs is underestimated compared to reanalyses, as is the zonal wavenumber 2 preconditioning of SSWs. This indicates that zonal wavenumber 2 forcing in UA-ICON(NWP) is underrepresented. The analysis of migrating diurnal and semidiurnal tides in temperature shows a good agreement of UA-ICON(NWP) with SABER-derived tides and the enhancement of the migrating semidiurnal tide during SSWs is well represented in UA-ICON(NWP). [ABSTRACT FROM AUTHOR]

This study reveals a remarkable interdecadal intensification in the interannual variability of summer synoptic-scale disturbances (SSDs) over the western North Pacific (WNP) since the late 1980s. This intensification can be explained by the remarkable difference in circulation anomalies associated with the SSD intensity variability during 1959–1987 (P1) and 1988–2022 (P2). That is, the lower-tropospheric cyclonic circulation anomalies are more significant during P2 than P1. Accordingly, the meridional shear of zonal winds over the WNP is significantly stronger in P2, and thus favors the enhancement of SSD intensity through the kinetic energy conversion from the mean flow to SSDs. Further analysis suggests that the SSD intensity variation is significantly related to the sea surface temperature (SST) anomalies in the equatorial central Pacific and maritime continent in P2, which can induce the cyclonic or anticyclonic circulation anomalies over the WNP, but the SST anomalies are vague in P1. Finally, the more pronounced influence of tropical SST anomalies on circulations and SSDs over the WNP is attributed to the much warmer SSTs over the maritime continent in P2, which intensifies the responses of atmospheric circulations to SST anomalies. [ABSTRACT FROM AUTHOR]

Interdecadal variations, since the middle of the 20th century, in the seasonal cycle of Southern Hemisphere extratropical synoptic scale weather systems, are studied and related to associated anomalies in Southern Australian rainfall over south-west Western Australia (SWWA) and southeast Australia (SEA). A data-driven method is employed in which atmospheric fluctuations, specified from 6-hourly lower-tropospheric reanalysis data, are spectrally analysed in space and time to determine the statistics of the intensity and growth rates of growing and decaying eddies. Extratropical storms, blocking and north-west cloud band weather types are investigated in two frequency bands, with periods less than 4 days and between 4 and 8 days, and in three growth rate and three decay rate bins. Southern Australian rainfall variability is found to be most related to changes in explosive storms particularly in autumn and winter. During the first 10 years of the Australian Millennium Drought (AMD), from 1997 to 2006, dramatic changes in rainfall and storminess occurred. Rainfall declines ensued over SEA in all seasons, associated with corresponding reductions in the intensity of fast-growing storms with periods less than 4 days. These changes, compared with the 20-year timespans of 1949 to 1968 and 1975 to 1994, also took place for the longer duration of 1997 to 2016, apart from summer. Over SWWA, autumn and winter rainfall totals have decreased systematically with time for each of the 10-year and 20-year timespans analysed. Southern Australian rainfall variability is also found to be closely related to the local, hemispheric or global features of the circulation of the atmosphere and oceans that we characterise by indices. Local circulation indices of sea level pressure and 700 hPa zonal winds are good predictors of SWWA and SEA annual rainfall variability particularly in autumn and winter with vertical velocity generally less so. The new Subtropical Atmospheric Jet (SAJ) and the Southern Ocean Regional Dipole (SORD) indices are found to be the most skilful non-local predictors of cool season SWWA rainfall variability on annual and decadal timescales. The Indian Ocean Dipole (IOD) and Southern Oscillation Index (SOI) are the strongest non-local predictors of SEA annual rainfall variability from autumn through to late spring, while on the decadal timescale, different indices dominate for different 3-month periods. [ABSTRACT FROM AUTHOR]

This study examines the role of winds in wintertime sporadic E layer intensification (WEsLI) in 2009 from a global viewpoint. Previous studies showed that sporadic E layer (EsL) intensity had increased for 20–30 days in some winters, although intense EsLs do not form generally in winter. A recent study found that vertical ion convergence (VIC) driven by intensified migrating semidiurnal (SW2) tides caused WEsLI at middle latitudes in 2009. However, no studies have investigated the global distributions and generation mechanisms of WEsLI in 2009. Herein, we employed FORMOSAT‐3/COSMIC radio occultations to investigate the global distributions of WEsLI in 2009. Distributions of VIC driven by winds obtained from the Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy were compared with global WEsLI distributions to elucidate the role of winds in WEsLI. We found that WEsLI in 2009 occurred at geomagnetic low/middle latitudes except between 60° $60{}^{\circ}$W and 80° $80{}^{\circ}$E. WEsLI was observed below 120 km altitudes, especially at 12–17 local times. WEsLI was attributable to VIC driven by SW2 tides, migrating diurnal tides, and eastward propagating diurnal tides with wavenumber 3. Tidal amplifications were possibly related to mesospheric/stratospheric atmospheric variations such as sudden stratospheric warming, zonal mean zonal winds, and quasi‐biennial oscillations. WEsLI in 2009 is further evidence of the coupling between EsLs and mesospheric/stratospheric atmospheric variations through tidal modifications. Key Points: Wintertime sporadic E layer intensification (WEsLI) in 2009 occurred at geomagnetic northern low/middle latitudes except at 60° $60{}^{\circ}$W–80° $80{}^{\circ}$EWEsLI in 2009 was observed intensively between 12 and 17 LTs, although general EsLs did not show a strong LT dependenceIntensified SW2, DW1, and DE3 tides drive the strong vertical ion convergence possibly leading to WEsLI in 2009 [ABSTRACT FROM AUTHOR]

We analyze the importance of cloud top temperature, dust aerosol, sea salt aerosol, and sea ice cover for the thermodynamic phase of low‐level, mid‐level, and mid to low‐level clouds observed by CloudSat/CALIPSO over the Arctic and the Southern Ocean using an explainable machine learning technique. As expected, the cloud top temperature is found to be the most important parameter for determining cloud phase. The results show also a predictive power of sea salt and sea ice on the phase of low‐level clouds, while in mid‐level clouds dust shows predictive power. Over the Southern Ocean, strong zonal winds coincide with the aerosol distribution. While they can produce high mixing ratios of sea spray at lower levels, the strong zonal winds may prevent the pole‐ward transport of dust. Sea ice may prevent the release of sea salt aerosols and marine organic aerosols leading to higher liquid fractions in clouds over sea ice. Plain Language Summary: The cloud phase describes whether a cloud consists of ice particles, liquid droplets, or both. The representation of the cloud phase in climate and weather models is uncertain, leading to radiation biases over the Southern Ocean and the Arctic Ocean. To investigate the impact of four different parameters on the cloud phase, we use an explainable machine learning technique. The parameters studied are the temperature of the cloud top, the sea ice coverage, and the concentration of sea salt aerosols and dust aerosols, both of which can act as ice nucleating particles and contribute to the ice formation in clouds. We find that temperature seems to be the most important factor in determining the cloud phase. Sea salt aerosol seems to be more relevant for low‐level clouds closer to the ocean surface, the source of sea salt aerosol. Sea ice may prevent the release of sea salt aerosol by covering the ocean and our analysis supports this hypothesis. Dust is typically transported over long distances and our analysis shows that dust aerosol is more important for mid‐level clouds, but persistent strong winds surrounding Antarctica may have an influence on the dust concentration and thus on cloud phase. Key Points: Cloud phase in polar regions can be predicted based on cloud top temperature, sea ice concentration, and sea salt and dust mixing ratiosCloud top temperature has the strongest impact, while sea salt/spray aerosol is relevant for low‐level, and dust for mid‐level cloud phaseSea ice coverage and Southern Ocean westerly winds may influence the aerosol distribution and thereby cloud phase [ABSTRACT FROM AUTHOR]

The movement and intensity indices of the South Asian High (SAH), an upper‐level anticyclonic‐circulation over the Tibetan Plateau, play a crucial role in Indian summer monsoon rainfall (ISMR) during June–September. The present study aims to document (i) the association between SAH and ISMR in the periods of June–July–August (JJA) and July–August–September (JAS), and (ii) the ISMR changes at different spatial and temporal scales. The Bayesian change‐point detection method identifies 1981 as change point and the dataset has been split into two groups (past climate:1940–1980 and current climate:1981–2020) to analyze temporal variations in ISMR. Results indicate that the northwest–southeast index (INW–SE), north–south index (INS) and intensity index (IINT) of the SAH are strongly correlated (˜0.67, ˜0.60, and ˜0.51, respectively) with ISMR whereas the east–west index (IEW) is negatively correlated (˜−0.52) during the JAS season. This relation was stronger in the past climate than the current climate, except for the IINT index during the JAS season. The INW–SE and INS indices are closely associated with all‐India, northwest India (NWI), and central India (CI) rainfall, whereas IINT is associated with south peninsular India (SPI) during the JAS season. The increased rainfall over NWI and SPI in the current climate is strongly associated with positive INS and IINT indices, respectively, during the JAS season. The northeast India (NEI) rainfall did not show any association with SAH indices; however, the IEW is strongly associated with increased NEI rainfall during El Niño years. A significant positive (negative) relation between meridional (zonal) wind shear and SAH indices except the IEW index is observed. Further, the positive (negative) SAH indices favor more (moderate) ISMR due to strong positive (negative) moisture anomalies. The study demonstrates the movement and intensity of the SAH in the dynamical shifts of seasonal rainfall patterns across the Indian homogeneous regions. [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]

The atmospheric circulation around the Tibetan Plateau (TP) exhibits a substantial 10–20‐day quasi–biweekly oscillation (QBWO), profoundly impacting weather and climate locally and remotely. Understanding the factors influencing the generation of QBWO over the TP (QBWOTP) and its physical mechanism is crucial. This study has investigated the influence of multi–timescale and land–atmosphere interactions on the generation of the QBWOTP in surface potential vorticity (SPV), a valuable tool for characterizing the mechanical and thermal variabilities in mountain forcing, based on a 2014 case study. Results indicate that in the free atmosphere, the summer monsoon onset over the Bay of Bengal induces a northward shift in the westerly jet toward the TP, manifested as an increase in low‐frequency zonal winds. This shift facilitates the propagation of wave trains, leading to atmospheric quasi–biweekly potential temperature anomalies (θa) over the TP through a multi‐timescale interaction. Additionally, the TP's surface thermal forcing and arrival of wave trains trigger anomalous upward motion and increase cloud cover. The resultant decrease in net short‐wave radiations and increase in net long‐wave radiations contribute to variations in surface potential temperature (θs) over the TP. As θa and θs evolve, the difference between them enlarges, resulting in the generation of the SPV QBWOTP. Given the relationship between the QBWOTP and downstream rainfall, this study could provide novel insights into understanding and predicting downstream rainfall QBWO. Plain Language Summary: The atmosphere around the Tibetan Plateau (TP) exhibits a substantial 10–20‐day oscillation known as the quasi–biweekly oscillation (QBWO). The QBWO over the TP (QBWOTP) significantly impacts weather and climate both locally and in distant regions, making it crucial to understand its generation. We conducted a case study to understand the QBWOTP and its generation mechanism in surface potential vorticity (SPV), a useful tool for examining the dynamic and thermodynamic variations of the TP. Our results indicate that the summer monsoon over the Bay of Bengal affects the TP by causing a northward shift in the westerly jet, leading to wave train propagation and atmospheric potential temperature variability. The surface thermal forcing of the TP and wave trains influence clouds and surface radiation, causing changes in the surface potential temperature. These combined effect of these processes results in the SPV QBWOTP generation, which has important implications for understanding and forecasting downstream rainfall QBWO. Key Points: SPV QBWOTP is dependent mostly on the vertical difference in the potential temperature (θs–θa)Onset of BOBSM influences the zonal wind, wave trains propagation, thereby θa during SPV QBWOTP generation via multi‐timescale interactionWave trains and TP land–air interaction regulate the surface energy balance, leading to the variation in θs during SPV QBWOTP generation [ABSTRACT FROM AUTHOR]

Using observation data and numerical simulations, we have demonstrated that the stratospheric Quasi-Biennial Oscillation (QBO) can predict extreme high temperatures (EHTs) in South Asia in June. The vertical structure of the QBO plays a crucial role in this prediction. When the QBO in June shows easterlies (westerlies) at 50 hPa and westerlies (easterlies) at 70 hPa, more (fewer) EHT events occur. This likely results from the QBO's vertical structure causing positive (negative) temperature anomalies in the lower stratosphere and negative (positive) static stability anomalies near the tropical tropopause. These anomalies enhance (weaken) convective activity over the equatorial Indian Ocean, leading to anomalous circulation with ascending (descending) air over the equatorial Indian Ocean and descending (ascending) air over northern and central South Asia. This suppresses (promotes) convection over northern and central South Asia, affecting cloud formation and precipitation. Consequently, more (less) solar radiation reaches the region, along with weaker (stronger) evaporative cooling effects, warming (cooling) the surface and creating a background state conducive to (against) EHT events. Additionally, the opposite zonal winds at 30 hPa and 50 hPa in April may serve as a reference factor for predicting the probability of EHT events in northern and central South Asia. This study provides a potential approach for forecasting tropospheric extreme weather events based on stratospheric signals. [ABSTRACT FROM AUTHOR]

Seasonal‐range predictability of summer climate in northwestern Europe is generally considered to be low. This is an increasing issue given the worsening impact of summer heatwaves, droughts and intense convective rainfall in a rapidly changing climate. In wintertime, predictive skill in the region is derived from a variety of sources, not least teleconnections with the El Niño‐Southern Oscillation (ENSO). Summer ENSO teleconnections, however, are often considered to be negligible. In this paper, we revisit the topic of summer teleconnections between ENSO and the North Atlantic‐European region. We build on previous work identifying upper tropospheric responses to tropical forcing, since dynamical teleconnections are most apparent at this level. Our results confirm that significantly increased geopotential heights are found stretching over the North‐Atlantic region and into western Europe when La Niña conditions are prevalent during summer. This pattern is part of the previously identified 'circumglobal' pattern of wider northern‐hemisphere height changes. We then look for these responses in a range of climate models used in operational seasonal prediction. While parts of the circumglobal pattern are weakly present, none of them produce the response seen over the North Atlantic, even when the effect of sampling on the observed teleconnection is accounted for. We additionally estimate the contribution of the previous (wintertime) phase of ENSO on the following summer. We find a significant delayed response, particularly in heights, to the earlier phase. The combination of the delayed and current responses gives height anomalies that are larger, on average, when ENSO changes phase from winter to summer. Finally, we show that a modest level of regional prediction skill from ENSO does exist. There is a contribution to skill in heights from the previous ENSO phase, but the equivalent contribution to the skill of zonal winds is smaller. [ABSTRACT FROM AUTHOR]

The North Pacific Oscillation (NPO) is an important intrinsic atmospheric pattern over the North Pacific. Observations have shown that the boreal winter NPO is a crucial precursor to the El Niño-Southern Oscillation (ENSO), with many ENSO events being preceded by the NPO. It is therefore imperative to assess the ability of current coupled climate models to reproduce the relationship between winter NPO and the subsequent winter ENSO. Previous studies have shown that most coupled climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) underestimate the influence of winter NPO on the subsequent winter ENSO. Simulations from CMIP6, representing the latest generation of climate models, are now available. This study shows a remarkable improvement of the CMIP6 models in simulating the influence of winter NPO on the subsequent ENSO development compared to the CMIP5 models. This improvement is due to the improved air-sea interaction over the subtropical North Pacific in CMIP6. The enhanced air-sea interaction over the subtropical North Pacific promotes the equatorward propagation of the NPO-induced wind and SST anomalies, resulting in enhanced surface zonal wind anomalies over the tropical western Pacific, which further exert a stronger influence on the subsequent winter ENSO development by triggering the tropical Bjerknes positive air-sea interaction. The enhanced subtropical air-sea interaction is thought to be related to a southward shift of the North Pacific intertropical convergence zone in CMIP6 compared to CMIP5. [ABSTRACT FROM AUTHOR]

This study focuses on the impact of the stratospheric quasi-biennial oscillation (QBO) on the early stage of the Indian summer monsoon (ISM) in May and June, which has thus far been an ambiguous topic of research. It is found that the 50-hPa QBO in the preceding winter and spring is significantly and negatively correlated with precipitation in the southern Arabian Sea and central India in May, which shifts northward to northern India in June. This correlation is nearly the opposite for the 10-hPa and 20-hPa QBO. An easterly phase of the 50-hPa QBO corresponds to a colder and higher tropopause over the subtropical ISM region which is related to vigorous convection over India. Meanwhile, the QBO-related meridional dipole pattern of zonal wind from the stratosphere to troposphere in the subtropics and mid-latitudes connects to an anomalous high in the upper troposphere across the subtropical land and the northern Arabian Sea, which causes an anomalous descent and in situ adiabatic heating. This heating supports an enhanced meridional land-sea thermal contrast and thus an early and strong ISM. The situation for westerly 50-hPa QBO is generally the opposite. The climate models from the Coupled Model Intercomparison Project Phases 6 (CMIP6) can generally reproduce the QBO–ISM relationship in June (but not in May), though with some discrepancies from the observation. Inter-model comparison demonstrates that better representation of the QBO–ISM correlation depends well on a better simulation of the QBO-related meridional dipole of zonal wind in the subtropical ISM region. [ABSTRACT FROM AUTHOR]

This study presents the first attempt to simulate a full cycle of the quasi‐biennial oscillation (QBO) in a global storm‐resolving model (GSRM) that explicitly simulates deep convection and gravity waves instead of parameterizing them. Using the Icosahedral Nonhydrostatic (ICON) model with horizontal and vertical resolutions of about 5km $5\,\mathrm{k}\mathrm{m}$ and 400m $400\,\mathrm{m}$, respectively, we show that an untuned state‐of‐the‐art GSRM is already on the verge of simulating a QBO‐like oscillation of the zonal wind in the tropical stratosphere for the right reasons. ICON shows overall good fidelity in simulating the QBO momentum budget and the downward propagation of the QBO jets in the upper QBO domain (25–35 km). In the lowermost stratosphere, however, ICON does not simulate the downward propagation of the QBO jets to the tropopause. This is the result of a pronounced lack of QBO wave forcing, mainly on planetary scales. The lack of planetary‐scale wave forcing in the lowermost stratosphere is caused by an underestimation of planetary‐scale wave momentum fluxes entering the stratosphere. We attribute this lack of planetary‐scale wave momentum fluxes to a substantial lack of convectively coupled equatorial waves (CCEWs) in the tropical troposphere. Therefore, we conclude that in ICON, simulating a realistic spatio‐temporal variability of tropical deep convection, in particular CCEWs, is currently the main roadblock toward simulating a reasonable QBO. To overcome this intermediate situation, we propose to aim at an improved explicit simulation of tropical deep convection by retuning the remaining parameterizations of cloud microphysics and vertical diffusion, and by increasing the horizontal resolution. Plain Language Summary: The quasi‐biennial oscillation (QBO) is a wind system located in the equatorial stratosphere between ∼17 ${\sim} 17$ and ∼35km ${\sim} 35\hspace*{.5em}\mathrm{k}\mathrm{m}$ and consists of westerly and easterly wind jets that alternately propagate downward with time. The QBO has been shown to influence surface weather, so it is important to simulate the QBO realistically in the computer models typically used for climate research. However, these models often struggle to simulate a realistic QBO because they represent the processes leading up to the QBO, that is, tropical rain showers and short atmospheric waves excited by these rain showers, only empirically through so‐called parameterizations. In this study, we attempt for the first time to simulate the QBO in a model that directly represents these processes through an ultra‐fine grid. We find that our model maintains QBO‐like stratospheric winds throughout the simulation, and in the central stratosphere, the model simulates the characteristics of the QBO reasonably well for the right reasons. However, in the lowermost stratosphere, the simulated QBO is not realistic and does not move downward with time as observed due to a misrepresentation of long waves in the tropical atmosphere. These results will guide future model development to improve the model's representation of the QBO. Key Points: The ability of the general circulation model ICON, which explicitly simulates deep convection and gravity waves, to simulate a quasi‐biennial oscillation (QBO) is testedICON simulates a reasonable downward propagation and momentum balance of QBO‐like jets in the upper QBO domain in the first simulation yearSimulated QBO‐like jets stall in lower QBO domain due to a lack of planetary wave forcing due to weak convectively coupled equatorial waves [ABSTRACT FROM AUTHOR]

ERA5 reanalysis data, precipitation data from China, and National Oceanic and Atmospheric Administration (NOAA) monthly sea surface temperature (SST) data are used to analyse the impact of the meridional position of the East Asian subtropical jet (EASJ) on summer precipitation in China and its correlation with Atlantic SST. The results indicate that when the EASJ significantly shifts northward, the western North Pacific subtropical high (WNPSH) weakens with an eastward displacement. Upper‐level convergence and moisture divergence, corresponding to descending motion, lead to decreased precipitation in the Yangtze River Valley (YRV). Meanwhile, upper‐level divergence occurs over South China (SC), the Hexi Corridor (HC), and Northeast China (NEC), where moisture converges and ascends, favouring an increase in precipitation. Conversely, when the EASJ undergoes a significant southward shift, the WNPSH strengthens and expands westward. Opposing atmospheric circulation patterns in these four regions result in reversed precipitation anomalies compared with those observed when the jet shifts northward. The meridional position of the EASJ is closely related to the summer subtropical Atlantic SST. The positive (negative) SST anomaly (SSTA) over the subtropical Atlantic induces negative (positive) geopotential height anomaly in the upper troposphere over the North Atlantic by modulating the atmospheric meridional circulation. Geopotential height anomalies trigger eastward‐propagating Rossby waves, generating anomalous cyclones and anticyclones over East Asia. These anomalous cyclones and anticyclones lead to zonal wind anomalies, which alter the strength of the westerlies on both sides of the climatological jet axis, thereby changing the jet's meridional position. Additionally, the difference in the propagation direction of wave activity flux between positive and negative SSTA alters the distribution of wave energy convergence and divergence in the EASJ region, further affecting the intensity of the average westerly winds on both sides of the climatological jet axis, ultimately producing the changes in the meridional position of the EASJ. [ABSTRACT FROM AUTHOR]

Changes in the activities of the Boreal Summer Intraseasonal Oscillation (BSISO) at the end of 21st century under the SSP5-8.5 scenario are assessed by adopting 17 CMIP6 models and the weak-temperature-gradient assumption. Results show that the intraseasonal variations become more structured. The BSISO-related precipitation anomaly shows a larger zonal scale and propagates further northward. However, there is no broad agreement among models on the changes in the eastward and northward propagation speeds and the frequency of individual phases. In the western North Pacific (WNP), the BSISO precipitation variance is significantly increased, at 4.62% K−1, due to the significantly increased efficiency of vertical moisture transport per unit of BSISO apparent heating. The vertical velocity variance is significantly decreased, at −3.51% K−1, in the middle troposphere, due to the significantly increased mean-state static stability. Changes in the lower-level zonal wind variance are relatively complex, with a significant increase stretching from the northwestern to southeastern WNP, but the opposite in other regions. This is probably due to the combined impacts of the northeastward shift of the BSISO signals and the reduced BSISO vertical velocity variance under global warming. Changes in strong and normal BSISO events in the WNP are also compared. They show same-signed changes in precipitation and large-scale circulation anomalies but opposite changes in the vertical velocity anomalies. This is probably because the precipitation anomaly of strong (normal) events changes at a rate much larger (smaller) than that of the mean-state static stability, causing enhanced (reduced) vertical motion. [ABSTRACT FROM AUTHOR]

There is limited understanding regarding the formation of multiple tropical cyclones (MTCs). This study explores the environmental conditions conducive to MTC formation by objectively determining the atmospheric circulation patterns favorable for MTC formation over the western North Pacific. Based on 199 MTC events occurring from June to October 1980–2020, four distinct circulation patterns are identified: the monsoon trough (MT) pattern, accounting for 40.3% of occurrences, the confluence zone (CON) pattern at 26.2%, the easterly wave (EW) pattern at 17.8%, and the monsoon gyre (MG) pattern at 15.7%. The MT pattern mainly arises from the interaction between the subtropical high and the monsoon trough, with MTCs forming along the monsoon trough and its flanks. The CON pattern is affected by the subtropical high, the South Asian high, and the monsoon trough, with MTCs emerging at the confluence zone where the prevailing southwesterly and southeasterly flows converge. The EW pattern is dominated by easterly flows, with MTCs developing along the easterly wave train. MTCs in the MG pattern arise within a monsoon vortex characterized by strong southwesterly flows. A quantitative analysis further indicates that MTC formation in the MT pattern is primarily governed by mid-level vertical velocity and low-level vorticity, while mid-level humidity and vertical velocity are significantly important in the other patterns. The meridional shear and convergence of zonal winds are essential in converting barotropic energy from the basic flows to disturbance kinetic energy, acting as the primary source for eddy kinetic energy growth. [ABSTRACT FROM AUTHOR]

During sudden stratospheric warming (SSW) events, significant modifications occur, not only in the neutral atmosphere, but also in the ionosphere. Specifically, sporadic E layers in the mesosphere and lower thermosphere regions significantly disrupt satellite communication. Although research has frequently focused on ionospheric alterations during SSW events, detailed studies on sporadic E layers remain limited. Examining these variations during SSW events could enhance our understanding of the interaction mechanisms between the ionosphere and the neutral atmosphere, and provide insights into the patterns of sporadic E layer alterations. This study analyzed the behavior of sporadic E layers during the 2008/2009 winter SSW period using data from three Japanese stations and satellite observations. The principal findings included the following: (1) The enhancement in the critical frequency of the sporadic E layers was most notable following the transition from easterly to westerly winds at 60° N at a 10 hPa altitude, accompanied by quasi 6-day and quasi 16-day oscillations in frequency. (2) The daily average zonal and meridional wind speeds in the MLT region also exhibited quasi 6-day and quasi 16-day oscillations, aligning with the observed periodicities in the critical frequency of the sporadic E layers. (3) Planetary waves were shown to modulate the amplitude of diurnal and semidiurnal tides, influencing the sporadic E layers. Furthermore, a wavelet analysis of foEs data with a time resolution of 0.25 h demonstrated that planetary waves also modulate the frequency of diurnal tides, thereby affecting the sporadic E layers. This research contributes to a deeper understanding of the formation mechanisms and prediction of sporadic E layer behavior. [ABSTRACT FROM AUTHOR]

Time-dependent meridional circulation and differential rotation in radiative zones are central open issues in stellar evolution theory. We streamline this challenging problem using the "downward control principle" of atmospheric science, under a geostrophic f-plane approximation. We recover the known stellar physics result that the steady-state meridional circulation decays on the length scale proportional to $ N/f \times \sqrt {Pr} $ N / f × Pr , assuming molecular viscosity is the dominant drag mechanism. Prior to steady-state, the meridional circulation and the zonal wind (= differential rotation) spread together via radiative diffusion, under thermal wind balance. The corresponding (fourth-order) hyperdiffusion process is reasonably well approximated by regular (second-order) diffusion on scales of order a pressure scale-height. We derive an inhomogeneous diffusion (equiv. advection-diffusion) equation for the zonal flow which admits closed-form time-dependent solutions in a finite depth domain, allowing for rapid prototyping of differential rotation profiles. In the weak drag limit, we find that the time to rotational steady-state can be longer than the Eddington-Sweet time and be instead determined by the longer drag time. Unless strong enough drag operates, the internal rotation of main-sequence stars may thus never reach a steady-state. Streamlined meridional circulation solutions provide leading-order internal rotation profiles for studying the role of fluid/MHD instabilities (or waves) in redistributing angular momentum in the radiative zones of stars. Despite clear geometrical limitations and simplifying assumptions, one might expect our thin-layer geostrophic approach to offer qualitatively useful results to understand deep meridional circulation in stars. [ABSTRACT FROM AUTHOR]

This study uses satellite measurements of plasma densities and thermospheric winds to analyze the effects of the November 2021 geomagnetic storm on ionospheric pre‐midnight topside plasma bubbles over South America. Using observations from the Ionospheric Connection Explorer (ICON) and the Global‐scale Observations of the Limb and Disk (GOLD) satellites, we find that pre‐midnight topside plasma bubbles were inhibited over eastern South America during the recovery phase of the storm. This is particularly notable because of the otherwise high occurrence rate of plasma bubbles at these longitudes during this season. This inhibition coincided with the recovery phase of the geomagnetic storm, marked by a northward turning of the z‐component of the interplanetary magnetic field (IMFBz) and quiet‐time values of the SuperMAG Auroral Electrojet Index (SME). We observed a westward turning of the zonal wind before the bubble inhibition, so we conclude the inhibition of topside plasma bubbles is likely related to a westward disturbance dynamo electric field (DDEF) causing a downward E×B $\mathbf{E}\times \mathbf{B}$ drift and suppress the growth of the instability responsible for bubble development. Contrary to theoretical predictions, we do not observe notable changes to the meridional wind during the event. These results provide new insights into the ionosphere‐thermosphere system's response to geomagnetic storms and highlight the role of wind patterns in inhibiting ionospheric irregularities, contributing to better predictive models for these phenomena. Plain Language Summary: Geomagnetic storms negatively affect communication and navigation systems by producing unpredictable changes in ionospheric densities. Plasma bubbles are irregularities in the ionosphere that occur mainly in the equatorial region and may be affected by geomagnetic storms. Studying plasma bubbles is crucial because they can significantly reduce the performance of communication and navigation systems, leading to signal loss or degradation. We conducted a study on how a geomagnetic storm in November 2021 affected the occurrence of pre‐midnight plasma bubbles over South America. Using data from two satellites (ICON and GOLD), we observed that these plasma bubbles disappeared during the storm's recovery phase. Our analysis suggests that this inhibition is likely linked to changes in wind patterns in the ionosphere. Specifically, a westward wind created conditions that prevented the generation of plasma bubbles. This study offers new insights into how winds during geomagnetic storms affect the ionosphere and helps improve predictions for communication and navigation systems affected by these disruptions. Key Points: Pre‐midnight topside plasma bubbles are inhibited over eastern South America during the recovery phase of a geomagnetic stormZonal wind measurements show evidence of wind‐driven electric fields inhibiting plasma bubble formationAlthough theoretical predictions also anticipate changes in the meridional wind, they are not observed [ABSTRACT FROM AUTHOR]

The Chukchi Sea is an open estuary in the southwestern Arctic. Its near-surface salinities are higher than those of the surrounding open Arctic waters due to the key inflow of saltier and warmer Pacific waters through the Bering Strait. This salinity distribution may suggest that interannual changes in the Bering Strait mass transport are the sole and dominant factor shaping the salinity distribution in the downstream Chukchi Sea. Using satellite sea surface salinity (SSS) retrievals and altimetry-based estimates of the Bering Strait transport, the relationship between the Strait transport and Chukchi Sea SSS distributions is analyzed from 2010 onward, focusing on the ice-free summer to fall period. A comparison of five different satellite SSS products shows that anomalous SSS spatially averaged over the Chukchi Sea during the ice-free period is consistent among them. Observed interannual temporal change in satellite SSS is confirmed by comparison with collocated ship-based thermosalinograph transect datasets. Bering Strait transport variability is known to be driven by the local meridional wind stress and by the Pacific-to-Arctic sea level gradient (pressure head). This pressure head, in turn, is related to an Arctic Oscillation-like atmospheric mean sea level pattern over the high-latitude Arctic, which governs anomalous zonal winds over the Chukchi Sea and affects its sea level through Ekman dynamics. Satellite SSS anomalies averaged over the Chukchi Sea show a positive correlation with preceding months' Strait transport anomalies. This correlation is confirmed using two longer (>40-year), separate ocean data assimilation models, with either higher- (0.1°) or lower-resolution (0.25°) spatial resolution. The relationship between the Strait transport and Chukchi Sea SSS anomalies is generally stronger in the low-resolution model. The area of SSS response correlated with the Strait transport is located along the northern coast of the Chukotka Peninsula in the Siberian Coastal Current and adjacent zones. The correlation between wind patterns governing Bering Strait variability and Siberian Coastal Current variability is driven by coastal sea level adjustments to changing winds, in turn driving the Strait transport. Due to the Chukotka coastline configuration, both zonal and meridional wind components contribute. [ABSTRACT FROM AUTHOR]

The present study uses reanalysis data to investigate how the East Asian cold surges are linked to the initiation of westerly wind anomalies prior to El Niño events and easterly wind anomalies prior to La Niña events in the western equatorial Pacific. Our analysis shows that East Asian cold surge events can be associated with the formation of an anomalous anticyclone that moves eastward across the North Pacific and cools the ocean surface prior to El Niño events. The outflow of the anomalous anticyclonic circulation is responsible for the initiation of westerly wind anomalies in the western equatorial Pacific. We also find that East Asian cold surge events are involved in the development of an anomalous cyclone that moves northeastward along the subpolar North Pacific and reinforces the horizontal temperature gradient prior to La Niña events. The cross-equatorial flow into the anomalous cyclone in the North Pacific contributes to the development of easterly wind anomalies in the western equatorial Pacific. We show that air-sea interactions can play a role in the propagation of anomalous circulations across the North Pacific, eventually initiating zonal wind anomalies in the western equatorial Pacific. [ABSTRACT FROM AUTHOR]