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The summer monsoon onset over the Bay of Bengal and South China Sea signals the beginning of the Asian summer monsoon, critical for local fisheries, agriculture and livelihoods, so communities are concerned about its potential changes under global warming. Previous projections have suggested a delay, but the extent of this delay remains uncertain, undermining the reliability of the projections. Here, we show a significant correlation between the projected shift in Bay of Bengal/South China Sea monsoon onset and present‐day sea surface temperature (SST) simulation over the western Pacific (WP). This emergent relationship arises from the spread of the precipitation response over the western‐central Pacific to WP SST, as more precipitation induces stronger tropical upper‐tropospheric warming, increasing westerly vertical shear near South Asia, and facilitating the onset delay. The rectified projections indicate that the delayed shift is almost halved compared to raw projections, and the intermodel uncertainty is reduced by 30%. Plain Language Summary: The summer monsoon onset over the Bay of Bengal and South China Sea is a crucial time for fishing, agriculture and livelihood, and is important for the further advance of the monsoon rains over tropical Asia. Previous studies have suggested that monsoon onset in these seas will be delayed in the future, but there is large uncertainty between models, undermining the reliability of the projection. This study finds that the projected shift in the onset is strongly related to a model's representation of sea surface temperatures over the western Pacific Ocean in the present day. Based on the relationship found in observations, we adjust or "constrain" the future projections of the monsoon onset. The delay to the monsoon onset using this adjustment is almost half of the original projection, and the uncertainty between models is also reduced. We suggest that a more accurate simulation of sea‐surface temperatures in the present‐day climate of the western Pacific Ocean is crucial for reliable future projections of the monsoon onset. Key Points: An observational constraint is identified for projected changes in summer monsoon onset date over the Bay of Bengal and South China SeaModels with a warmer sea surface temperature bias over the tropical western Pacific tend to project a more delayed monsoon onset in the futureThe emergent constraint indicates that the delay of monsoon onset over the Bay of Bengal and South China Sea is overestimated [ABSTRACT FROM AUTHOR]

Trends in the coupled stratosphere‐troposphere system during the 1979–2022 period are investigated in the Northern Hemisphere using reanalysis datasets. More upward planetary wave propagation in December is shown to precede the deceleration of the stratospheric polar vortex in January. This deceleration prevents the waves from continuing to propagate upward in February and favors an acceleration of the stratospheric polar vortex in March. This is associated with an increased Northern Hemisphere annular mode in March in the stratosphere and the troposphere. Trends show a moderate significance level because of strong interannual variability. Recent seasons whose anomalies project onto the trends are those for which wave‐1 anomaly constructively interferes with wave‐1 climatology in December, which occurs when there is warming in an area extending from Eastern Canada to Greenland and slight cooling over Eurasia. It shows the potential for predicting the springtime stratospheric polar vortex from wintertime wave‐1 anomalies. Plain Language Summary: In winter the stratosphere hosts strong westerly winds forming the stratospheric polar vortex. Variations in the vortex strength can affect weather patterns in the underlying troposphere. Vice‐versa, the stratospheric vortex can also be disrupted by tropospheric waves that propagate upwards into the stratosphere. Sea‐ice loss from Arctic warming is amongst the forcings that have been proposed to produce tropospheric waves leading to a weakening of the polar vortex. However, there is a strong dependence of the stratospheric signal on the region of sea ice loss. In the present study, we show that mid‐winter to early spring stratospheric trends during the 1979–2022 period are linked to large‐scale surface temperature trends in December. Specifically, strong warming from Eastern Canada to Greenland and relative cooling over Eurasia in December are associated with enhanced upward wave propagation and a deceleration of the stratospheric polar vortex in January. In mid‐winter a weaker than usual vortex inhibits wave propagation to the stratosphere, allowing for a recovery in vortex strength by March. In conclusion, our work shows how the opposite mid‐winter and early spring trends in the stratosphere can be connected with stronger wave propagation from the surface in December and with its time‐evolving effects on troposphere–stratosphere coupling. Key Points: January and March exhibit opposite trends in the stratospheric Northern Annular Mode (NAM) during 1979–2022The negative (positive) trend in the NAM in January (March) is explained by more (less) upward wave propagation in December (February)More upward wave propagation in December is linked to warming over Eastern Canada and cooling over Siberia [ABSTRACT FROM AUTHOR]

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

ABSTRACT Current East Asian winter monsoon (EAWM) indices effectively depict the associated high‐ and low‐latitude atmospheric circulations. However, the spatial dynamics of the winter coldness within the monsoon domain are not well adequately represented by EAWM indices. We introduce a novel approach to classify winter temperatures based on both their co‐variability and their mean values. We classified the EAWM domain into three distinct modes: northern (ranging from −27°C to −15°C), central (−14°C to 5°C), and southern (6°C to 27°C). The northern mode, characterised by intense coldness, correlates with a strengthened westerlies that traps Arctic cold air masses during the positive phase of the Arctic Oscillation (AO). In contrast, the southern mode is primarily influenced by low‐latitude oceanic and atmospheric patterns, particularly for near‐coast areas. The central mode, representing an interplay of both high and low‐latitude processes, encapsulates the comprehensive characteristics of the EAWM. Our analysis reveals a notable shift in the relationships among the northern, central, and southern modes around 1990. Prior to this year, the EAWM was predominantly influenced by northern atmospheric patterns, while there is a discernible increase in the influence of low‐latitude drivers afterwards. This shift may be linked to the significant warming in the western Pacific and Indian Oceans, underscoring the heightened role of low‐latitude drivers on the EAWM. [ABSTRACT FROM AUTHOR]

The Shyok Valley, within the Upper Indus Basin in Trans-Himalayas, lies in the cold and arid region. It is fed by the Siachen glacier; the largest glacier in the third pole. The fluctuations in this glacier are attributed to varying intensities of the two weather regimes namely mid-latitude westerlies and Southwest Monsoon. The reconstruction of the magnitude, timing, and landscape impact of glaciers in Nubra-Shyok valley have been explored in past decade yet they are contentious and uncertain. Therefore, the present study investigates the sediments from the key sites (Agham, Khardung, Changmar, Chalunka) in the Shyok valley using Optically Stimulated Luminescence (OSL) dating technique. The diverse geomorphology, including moraines, sand dunes, mass movement zone, alluvial fans are mapped to represent the geomorphic setting of the region. The chronological ages from the lower Shyok Valley (Agham section: 18.4 ± 2.2 ka) suggest extensive glacier expansion beyond the present terminus of the Siachen Glacier during the Marine Isotopic Stage (MIS)-2. The Khardung section has glaciofluvial deposits: 24.0 ± 2.4 ka (KHG-1), younger moraine: 8.2 ± 1.0 ka (KHG-9) and lacustrine deposits (KHGL-5, KHGL-6, KHGL-7): 14.1 ± 2.0 ka, 12.1 ± 1.1 ka, 10.7 ± 1.3 ka. The glacial events that occurred around 8.2 ka and 12.0 ka suggest glacial advancement which are likely due to a period of lower temperature. The Changmar section displays lacustrine deposits and debris flow events dated to the late glacial (14.5 ± 1.7 ka, CHG-1; 13.2 ± 1.6 ka, CHG-2) and deglaciation period (6.5 ± 1.2 ka, CHG-3; 5.5 ± 1.0 ka, CHG-4). The presence of the Holocene-aged lateral moraines (20 km away from Chalunka village: 14.3 ± 1.3 ka, CLM-2; 5.7 ± 0.9 ka, CLM-3; 0.4 ± 0.04 ka, CLM-4) and CHG-8: 2.1 ± 0.3 ka; outwash plain deposits: COP-7: 2.4 ± 0.4 ka aligns with the previous findings in the Nubra Valley, suggesting glacial advance in Little Ice Age in the whole valley. The glacial event occurring between 2.4 and 2.1 ka corresponds to the Neoglacial epoch, characterized by a glacial advance likely caused by a decrease in temperature during the late Holocene period. The intensified mid-latitude westerlies during MIS-2 are inferred to be the key factor in increased moisture to the Shyok Valley which led to the glacier expansion. These findings enhance our understanding of past climate changes in this high-altitude region and serve as a valuable baseline for future studies on glacial response to climate variations. [ABSTRACT FROM AUTHOR]

Based on the observed maximum temperature (Tmax), relative humidity (RH) and NCEP/NCAR reanalysis data during 1981–2021, basic temporal features and dominant atmospheric circulation patterns of dry‐type and humid‐type heatwaves in North China are investigated and compared. Statistical results indicate the dry heatwaves occur mainly in early summer (from early June to early July), that is, before the rainy season of North China, while the humid heatwaves have a high frequency in mid‐July to mid‐August. During the research period, the increasing trend of dry heatwaves is 0.67 days·decade−1, while the humid heatwaves increase at a greatly higher rate of 1.85 days·decade−1. For the dry heatwave, a high ridge in the subtropical westerlies plays the main role, and the northerly wind in the east of the ridge reduces the air moisture convergence over the region. However, for the humid heatwave, the westward and northward propagations of the western Pacific subtropical high (WPSH) may make the major contribution, and the southerly wind anomalies in the west of the WPSH enlarge the water vapour to the region. The adiabatic heating in subsiding air at all levels and horizontal temperature advection at lower troposphere are stronger for dry heatwaves than for humid heatwaves, which cause a higher Tmax for the former type. These results highlight the diversity of the heatwaves in North China, which suggests that multiple local and large‐scale subseasonal circulations should be considered to improve the subseasonal to seasonal forecast skills for heat extremes. [ABSTRACT FROM AUTHOR]

Westerly wind bursts (WWBs) in the western–central equatorial Pacific are critical in El Niño–Southern Oscillation (ENSO) dynamics. Understanding how they may change with global warming has important implications for future projections of El Niño. In this study, we investigate how the enhanced eastern equatorial Pacific warming pattern, emerging in future climate projections, can influence WWB characteristics in an atmospheric general circulation model, CAM6. Changes in three main factors affecting WWBs—El Niño-conditions mean westerly wind stress anomalies, the Madden–Julian oscillation (MJO), and tropical cyclones (TCs)—are analyzed. We find that during El Niño onset (December–April), the WWB wind stress intensity remains largely unchanged but WWBs shift westward by about 20° of longitude. In contrast, during El Niño development (May–November), the WWB intensity increases by 41% or 79% in two warming scenarios considered [shared socioeconomic pathways (SSP5-8.5) and SSP2-4.5, respectively], which is mainly caused by a higher frequency of TC occurrence in the central tropical Pacific within 15°N/S. Further, we find that westerly wind speed anomalies associated with El Niño and the MJO also increase during El Niño development. However, as trade winds weaken in the central-eastern equatorial Pacific, the mean strength of the resulting westerly wind stress anomalies does not change much, and no significant eastward shift of WWBs is observed. Thus, our atmospheric model simulations suggest a strong TC-driven increase in WWB wind stress anomalies during El Niño development and hence a more important role of TCs in WWB generation, which in a coupled system could lead to stronger ENSO events, enhanced TC–ENSO coupling, and even greater intensification of WWBs. [ABSTRACT FROM AUTHOR]

The interdecadal change in the relationship between the Antarctic Oscillation (AAO) and autumn rainfall in the Yunnan–Guizhou Plateau of Southwest China is investigated by using the observed autumn rainfall data at 119 stations and the National Centers for Environment Prediction/National Center for Atmospheric Research reanalysis data for the period 1961–2021. Results show the AAO correlates well with the autumn rainfall in the Yunnan–Guizhou Plateau for the second period (2002–2021) because the AAO becomes stronger. The possible influencing mechanism of AAO on autumn rainfall in the Yunnan–Guizhou Plateau during 2002–2021 is related to the meridional teleconnection pattern and associated convection over the Philippine Sea. The positive AAO can trigger a meridional teleconnection pattern in the upper troposphere to propagate from the southern Pacific to northern Pacific and cause anomalous westerly over the tropical west Pacific, which inhibits the convection over the Philippine Sea. On the one hand, the weakened convection over the Philippine Sea causes the anomalous ascending motion over the Yunnan–Guizhou Plateau; on the other hand, it results in an anomalous anticyclone over the tropical Northwest Pacific and strengthens the transport of water vapor from the tropical Pacific to the Yunnan–Guizhou Plateau. [ABSTRACT FROM AUTHOR]

In this study, available large ensemble datasets in the Coupled Model Intercomparison Phase 6 (CMIP6) archive were used to provide the first multi-variate overview of the evolution of extreme seasons over Antarctica and the Southern Ocean during the 20th and 21st centuries following medium-to-high radiative forcing scenarios. The results show significant differences between simulated changes in background mean climate and changes in low (10th percentile) and high (90th percentile) extreme seasons. Regional winter warming is most pronounced for cold extremes. In summer, there are more pronounced increases in high extremes in precipitation and westerly wind during the ozone hole formation period (late 20th century), affecting coastal regions and, in particular, the Antarctic Peninsula. At midlatitudes, there is a reduction in the range of summer season wind extremes. Suggested mechanisms for these differences are provided relating to sea ice retreat and westerly jet position. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms driving hydrological change in arid Central Asia over a range of time scales is crucial for making predictions for future changes in fragile desert-lake ecosystems. As of yet, the drivers of hydrological changes in lake systems of arid Central Asia over the Holocene remain largely unexplored. Aibi Lake, fed by rivers originating from the glaciated Tianshan Mountains and terminating in the arid Junggar Basin of northwestern China, presents a perfect natural laboratory to explore lake evolution in context to Holocene climate evolution in arid Central Asia. Here, a single-grain K-feldspar dating method was used to effectively date 20 paleolake shorelines with poorly bleached sediment to constrain lake level evolution over the past 18 k.y. Results indicate that Aibi Lake experienced a rapid increase in water levels, reaching a peak of ~36 m during the early to mid-Holocene period (10-7 ka). Subsequently, the lake level may have shown a general decline during the middle Holocene (7-4 ka), with the lake reaching a low level of less than 10 m at ca. 4 ka. In the late Holocene, lake levels fluctuated by 10-30 m above modern levels during 4-1 ka, with generally low levels of <9 m after 1 ka. The evolution of Aibi Lake underlines a clear out-of-phase relationship between Central Asian lake evolution and Westerlies precipitation changes, where Holocene lake changes were instead more directly controlled by the flux of glacial meltwater from the Tianshan Mountains, driven by change in Northern Hemisphere summer insolation. Glacier meltwater, in combination with variable delivery of Westerlies and East Asian summer monsoon precipitation, are responsible for asynchronous lake evolution trends across Central to East Asia. [ABSTRACT FROM AUTHOR]

We present monsoon variability records for the Holocene using multi-proxy approach (environmental magnetism, carbon isotope, and total organic carbon) from a 146 cm thick sedimentary profile in the Kotikanasar meadow (Chakrata), Northwest Himalaya. The chronology of the record was constrained by five AMS 14C ages. The carbon isotope (δ13C) and Total Organic Carbon (TOC) data highly variable which vary between −26.62‰ and −22.46‰ (C 3 -plants) and 0.1–∼4%, respectively, indicating paleo-vegetation history and productivity of the studied area. The environmental magnetism is highly fluctuating in the Early Holocene with high concentrations of magnetic minerals during the high monsoon conditions and vice-versa. Intense Indian Summer Monsoon (ISM) phases were identified during the Early and Late Holocene i.e., ∼9.2 to 7.4 ka, and ∼4.8 ka to Modern which shows warm and wet climate. While decline in the ISM intensity during ∼7.4 to 4.8 ka which indicates cold and dry climatic condition in the Northwest Himalayan regions. From ∼9.2 to 7.4 ka, highly fluctuating climate linked with the Early Holocene warming. Sediment profile exhibits aridity in climate accompanying with the high influence of mid-latitude westerlies during ∼7.4 to 4.8 ka from Northwest Indian regions. Hence the long-term fluctuation in the climate governed by the changes in the North Atlantic Ocean circulation as well as variations in the incoming solar radiations. [ABSTRACT FROM AUTHOR]

Understanding precipitation over complex terrain, such as southwestern China, requires the consideration of both multiscale circulation and topography. First, the dominant synoptic system must be clarified, as it determines how multiscale topography affects precipitation. Here, based on a self-organizing map, large-scale winds are categorized into anomalous-westerly types, anomalous-easterly types, and transitional types. Four synoptic-scale systems (vortex type, cold-front type, tropical-depression type, and weak-synoptic-forcing type) dominate the summer precipitation. The vortex type occurs with strengthened large-scale westerlies, and its precipitation is distributed within the moisture convergence region. The cold-front type, tropical-depression type, and weak-synoptic-forcing type exhibit large-scale easterly anomalies. For the cold-front type, a low-level northeasterly blocked by topography shapes the northwest–southeast-oriented front zone at the upper highland slope. The precipitation frequency and intensity are high within the frontal zone, while the intensity is weak on both sides. For the tropical-depression type, moist low-level easterlies uplifted by westward-rising topography anchor precipitation at the lower slope. Large precipitation for the tropical-depression type is attributed to a high frequency. Large-scale horizontal winds are the weakest for the weak-synoptic-forcing type, and the local topography influences the scattered precipitation distribution. Both the frequency and intensity are high for the weak-synoptic-forcing type. Overall, long-lasting nocturnal events dominate the precipitation of the four synoptic types, while large-scale easterlies favor precipitation events with shorter durations and earlier peaks. For obvious synoptic systems, large-scale topography influences precipitation via a dynamic blocking effect, while the thermodynamic role of local topography is important with a weak-synoptic-forcing. Significance Statement: Clarifying dominant synoptic systems is highly important for understanding precipitation over complex terrain, as the effect of topography on precipitation varies with different synoptic backgrounds. Taking southwestern China as a representative of complex terrain, this study objectively identified the dominant synoptic systems associated with summer precipitation. The distribution and fine-scale characteristics of precipitation have been further analyzed considering the combined influence of multiscale circulation and topography. In addition to advancing our understanding of precipitation in southwestern China, this study provides a reference for analyzing precipitation in other regions with complex terrains. [ABSTRACT FROM AUTHOR]

We analyze biases in subseasonal forecast models and their effect on Southwest United States (SWUS) precipitation prediction (2–6-week time scale). Cluster analyses identify three primary wave trains associated with SWUS precipitation: a meridional El Niño–Southern Oscillation (ENSO)–type wave train, an arching Pacific–North American (PNA)–type wave train, and a circumglobal zonal wave train. Compared to reanalysis, the models overrepresent the arching pattern, underrepresent the zonal pattern, and produce mixed results for the meridional pattern. The arching pattern overrepresentation is linked to model mean flow biases in the midlatitude–subpolar North Pacific, which cause a westward retraction of the region of forbidden linear Rossby wave propagation. The zonal pattern underrepresentation is linked to westerly biases in the subtropical jet, which cause a westward retraction of the waveguide in the midlatitude eastern North Pacific and divert wave trains southward. These results are confirmed using linear, barotropic ray-tracing analysis. In addition to mean state biases, the models also contain errors in their representation of the Madden–Julian oscillation (MJO). Tropical convection anomalies associated with the MJO are too weak and incoherent at lead times greater than 2 weeks when compared to reanalysis. Additionally, there is a strong SWUS precipitation signal as far out as 5 weeks after a strong MJO in reanalysis, associated with its persistent eastward propagation, but this signal is absent in the models. Our results indicate that there is still significant room for improvement in subseasonal predictions if we can reduce model biases in the background flow and improve the representation of the MJO. [ABSTRACT FROM AUTHOR]

This study reveals the remarkable interdecadal changes in the influence of boreal winter Arctic sea ice concentration (SIC) anomalies (ASICAs) in the Greenland–Barents Seas on the subsequent El Niño–Southern Oscillation (ENSO) development. Winter ASICA is strongly associated with the subsequent winter ENSO before the late 1980s and after the late 2000s, but their connection is weak during the 1990s and the 2000s. The interdecadal variations in the influence of ASICA on ENSO are associated with changes in the spatial structure of the ASICA-induced North Pacific atmospheric anomalies. During high-correlation periods, winter SIC increases in the Greenland–Barents Seas lead to tropospheric cooling via the suppression of upward surface heat fluxes, which further trigger an atmospheric teleconnection from the Arctic to the North Pacific. The accompanying North Pacific Oscillation–like atmospheric anomalies result in sea surface temperature (SST) warming in the subtropical North Pacific, which extends southward into the tropical Pacific via wind–evaporation–SST feedback and leads to surface westerly anomalies over the tropical western Pacific in the following summer. The tropical western Pacific westerly wind anomalies impact the subsequent ENSO development via triggering positive Bjerknes air–sea interaction. During low-correlation periods, atmospheric anomalies over the North Pacific generated by the winter ASICA are located more northward and cannot induce marked subtropical North Pacific SST anomalies and thus have a weak impact on the following ENSO development. Numerical experiments suggest that the interdecadal variation in the spatial structure of the North Pacific atmospheric anomalies induced by the winter ASICA is partly attributed to change in the atmospheric mean flow. Significance Statement: Arctic sea ice is an important component of the global climate system. Studies have shown that Arctic sea ice has decreased significantly in recent decades, which is considered to be one of the most important responses of Earth's climate system to global climate change. A number of studies have shown that Arctic sea ice anomalies have significant impacts on weather and climate over mid–high latitudes through tropospheric and stratospheric processes. Recent studies have suggested that the effects of Arctic sea ice anomalies could extend to the tropics via air–sea interactions. El Niño–Southern Oscillation (ENSO) is the strongest air–sea coupled system in the tropics and can have a significant impact on climate over the globe. ENSO is also considered to be one of the most important sources of subseasonal–seasonal climate predictability over many parts of the globe. It is therefore important to study the factors for the ENSO occurrence. A recent study showed that Arctic sea ice anomalies during boreal winter in the Greenland–Barents Seas have a significant impact on the following winter ENSO. In this study, we further reveal that the influence of winter Arctic sea ice anomalies in the Greenland–Barents Seas on the subsequent ENSO is unstable and has undergone significant interdecadal variation. We then investigate the mechanisms underlying the interdecadal variation via observational analysis and numerical simulations. The results of this study not only have implications for improving the prediction of ENSO but also could improve our understanding of the physical link between high-latitude climate systems and tropical air–sea coupling systems. [ABSTRACT FROM AUTHOR]

We present an analysis of magnitude clustering of microfractures inferred from acoustic emissions (AEs) during stick‐slip (SS) dynamics of faulted Westerly granite samples in frictional sliding experiments, with and without fluids, under triaxial loading with constant displacement rate. We investigate magnitude clustering in time across periods during, preceding and after macroscopic slip events on laboratory faults. Our findings reveal that magnitude clustering exists such that subsequent AEs tend to have more similar magnitudes than expected. Yet, this clustering only exists during macroscopic slip events and is strongest during major slip events in fluid‐saturated and dry samples. We demonstrate that robust magnitude clustering arises from variations in frequency‐magnitude distributions of AE events during macroscopic slip events. These temporal variations indicate a prevalence of larger AE events right after (0.3–3 s) the SS onset. Hence, magnitude clustering is a consequence of non‐stationarities. Plain Language Summary: Can we determine the size of a future earthquake based on the size of past earthquakes? This fundamental question has been controversially debated over the years, without an agreed‐upon answer. Here, we tackle this question under controlled conditions in a lab setting by studying the frictional stick‐slip dynamic of rough granite faults, which gives rise to mm‐scale seismic events. We find that the sizes of these seismic events are not independent during periods containing a macroscopic slip but instead are clustered such that larger seismic events tend to directly follow other large seismic events. We show that this can be explained by temporal changes in the frequency of occurrence of seismic events associated with the sliding motion of the fault. Our findings link the properties of mm‐scale seismic events with macroscopic slip on lab faults. Key Points: Magnitude clustering of acoustic emissions (AEs) is strongest during major slip events in fluid‐saturated samples of Westerly graniteVariations in the frequency‐magnitude distribution of AEs during slip events explain the strong magnitude clusteringThese variations indicate a prevalence of larger AEs right after the onset of slip events [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]

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

The Arctic Oscillation (AO) has been observed to undergo distinct decadal structural fluctuations that significantly influence regional weather and climate. Understanding the drivers and mechanisms behind the AO's spatial nonstationarity is critical for improving climate predictions related to the AO. We present evidence that the Atlantic Multidecadal Oscillation (AMO) plays a pivotal role in modulating AO's Pacific center in recent decades. The poleward amplified cooling associated with negative AMO enhances the north-south temperature gradient and results the strengthened westerly winds and stratospheric polar vortex (SPV) responses, which reflects more planetary waves from the North Pacific to the North Atlantic. This enhances the atmospheric coupling between these regions and leads to a more pronounced Pacific center within the AO pattern. Numerical simulations from ECHAM5 and 35 CMIP6 models further corroborate the essential role of the AMO. These findings advance our understanding of the mechanisms driving the variability of the AO pattern. [ABSTRACT FROM AUTHOR]

This study analyzes the characteristics of near-surface atmospheric CO2 concentrations on the southern slope of Tomur Peak from 2013 to 2022. Utilizing the Boughton two-parameter algorithm, we determined the CO2 background and non-background concentrations. Additionally, the Hysplit model was employed to investigate the potential contributing areas of the CO2 non-background concentration. Our findings that the CO2 concentration in the study area was approximately 17% lower than the global average, with an annual growth rate that was only about 47% of the global rate. The CO2 non-background concentration accounted for only 6.25% of the observed concentration, but was largely unaffected by near-ground meteorological factors. The source of the CO2 non-background was mainly controlled by the westerly belt, with potential source area mainly located in Central Asia. However, the WPSCF values in the modern ice and snow distribution area south of Tomur Peak and west of the monitoring station were significantly elevated, suggesting a strong wind transport of CO2 after accumulation under the influence of the elevated terrain. The local supplying path accounted for only 12.87%, indicating that human activities in the oasis areas of the Tarim Basin had a limited influence on the study area. Due to the blocking effect of the Baikal high-pressure system and the towering topography of the Tianshan Mountains, the influence of human emissions from higher latitudes was likely negligible. [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]

Unprecedented extreme Saharan dust (duxt) events have recently expanded northward from subtropical NW Africa to the Atlantic and Europe, with severe impacts on the Canary Islands, mainland Spain and continental Portugal. These six historic duxt episodes occurred on 3–5 and 22–29 February 2020, 15–21 February 2021, 14–17 January 2022, 29 January–1 February 2022, and 14–20 March 2022. We analyzed data of 341 governmental air quality monitoring stations (AQMSs) in Spain (330) and Portugal (11), where PM10 and PM2.5 are measured with European norm (EN) standards, and found that during duxt events PM10 concentrations are underestimated due to technical limitations of some PM10 monitors meaning that they can not properly measure extremely high concentrations. We assessed the consistency of PM10 and PM2.5 data and reconstructed 1690 PM10 (1 h average) data points of 48 and 7 AQMSs in Spain and Portugal, respectively, by using our novel "duxt-r" method. During duxt events, 1 h average PM10 and PM2.5 concentrations were within the range 1000–6000 µgm-3 and 400–1200 µgm-3 , respectively. The intense winds leading to massive dust plumes occurred within meteorological dipoles formed by a blocking anticyclone over western Europe and a cutoff low located to the southwest, near the Canary Islands and Cape Verde, or into the Sahara. These cyclones reached this region via two main paths: by deviating southward from the Atlantic mid-latitude westerly circulation or northward from the tropical belt. The analysis of the 2000–2022 PM10 and PM2.5 time series shows that these events have no precedent in this region. The 22–29 February 2020 event led to (24 h average) PM10 and PM2.5 concentrations within the range 600–1840 and 200–404 µgm-3 , respectively, being the most intense dust episode ever recorded on the Canary Islands. The 14–20 March 2022 event led to (24 h average) PM10 and PM2.5 values within the range 500–3070 and 100–690 µgm-3 in southeastern Spain, 200–1000 and 60–260 µgm-3 in central Spain, 150–500 and 75–130 µgm-3 in the northern regions of mainland Spain, and within the ranges 200–650 and 30–70 µgm-3 in continental Portugal, respectively, being the most intense dust episode ever recorded in these regions. All duxt events occurred during meteorological anomalies in the Northern Hemisphere characterized by subtropical anticyclones shifting to higher latitudes, anomalous low pressure expanding beyond the tropical belt and amplified mid-latitude Rossby waves. New studies have reported on recent record-breaking PM10 and PM2.5 episodes linked to dipole-induced extreme dust events from North Africa and Asia in a paradoxical context of a multidecadal decrease in dust emissions, a topic that requires further investigation. [ABSTRACT FROM AUTHOR]

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

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]

A major challenge for climate system models in simulating the impacts of El Niño–Southern Oscillation (ENSO) on the interannual variations of East Asian rainfall anomalies is the wide inter-model spread of outputs, which causes considerable uncertainty in physical mechanism understanding and short-term climate prediction. This study investigates the fidelity of 40 models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) in representing the impacts of ENSO on South China Spring Rainfall (SCSR) during the ENSO decaying spring. The response of SCSR to ENSO, as well as the sea surface temperature anomalies (SSTAs) over the tropical Indian Ocean (TIO), is quite different among the models; some models even simulate opposite SCSR anomalies compared to the observations. However, the models capturing the ENSO-related warm SSTAs over TIO tend to simulate a better SCSR-ENSO relationship, which is much closer to observation. Therefore, models are grouped based on the simulated TIO SSTAs to explore the modulating processes of the TIO SSTAs in ENSO affecting SCSR anomalies. Comparing analysis suggests that the warm TIO SSTA can force the equatorial north–south antisymmetric circulation in the lower troposphere, which is conducive to the westward extension and maintenance of the western North Pacific anticyclone (WNPAC). In addition, the TIO SSTA enhances the upper tropospheric East Asian subtropical westerly jet, leading to anomalous divergence over South China. Thus, the westward extension and strengthening of WNPAC can transport sufficient water vapor for South China, which is associated with the ascending motion caused by the upper tropospheric divergence, leading to the abnormal SCSR. [ABSTRACT FROM AUTHOR]

This study explored the interannual characteristics of the out‐of‐phase pattern of summer precipitation over northern China during the past years, as well as the possible underlying mechanisms. The out‐of‐phase pattern is characterized by the positive precipitation anomaly in Northwest China (NWC) and the negative anomaly in North China (NC). Our analyses indicate that the variation of Asian westerly jet (AWJ) is found evidently associated with this out‐of‐phase pattern of summer precipitation. The meridional displacement of AWJ induces opposite trends in water vapour transport and circulation anomalies between NWC and NC, leading to the out‐of‐phase pattern. Further analyses suggest that the anomalies of Arctic sea ice concentration (SIC), North Atlantic and Northwest Pacific sea surface temperature (SST) can impact the variation of AWJ by triggering the Eurasian (EU), polar‐Eurasia (POL) and Pacific‐Japan (PJ) teleconnection patterns, respectively, which in turn modulate the formation of this pattern. The precipitation in NWC is primarily affected by the North Atlantic and Northwest Pacific SST anomalies, whereas the precipitation in NC is notably influenced by both SST anomalies and the Arctic SIC anomaly. However, the role of SST anomalies on precipitation in NWC and NC exhibits an evident contrast, leading to the out‐of‐phase pattern of summer precipitation. [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]

The Asian summer monsoon anticyclone (ASMA) plays a vital role in the transport and redistribution of the tracers in the upper troposphere and lower stratosphere during transient monsoon conditions. The ASMA can be identified using various diagnostics such as winds and divergence, potential vorticity, the Montgomery stream function or geopotential height. Its representation based on the commonly‐used existing method that takes a climatological range of the geopotential height (GPH) at a given pressure level is not robust and fails in several circumstances. In this study, we propose a new GPH method to define the ASMA for the active and break phases of the monsoon based on the GPH values at the tropical easterly jet (TEJ) and subtropical westerly jet (STJ) locations. The proposed method compares the GPH values at the locations corresponding to the wind speed maxima of TEJ and STJ at 100 hPa with the maximum GPH at 100 hPa and selects the one with the minimum difference to define the ASMA extent. The same procedures were applied to obtain the ASMA extent at 150 hPa. The ASMA extents are obtained using the method proposed here and the existing fixed GPH method for the 30 active and 27 break phases from 2005 to 2023. The fixed GPH method provides a larger ASMA extent for 24 active and 21 break phases, which appears to be an unrealistic estimation compared to the proposed method. Our proposed method identifies the ASMA extent that is larger and centred around the Iranian side during the active phases while it is smaller and centred around the Tibetan side during the break phases. Furthermore, the minimum concentration of ozone (O3), and maximum concentrations of carbon monoxide (CO) and water vapour (H2O) are also found to be centred near the Iranian side during the active phases and on the Tibetan side during the break phases. The tracer concentrations are generally higher for CO, H2O and O3 during the active phase than in the break phase. Tracer concentrations within the ASMA extent using the fixed GPH method are higher for O3 while lesser for CO and H2O when compared to the proposed method. [ABSTRACT FROM AUTHOR]

Western Himalayas (WH) receives precipitation through eastward propagating synoptic weather systems, Western Disturbances (WDs), embedded in large scale subtropical westerly jet (SWJ) during Northern Hemispheric boreal winter (December, January and February; DJF). In the recent decade, WDs have undergone certain changes in their characteristic's viz., decreased associated precipitation, no significant changes in the WDs frequency and intensity, etc. However, Non-WDs days and associated precipitation gained its importance as it contributes ~ 20% of winter precipitation over WH. In this study, structure, dynamics and moisture availability associated with changing WDs and Non-WDs precipitating days are analyzed. Robust statistical methods and Theil-Sen slope on ERA5 data during 1987–2020 is considered to comprehend why active WDs' (Non-WDs) precipitation frequency is declining (notably increasing). Interestingly, it is as well found that all the WDs do not precipitate and there are Non-WDs days when precipitation occur. It shows declining trend in WD precipitation and the increasing importance of Non-WDs. The findings show formation of 'Ω shape' flow - where high pressure is locked between two low pressure systems - over Indian subcontinent during Non-WDs days. Such dynamical wind col formation ('Ω shape') from lower (850Hpa) to upper (200 hPa) troposphere provides conducive situation for enhanced moisture incursion from Bay of Bengal (BoB) towards and over the WH during Non-WDs days leading to precipitation. This new finding needs to be looked from sustaining and replenishment of glaciers; agriculture; socioeconomic benefits during winters and needs to be addressed. [ABSTRACT FROM AUTHOR]

Previous study has revealed the possible linear superposition of the El Niño-Southern Oscillation (ENSO) and Quasi-Biennial Oscillation (QBO)'s impacts on the northern winter stratosphere. This study first assesses the reproducibility of stratospheric ENSO and QBO teleconnections by 18 models able of producing QBO from the Coupled Model Intercomparison Project Phase (CMIP)-5 and CMIP6. Based on different teleconnection indices, high-skill models are adopted to reexamine the extratropical response to four ENSO-QBO configurations (El Niño-QBO easterly (EQBO), El Niño-QBO westerly (WQBO), La Niña-QBO easterly, and La Niña-QBO westerly). The modelling evidence validates that the stratospheric ENSO (QBO) signal can be separated by selecting major events based on the corresponding index and subtracting the QBO (ENSO) signal from model outputs through the regression method. The composite stratospheric response to a specific ENSO-QBO configuration is almost identical to the summation of impacts of individual QBO and ENSO, which validates the linearity. This study offers novel model evidence that stratospheric ENSO and QBO's teleconnections exhibit strong linearity in the northern winter. This linearity also exists in the troposphere to some degree. During the composite compound El Niño-EQBO conditions, the stratospheric polar vortex (SPV) weakens and is warmer, and its strengthening and cold anomalies also enlarge for the composite compound La Niña-WQBO conditions due to their cooperative amplification of signals by both QBO and ENSO. In contrary, the sign of the composite polar stratospheric total response is mainly determined by the QBO phase in both El Niño-WQBO and La Niña-EQBO composites. [ABSTRACT FROM AUTHOR]

This work evaluates the performances of climate models in simulating the Southern Ocean (SO) sea surface temperature (SST) by a large ensemble from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). By combining models from the same community sharing intermodel variations in the zonal-mean SST biases is characterized as a meridional uniform warm bias pattern, explaining 79.1% of the intermodel variance and exhibiting positive principal values for most models. The ocean mixed layer heat budget further demonstrates that the SST biases at 70°−50°S primarily result from the excessive summertime heating effect from surface net heat flux. The biases in surface net heat flux south of 50°S are largely impacted by surface shortwave radiation from cloud and clear sky components at different latitudes. North of 50°S, the underestimated westerlies reduce the northward Ekman transport and hence northward cold advection in models, leading to warm SST biases year-round. In addition, the westerly biases are primarily traced back to the atmosphere-alone model simulations forced by the observed SST and sea ice. These results disclose the thermal origin at the high latitude and dynamical origin at the low latitude of the SO SST biases and underscore the significance of the deficiencies of atmospheric models in producing the SO SST biases. [ABSTRACT FROM AUTHOR]

Current climate models suggested that Antarctic sea ice cover would decrease substantially under cumulative CO2 emission, but little is known whether large decrease in Antarctic sea ice can influence the occurrence of strong El Niño. Using time slice coupled and uncoupled model experiments, we show that in response to half reduction of Antarctic sea ice projected near the end of the 21st century, the frequency of strong El Niño would be increased by ~40%. It is contributed by enhanced thermocline, Ekman, and zonal advective positive feedbacks that are partly offset by enhanced thermodynamic damping. The strong warming and weakened westerly winds in the southeastern Pacific generate an anomalous Rossby wave propagating into the eastern subtropical and tropical Pacific, favoring stronger El Nino, and air-sea coupling and ocean feedbacks play a critical role in the teleconnection. Unexpectedly, compare to halved Antarctic sea ice, the ice-free Antarctic leads to a decrease in the frequency of strong El Niño, which is largely due to a substantial increase in thermodynamic damping. We also show that a large portion of the increase of strong El Niño events under greenhouse warming might be connected with Antarctic sea-ice loss, though increased greenhouse gas plays an important role. [ABSTRACT FROM AUTHOR]

In this study, changes in westerly waves and their connections to increased global warming under the influence of greenhouse gases were investigated via a Caputo fractional four-dimensional atmospheric system. The idea of the existence of chaotic behavior in the westerly wind's motion was depicted. It has been noted that westerlies are becoming stronger due to rising air temperatures. An analysis of the existence, uniqueness, boundedness, stability of equilibrium points, and conservative behavior of the solutions was conducted. To prove the existence of chaos in the modified model, the Lyapunov exponents, Poincaré map, and bifurcation were computed. A sliding mode controller to control the chaos in this novel fractional-order system was designed, and conditions for the global stability of the controlled system with and without external disturbances and uncertainties were derived. The finite-time interval for the system to reach the sliding surface was computed. The developed controller's performance was evaluated with respect to both commensurate and non-commensurate fractional derivatives. In each scenario, the impact of fractional orders was investigated. Numerical simulations were used to support theoretical statements about how the controller affects the system. [ABSTRACT FROM AUTHOR]

In Lake Kinneret (the biblical Sea of Galilee), Israel, internal waves of significant amplitude are induced by westerly winds. These waves give rise to upwelling into the surface mixed layer of colder, oxygen‐depleted water from the hypolimnetic and metalimnetic layers. If upwelling occurs soon after the onset of annual thermal stratification, when surface mixed layer extends over a narrow depth range, but the hypolimnion is already anoxic, there is a potential for massive fish kills as fish cannot escape the anoxic water that intrudes into the surface mixed layer along the western shore. This study uses a coupled three‐dimensional atmosphere‐lake model to elucidate the mechanisms behind these infrequent major fish kills in Lake Kinneret. Remarkably, nowadays fish‐kill events happen at the same location in the lake where the biblical Miracle of Loaves and Fishes and presumably the Miraculous Catch of Fish occurred two millennia before the present and may explain the appearance of large numbers of easy‐to‐collect fish close to the shore described in the biblical narratives. Key Points: A 3D atmosphere‐lake model elucidated mechanisms for the 2012 fish kill at Lake Kinneret where 'miraculous catches' occurred 2000 years agoUpwelling of low‐oxygen water caused by wind‐driven internal waves was identified as the mechanism for the fish killsFive prerequisite conditions were identified for the fish kill to occur [ABSTRACT FROM AUTHOR]

Using 11 years of hourly merged rainfall records and ERA reanalysis data, this paper reveals two major circulation modes leading to two types of short‐term heavy rainfall (STHR) along the Taihang Mountains' eastern foothills, and further explains their mechanisms. One circulation mode has a distinct warm anomaly at 300 hPa covering most areas of North China, together with the boundary‐layer westerly anomaly occurring in North China and its southern region (UTWA‐BLWA). UTWA‐BLWA effectively contributes to the reinforcement of the upper‐level divergence and the low‐level moisture convergence by promoting the strengthening of upper anticyclonic and low‐level southwesterly anomalies. The combined effects of low‐level jet (LLJ) and topographic uplift form the central‐northern STHR pattern. The other circulation structure has a 300‐hPa warm anomaly located to the southeastern Russia and a prominent 300‐hPa cold anomaly covering the south, together with the boundary‐layer easterly anomaly occurring over the whole region of eastern China (UTCA‐BLEA). The southern STHR pattern is attributed to the exit of the boundary‐layer jet (BLJ) over lower elevations due to moist transport and dynamic uplift associated with the easterly anomaly. The results indicate that the locations of STHR are related to the direction, intensity, and height of the LLJ. The findings highlight that the upper‐tropospheric temperature anomalies (UTTA) and boundary‐layer easterly flow jointly modulate heavy rainfall. Analysis of the coupled upper troposphere and boundary layer could help understand and forecast heavy rainfall. Plain Language Summary: Short‐term heavy rainfall (STHR) frequently occurs along the eastern foothills of the Taihang Mountains, which is difficult to forecast accurately and causes significant damage. Operational weather analysis and previous studies have suggested that the atmospheric conditions at the middle‐ and lower troposphere are the most important for STHR. Our findings highlight that the upper‐tropospheric temperature anomalies (UTTA) could represent the evolution of atmospheric structures favorable for heavy rainfall, and the boundary‐layer flow can refine the intensity and locations of orographic rain. The coupled UTTA and boundary‐layer easterly flow can jointly represent the major patterns of STHR over the eastern slope of Mt. Taihang. The results may provide helpful information for weather forecasts and warning issuing and give a better understanding of heavy rainfall in North China. Key Points: Upper‐tropospheric temperature anomalies (UTTA) can contribute to the favorable atmospheric conditions for short‐term heavy rainfall (STHR)The locations of STHR correspond to the direction, intensity, and height of the low‐level jetCoupling UTTA and boundary‐layer jet provides the two major patterns of STHR [ABSTRACT FROM AUTHOR]

In this study, the mechanisms for the long‐lived and medium‐lived regional consecutive cloudy‐rainy events (RCCREs) in the Yangtze‐Huaihe River valley (YHRV) are investigated by comparing the influences of 10–30‐day and 30–90‐day intraseasonal oscillations (ISOs). The evolution of medium‐lived RCCREs is generally dominated by the 10–30‐day ISO, while the 10–30‐day and 30–90‐day ISOs synergistically contribute to the occurrence of long‐lived RCCREs. Further analysis reveals that, when the East Asian westerly jet (EAWJ) shifts northward to the north of YHRV, a RCCRE in the YHRV occurs in 2 days later. In the medium‐lived RCCREs, the EAWJ retreats southward rapidly; while in the long‐lived RCCREs, the stronger 30–90‐day ISO prevents the southward retreat of EAWJ, which favors the persistence of the RCCREs in the YHRV. The northward shifted EAWJ facilitates the RCCREs by modulating the secondary atmospheric circulation, consequently enhancing the moisture convergence and ascending motion anomalies over the YHRV. The RCCREs‐related precursor wave train originates from northern Europe with at least a 4‐day lead. The wave train propagates southeastward to excite an anomalous high over East Asia, which contributes to the northward shift of EAWJ and subsequently the occurrence of RCCREs. Different from the rapid weakness of the precursor wave train associated with the medium‐lived RCCREs, the precursor wave train associated with the long‐lived RCCREs can be maintained by the 30–90‐day ISO, which consequently leads to a persistence of the northward located EAWJ and long‐lived RCCREs in the YHRV. Plain Language Summary: Regional consecutive cloudy‐rainy events (RCCREs) have a disastrous impact on economy, agriculture, and natural environment over the Yangtze‐Huaihe River valley (YHRV). This study investigates the mechanisms contributing to the long‐ and medium‐lived RCCREs in the YHRV by comparing the role of 10–30‐day and 30–90‐day intraseasonal oscillations (ISOs). The medium‐lived RCCREs are dominated by the 10–30‐day ISO, while the long‐lived RCCREs are caused by a combination of the 10–30‐day and 30–90‐day ISOs. The East Asian westerly jet (EAWJ) plays a crucial role in RCCRE occurrence. When the EAWJ shifts northward, it triggers RCCREs by enhancing moisture convergence and ascending motion over the YHRV. The EAWJ retreats southward rapidly in the medium‐lived RCCREs. Conversely, the stronger 30–90‐day ISO prevents its southward retreat in the long‐lived RCCREs, favoring the persistence of RCCRE in the YHRV. Additionally, the precursor wave train originating from northern Europe contributes to the northward shift of the EAWJ and the occurrence of the RCCREs over the YHRV. Although both the long‐ and medium‐lived RCCREs exhibit similar precursor wave train, the influence of different ISOs varies, with the 30–90‐day ISO showing a larger impact on the precursor wave train and northward shift of the EAWJ in the long‐lived RCCREs. Key Points: Medium (long)‐lived RCCREs are mainly influenced by 10–30‐day intraseasonal oscillation (ISO) (both 10–30‐day and 30–90‐day ISOs)In the long (medium)‐lived RCCREs, the East Asian westerly jet (EAWJ) remains stable (retreats southward rapidly) after shifting northwardThe precursor wave train of the medium‐lived RCCREs weakens rapidly, while that of the long‐lived RCCREs is maintained by 30–90‐day ISO [ABSTRACT FROM AUTHOR]

As global warming intensifies, the coupling relationship between negative Indian Ocean Dipole (nIOD) and La Niña has substantially changed. However, the characteristics and mechanisms of these changes are not yet fully understood. Here, we find that the impact of La Niña on nIOD has considerably weakened since 1999, with the frequency of nIOD occurrences during La Niña years plummeting to a mere one‐third of the pre‐1999 levels. This is primarily attributed to the early onset of Indian summer monsoon and the decrease in La Niña intensity, while the effect of variations in Bjerknes feedback is relatively minor. Model simulations suggest that the influence of La Niña on nIOD will continue to weaken under future global warming through similar mechanisms as in the observations, increasing the complexity of air‐sea coupling in the Indian Ocean. Plain Language Summary: The negative Indian Ocean Dipole (nIOD) has significant impacts on both local and global climate change. La Niña is a major trigger for nIOD. However, as the globe warms up, the connection between nIOD and La Niña has changed a lot. This study analyze observation and reanalysis data from the 1950s onwards to see how nIOD and La Niña are linked. We find that La Niña has had less impact on nIOD since 1999. This is because the summer monsoon starts earlier, causing changes in wind patterns over the Indian Ocean. La Niña normally brings westerly wind anomalies in the Indian Ocean, but these winds are weakened because of the early monsoon. This makes it harder for nIOD to form. The feedback loop known as the Bjerknes feedback doesn't play a big role in maintaining nIOD. Furthermore, the reduction in La Niña intensity also contribute to weakening the relationship between La Niña and nIOD. Models predict that La Niña's impact on nIOD will continue to weaken under future global warming, increasing the complexity of air‐sea coupling in the Indian Ocean and leading to more challenges in forecasting. Key Points: The relationship between La Niña and negative Indian Ocean Dipole has weakened significantly in this centuryThe spring‐summer easterly anomaly induced by the premature summer monsoon weakens the La Niña‐induced westerly anomalyThe impact of La Niña on negative Indian Ocean Dipole will continue to weaken under future global warming [ABSTRACT FROM AUTHOR]

Eulerian and Lagrangian numerical moisture tracking models, which are primarily used to quantify moisture contributions from global sources to specific regions, play a crucial role in hydrology and (paleo)climatology studies on the Tibetan Plateau (TP). Despite their widespread applications in the TP region, potential discrepancies in their moisture tracking results and their underlying causes remain unexplored. In this study, we compare the most widely used Eulerian and Lagrangian moisture tracking models over the TP, i.e., WAM2layers (the Water Accounting Model – 2 layers) and FLEXPART-WaterSip (the FLEXible PARTicle dispersion model coupled with the "WaterSip" moisture source diagnostic method), specifically focusing on a basin governed by the Indian summer monsoon (Yarlung Zangbo River basin, YB) and a westerly-dominated basin (upper Tarim River basin, UTB). Compared to the bias-corrected FLEXPART-WaterSip, WAM2layers generally estimates higher moisture contributions from westerly-dominated and distant sources but lower contributions from local recycling and nearby sources downwind of the westerlies. These differences become smaller with higher spatial and temporal resolutions of forcing data in WAM2layers. A notable advantage of WAM2layers over FLEXPART-WaterSip is its closer alignment of estimated moisture sources with actual evaporation, particularly in source regions with complex land–sea distributions. However, the evaporation biases in FLEXPART-WaterSip can be partly corrected through calibration with actual surface fluxes. For moisture tracking over the TP, we recommend using high-resolution forcing datasets, prioritizing temporal resolution over spatial resolution for WAM2layers, while for FLEXPART-WaterSip, we suggest applying bias corrections to optimize the filtering of precipitation particles and adjust evaporation estimates. [ABSTRACT FROM AUTHOR]

The last glacial maximum (LGM) is widely acknowledged as the most recent cold period representing maximum global ice conditions. However, substantial warming is observed over Northern Hemisphere. Here, we show that the LGM climate shifted from very cold to fairly warm, followed by less cold conditions in the early Heinrich Stadial 1 (HS1) phases. Our synthesis of accurate AMS 14C dates refines the exact timing of Laurentide Ice Sheet (LIS) advances during the early LGM/HS1, constraining the chronology of the LIS decay during the late LGM. The summertime soil temperatures near ice fronts were found to increase by 1.3 °C from the early to late LGM and to decrease by 0.5 °C to the early HS1 phases, consistent with the cold-warm-cool climate patterns. The early/late LGM and early HS1 climates are found to be characterized by frequent cold/warm summers and cold winters since the world's largest LIS began to decay. [ABSTRACT FROM AUTHOR]

Drought, which is severe and recurring in Northern China, is the most significant meteorological disaster on a worldwide scale. However, a definitive conclusion has not yet been reached on the multi-scale drought features in Northern China under a warming context. Drought is affected by several circulation-related variables and a deeper scientific comprehension of their synergistic effects is required. This research addresses the spatiotemporal aspects of summer drought and its causes in the northern drought-prone belt of China (NDPB), based on meteorological observation and data reanalysis. The primary findings indicate that the NDPB is very vulnerable to drought, with the uniform mode being the region's most significant spatial mode associated with summer droughts. Inter-annual variations are the primary cause of the summer drought in the NDPB when considering multi-scale changes. Regional drought has typically declined during this century because of the effect of multi-decadal scale changes, which cannot be disregarded. Considering the circulation background field, during summer drought years in the NDPB, the South Asian high is weaker and moves eastward, while the high-level westerly jet is weaker and more northerly. From Lake Baikal to Northern China, there is an anomalous anticyclonic circulation, and the mid-level subtropical high in the western Pacific is more southerly and easterly. As a result, the NDPB is governed by the unusual northerly circulation. The Mongolian and northeastern surface cyclone activity is decreasing, the low-level warm and humid airflow is feeble, and the NDPB is not experiencing anomalous water vapor transport. In the high-, middle-, and low-level arrangement, the NDPB circulations diverge at low levels and converge at high levels--unusual sinking in the vertical field results in regular droughts. Multiple circulation parameters on inter-annual, inter-decadal, and multi-decadal scales interact synergistically to impact summer droughts in NDPB. However, the synergistic elements that mainly impact various timescales differ. They are primarily influenced by northerly circulation, westerly winds, and the combined effect of the South Asian summer monsoon on inter-annual timescales. The combined impact of the upper westerly winds, northerly circulation, and East Asian, South Asian, and plateau summer monsoons significantly influences their inter-decadal timescales. The westerly winds, northerly circulation, and East Asian, plateau summer monsoons are the primary multi-decadal factors. [ABSTRACT FROM AUTHOR]

The Indian Central Himalaya Region (ICHR), the northern topographic front of the Indian summer monsoon (ISM), is an ideal location to study topography-climate interactions because two weather systems—the ISM and mid-latitude westerlies (MLW)—create distinct eco-climatic regimes from tundra (north) to tropical (south). The region's paleoclimatic studies show considerable climatic variations since the late Pleistocene. We evaluated 29 paleoclimatic records from the region and synthesized the results semi-quantitatively using the weighted palaeoclimate index (WApCI) to better understand the important climatic events and their driving mechanisms. According to the WApCI, the region has at least six enhanced monsoonal periods and eight drier spells during the past 34 ka. The cold-dry climatic events, such as the last glacial maxima (LGM), Younger Dryas (YD), 8.2 ka, and 4.2 ka, are associated with northern-hemisphere (NH) climate dynamics and propagated via MLWs. While, the warmer phases are dictated by the insolation-driven ISM dynamics. The WApCI's reconstructed rainfall anomaly aligns with paleoclimatic-model experiments for dynamically generated Paleoclimate Modeling Intercomparison Projects (PMIP3/PMIP4) rainfall for chosen time-slices (last-millenium, historical, mid-Holocene, and LGM). Finally, the Granger causality test determines the temporal relationship between the climatic drivers/forcing indices and primary meteorological parameters. The results showed that summer and post-monsoon precipitation is primarily influenced by total solar irradiation, winter precipitation is driven by a complex mix of variables, and pre-monsoon precipitation is driven by the Arctic oscillation. Based on the facts, we hypothesize that past climate variability demonstrates a complex interplay of local and hemisphere teleconnections in ICHR's climate dynamics. [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]

The onset of the rainy season in Vietnam displays distinct characteristics compared to other Asian summer monsoon (ASM) regions due to its elongated shape running from south to north, coupled with its complex terrain and the influences of multiple weather systems. Determining the onset is challenging, although numerous empirical indices exist. Here, we proposed an alternative approach to determine the onset by examining the large-scale weather patterns associated with the abrupt increase in rainfall in the country. In this new approach, the evolutions of the ASM system were interpreted as consecutive occurrences of multiple weather patterns, detected by a clustering algorithm Self Organizing Maps (SOMs). Specifically, SOMs were applied on outgoing longwave radiation, 850 hPa wind, and geopotential height over the ASM regions from April to July. The primary features of these atmospheric variables were generalized to 16 patterns. A high-resolution Vietnam Gridded Precipitation (VnGP) dataset was employed to investigate the linkage between these large-scale patterns and local-scale rainfall. The large-scale patterns associated with the onset were distinguished, which are characterized by the development of westerly winds and large-scale convections. Then, the onset of summer rainfall in Vietnam is identified based on the initial appearance of wet-condition weather patterns. This approach offers several advantages: it is objective and does not require additional thresholds of rain or wind to determine the monsoon onset. It also more effectively captures the characteristics of large-scale circulation and deep convection compared to relying on predefined monsoon indices alone. Additionally, this approach helped to elucidate two types of weather patterns associated with the onset: those associated with the northeastward development of deep convection and westerly wind from the Indian Ocean to the Indochina Peninsula and activities of the Boreal Summer Intraseasonal Oscillation. [ABSTRACT FROM AUTHOR]

We evaluate and compare the simulation of the main features (low-level westerlies (LLWs) and the Congo basin (CB) cell) of low-level circulation in Central Equatorial Africa (CEA) with eight climate models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) and the corresponding eight previous models from CMIP5. Results reveal that, although the main characteristics of the two features are reasonably well depicted by the models, they bear some biases. The strength of LLWs is generally overestimated in CMIP5 models. The overestimation is attributed to both divergent and rotational components of the total wind with the rotational component contributing the most in the overestimation. In CMIP6 models, thanks to a better performance in the simulation of both divergent and rotational circulation, LLWs are slightly less strong compared to the CMIP5 models. The improvement in the simulated divergent component is associated with a better representation of the near-surface pressure and/or temperature difference between the Central Africa landmass and the coastal Atlantic Ocean. Regarding the rotational circulation, and especially for HadGEM3-GC31-LL and BCC-CSM2-MR, a simulated higher 850 hPa pressure is associated with less pronounced negative vorticity and a better representation of the rotational circulation. Most CMIP5 models also overestimate the CB cell intensity and width in association with the simulated strength of LLWs. However, in CMIP6 models, the strength of key cell characteristics (intensity and width) are reduced compared to CMIP5 models. This depicts an improvement in the representation of the cell in CMIP6 models and this is associated with the improvement in the simulated LLWs. [ABSTRACT FROM AUTHOR]

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

Enhanced warm pool eastward extension (WPEE) over the western‐central Pacific prior to the onset of El Niño has been observed following Madden‐Julian Oscillation (MJO) precipitation events. However, the mechanism driving the WPEE after the MJO precipitation remains unclear. This study investigates how MJO precipitation affects WPEE by analyzing the zonal momentum budget of two coupled atmosphere‐ocean model simulations with and without freshwater from Kerns and Chen (2021, https://doi.org/10.1016/j.ocemod.2021.101892). It is found that the zonal pressure gradient (ZPG) force created by decreasing salinity and density from MJO precipitation is a key driving force of the WPEE during the onset of El Niño in 2018. The ZPG forces eastward density currents, transporting warmer and fresher waters against persistent easterly winds. Eastward density currents can persist for up to 30 days post‐MJO precipitation. This new mechanism works in concert with two other well‐known MJO‐induced processes in the upper ocean: (a) deepening of thermocline by oceanic Kelvin waves excited by the MJO's westerly winds, and (b) increased SST due to the barrier layer formation. The cumulative effects of these interactive processes during consecutive MJO events reduce the Pacific basin‐scale east‐west temperature gradient, enhancing the WPEE. These processes can contribute to flattening the east‐west thermocline, decreasing in upwelling in the central Pacific, and warming in the eastern equatorial Pacific. Plain Language Summary: It is well known that the Madden‐Julian Oscillation (MJO) and El Niño‐Southern Oscillation (ENSO) affect global high‐impact weather and climate extreme events. In this study, we found that the MJO contributes to the start of El Niño by increasing sea surface temperature in the western Pacific and moving its warm water eastward in favor of developing El Niño. We use a high‐resolution coupled atmosphere‐ocean model representing the MJO precipitation realistically and its effects on the upper ocean. We show that rainfall from the MJO injects a large amount of freshwater into the ocean. It helps maintain surface warm water by forming a barrier layer that prevents the upwelling of cooler water from the deeper ocean. Furthermore, the warmer surface water propagates eastward for weeks after the MJO rainfall. This happens because freshwater increases the zonal density gradient and pressure difference, which forces eastward surface currents to carry warm water farther east than the MJO. Key Points: Madden‐Julian Oscillation (MJO) precipitation and freshwater forcing induce ocean density currents that persist for weeks after the MJO rainfallOcean density currents are mainly driven by zonal pressure gradients due to decreased salinity and increased east‐west density differencesThe long‐lasting eastward density currents contribute to the warm pool eastward extension prior to the onset of El Niño [ABSTRACT FROM AUTHOR]

The spatiotemporal characteristics of the south Sri Lanka coastal current (SSLCC) during summer are examined in this study. Climatologically, the SSLCC flows eastward as a part of the southwest monsoon current during summer. However, westward SSLCC occurred lasting more than 20 days in the summer of 2013, 2016, 2017, and 2018 based on reanalysis data, implying significant interannual variability of the SSLCC. The analysis on the summer extreme westward SSLCC indicates that the intra‐seasonal wind associated with atmospheric boreal summer intra‐seasonal oscillation (BSISO) is the main factor leading to the westward SSLCC. Firstly, the northward propagation of the BSISO induces the westerly wind anomaly and positive wind stress curl anomaly along the south and east coast of Sri Lanka, which induces the westward SSLCC. Secondly, driven by equatorial Indian Ocean intra‐seasonal wind, the low sea level anomaly associated with upwelling Rossby wave reflected from the west coast of Sumatra Island propagates westward. This propagation significantly influences the cyclonic circulation off the south coast of Sri Lanka, thus facilitating the occurrence of the westward SSLCC. Plain Language Summary: Climatologically, the south Sri Lanka coastal current (SSLCC) transports high‐salinity water from the Arabian Sea to the Bay of Bengal as it flows eastward during summer. However, extreme westward SSLCC occurred in the summer of 2013, 2016, 2017, and 2018, lasting roughly 20 days and affecting the salinity balance of the Northern Indian Ocean. Our study aims at investigating the characteristics of the SSLCC and explores the mechanism of the summer westward SSLCC. The findings suggest that the westward SSLCC is primarily regulated by the intra‐seasonal component (30–105 days). The regulation of this intra‐seasonal signal involves a combination of factors, including a positive wind stress curl anomaly along the south and east coast of Sri Lanka and the presence of upwelling Rossby waves along the 5°N section. Key Points: The south Sri Lanka coastal current (SSLCC) exhibits significant intra‐seasonal, semi‐annual, and annual componentsIntra‐seasonal signal associated with boreal summer intra‐seasonal oscillation dominates the emergence of the summer westward SSLCCThe synergistic effect of the cyclonic wind stress curl anomaly on the southern Sri Lanka coast and upwelling Rossby waves reflected from Sumatra Island facilitates the summer westward SSLCC [ABSTRACT FROM AUTHOR]

Coastal ocean temperatures can respond to different atmospheric and oceanic processes at local spatial scales or through remote teleconnections. This study focused on subsurface ocean temperatures (subT) at 10 m depth in Maxwell Bay, northern Antarctic Peninsula from February 2017 and January 2022. It investigated extreme warming events during austral summers and their interaction with atmospheric and oceanic conditions regionally and locally. The analysis identified active and extreme Marine Heat Waves (MHWs) in March 2017 and January‐February 2020 associated with a significantly negative Southern Annular Mode index observed 3–4 months before the temperature increase. In March 2017, temperatures exceeded the climatological mean by over 1°C. This anomaly was linked to a strengthened Amundsen Sea Low and a blocking anticyclone moving between the Scotia Sea and the South‐West Atlantic Ocean that deflected westerly winds and facilitated the anomalous transport of warmer northern air masses to the AP. In January‐February 2020, the highest recorded subT was observed (2.97°C), although air‐sea heat fluxes did not show a similar pattern. In February 2020, one of the most intense atmospheric heatwaves ever recorded in West Antarctica was observed. This heatwave corresponded with maximum subT and positive sea surface temperature anomalies extending throughout the western region of the Southern Ocean, related to an extremely negative Southern Annular Mode. This study provides valuable insights into the impact of strong MHWs, a phenomenon that has been less documented in Antarctic coastal regions. Plain Language Summary: Understanding how ocean temperature varies and responds to different local and remote forcings is crucial for quantifying the potential impact of temperature fluctuations on the coastal Antarctic environment. Between 2017 and 2022 in Maxwell Bay, Antarctic Peninsula, the response of subsurface ocean temperatures (subT, temperature at 10 m depth) to extreme warming events was recorded and analyzed in relation to atmospheric and oceanic conditions. Extreme warming events in the ocean, identified as Marine Heat Waves, occurred in March 2017 and in January and February 2020. In March 2017 subT exceeded the historical averaged temperature by over 1°C. The increase in subT was associated with the deflection of the typically dominant westerly winds to the south, which transported warmer northern air to the Peninsula. In January 2020, the highest recorded subT was observed (2.97°C). Observations suggested this warming was related to the negative Southern Annular Mode in 2019. This study provides valuable insights into the environmental variability and temperature ranges that affect Antarctic marine communities. Key Points: Subsurface ocean temperature (subT, temperature at 10 m depth) in a coastal Antarctic embayment responded to extreme warming eventsSubT recorded Marine Heat Waves in summers 2017 and 2020, identified as years of extreme negative Southern Annular Mode prior to extremely high subTThe main dominant modes of subT variability were related to the Amundsen Sea Low and the Southern Annular Mode [ABSTRACT FROM AUTHOR]