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The Indochina Peninsula, located between China and the South Asian subcontinent, serves as a convergence zone for the South Asian monsoon, the East Asian monsoon, and the South China Sea monsoon. Our study reveals that the interannual variability of spring near-surface temperature in the Indochina Peninsula is influenced by the westward extension of the western North Pacific subtropical high (WNPSH). Specifically, the westward extension of the WNPSH leads to a decrease in the low cloud cover over the Indochina Peninsula, resulting in radiation warming and thus an increase in the near-surface temperature. Furthermore, this westward extension of the latter is modulated synergistically by El Niño and the Aleutian Low. El Niño reduces rainfall near the Philippine Sea by weakening the Walker circulation, which acts as a cooling forcing and triggers an anticyclonic anomaly on its northwest side. This contributes to the westward motion of the WNPSH. In addition, the Aleutian Low promotes this westward shift of the WNPSH through a southwesterly flow at its periphery. Further results indicate that there is an 87.5% probability for the WNPSH to move westward with a synergistic modulation by El Niño and the Aleutian Low, compared to 50% without such modulation. Based on these findings, we have developed a prediction model for predicting potential (westernmost point of the WNPSH) of spring near-surface temperature in the Indochina Peninsula using January Niño3.4 index and Aleutian Low index. The correlation coefficient between hindcasting potential during spring from 2014 to 2023 using this model and observations is 0.67.

The impact of sea surface temperature anomalies (SSTAs) over the North Pacific (32°–45° N, 140° E–150° W) and North Atlantic (north: 52°–68° N, 60°–20° W; south: 0°–30° N, 100°–40° W) on the summer surface air temperature (SAT) over the Mongolian Plateau (MP) is studied using NCEP/NCAR and Climatic Research Unit (CRU) reanalysis data. The results show that the circumglobal teleconnection (CGT) wave train related to the SSTAs in the North Atlantic propagates along with the westerly jet, resulting in a geopotential height anomaly at 200 hPa that favors an SAT anomalies (SATAs) on the MP. In addition, an anticyclonic (cyclonic) circulation anomaly with the characteristics of an atmospheric ultralong wave at 200 hPa over the Eurasia–Pacific region associated with the SSTAs in the North Pacific is also responsible for the positive (negative) SATAs on the MP. The results further reveal that the SSTAs in the North Pacific and the northern North Atlantic act synergistically on the SATAs on the MP. If there are strongly positive (negative) SSTAs in the North Pacific and the northern North Atlantic at the same time, the probability of a strongly positive (negative) SATAs on the MP is 67% (100%) for the research period of this paper. However, if the strongly positive (negative) SSTAs only appear in the North Pacific or the northern North Atlantic, the probability of a strongly positive (negative) SATAs on the MP is less than 34% (31%). The synergism of the SSTAs in the North Pacific and the northern North Atlantic enhances Rossby wave energy in Eurasia, with more Rossby wave energy spreading to the MP that in turn causes the circulation anomaly on the MP. Meanwhile, baroclinic conditions at 850 hPa north of the North Pacific, the northern and southern North Atlantic favor a northward shift of the westerly jet that guides the Rossby wave downstream so that the circulation anomaly that produces the SATAs on the MP is located in the mid–high latitudes.

El Niño‐Southern Oscillation (ENSO) is the dominant atmosphere‒ocean coupling system over equatorial Pacific and has substantial influences on global climate/weather. Here, we find that the west‒east sea surface temperature (SST) gradient over the northeastern Atlantic can influence the remote development of ENSO through the wave train along the subtropical westerly jet in boreal winter. Specifically, a positive SST gradient favors a positive precipitation anomaly over the northeastern Atlantic by inducing a cyclonic anomaly. Diabatic heating induced by anomalous rainfall excites an anticyclonic response in the upper troposphere, which propagates eastward along the subtropical westerly jet and eventually leads to an anticyclonic anomaly over the eastern tropical Pacific. This anticyclonic anomaly tends to cause an anomalous divergence there, which promotes atmospheric convection. As a result, the low‐level westerly winds over the equator strengthen (i.e., 850 hPa), contributing to the development of an El Niño‐like SST anomaly.

The El Niño–Southern Oscillation (ENSO) teleconnections over East Asia display inconsistency between early and late winter, yet the undering physical mechanisms remain unclear. Employing causal inference analysis, including Liang-Kleeman information flow and PC-MCI causal discovery, our study reveals a stable information flow from ENSO to the upper tropospheric atmospheric circulation (250 hPa) over southern East Asia. Building on this, we explore the reason behind the differences in ENSO teleconnections during early and late winter. Findings indicate that Siberian snow water equivalent acts as a ‘land bridge’, storing ENSO signals from early to late winter, which are then released via an ‘atmospheric bridge’ in late winter. Specifically, reduced snow water equivalent from early to late winter facilitates lower surface temperatures in late winter, serving as a cooling source to initiate an ENSO-related wave train over Eurasia. This leads to an inconsistent ENSO teleconnection in late winter, contrasting with early winter, and vice versa.

Tropical atmospheric convection generated by the El Niño–Southern Oscillation (ENSO) plays a crucial role in affecting the western Pacific pattern (WP) in the boreal winter by triggering an atmospheric teleconnection. Here we show from analysis of observations and model simulations that the Atlantic Multidecadal Oscillation (AMO) asymmetrically modulates the relationship between ENSO and WP. We find a significant modulatory effect of AMO on the relationship between wintertime El Niño and WP. A robust El Niño−WP relation can be attributed to the negative AMO phase (−AMO), yet a weak relationship during the positive AMO phase (+AMO). In contrast, the relationship between La Niña and WP is independent of AMO modulation. Furthermore, during the −AMO period, stronger El Niño amplitudes lead to stronger atmospheric convection anomalies over the tropical western North Pacific, which excites stronger atmospheric teleconnection and thus has a more significant effect on WP than during the +AMO period. Plain Language Summary The western Pacific pattern (WP) is one of the most crucial teleconnection patterns in boreal winter over the Northern Hemisphere, which exerts substantial impacts on weather and climate in Eurasia and North America. El Niño–Southern Oscillation (ENSO) is the most prominent air‐sea coupling system in the tropics, considered to exert great impacts on the WP via triggering atmospheric teleconnection. In this study, we revealed that the Atlantic Multidecadal Oscillation (AMO) has a significant asymmetrical modulation on the relation between the ENSO and WP. In particular, a significant El Niño‒WP connection only occurs in the negative AMO phase (‒AMO). Physical mechanisms of the modulation effects of AMO on the ENSO–WP relationship are further analyzed. The results suggest that stronger El Niño amplitude during the −AMO phase leads to larger atmospheric convection anomalies in the equatorial central Pacific and tropical western North Pacific compared to those during the +AMO phase. As such, stronger atmospheric convection anomalies during El Niño events have a greater impact on WP events during the −AMO phase; however, for La Niña events, there is no significant impact on WP events during the −AMO phase. Results obtained in this study may help to improve our understanding of the WP variability. Key Points The connection between El Niño–Southern Oscillation (ENSO) and the western Pacific pattern (WP) varies markedly in different Atlantic Multidecadal Oscillation (AMO) phases A robust connection between ENSO and the WP in the boreal winter can only be observed during the negative AMO phase The AMO influences the ENSO‐WP relationship via modulating the ENSO amplitude and the associated change in atmospheric convection

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

The Siberian High (SH) is one of the most prominent wintertime circulation systems in the Northern Hemisphere. The variability of the intensity of the SH during the boreal winter has been extensively studied, while the reasons behind its formation remain largely elusive. This study investigates the interannual variation and underlying mechanisms of the SH formation, especially the three-dimensional circulation and associated surface boundary conditions. Early SH formation is usually accompanied by a Eurasian teleconnection (EU)-like pattern and a strengthened subtropical westerly jet, and opposite conditions are true for a delayed SH formation. Boundary conditions, including North Atlantic sea surface temperature (NASST) anomalies and the high-latitude Central Eurasia snow-cover extent (HCESCE) anomalies could modulate SH formation by affecting the overlying atmospheric circulations. Specifically, anomalously cold NASSTs can result in an anomalous equivalent barotropic anticyclone via diabatic heating and transient eddy forcing. Such an anomalous anticyclone and related divergence anomalies in the upper troposphere act as effective atmospheric Rossby sources, triggering a downstream propagating atmospheric teleconnection with a pattern similar to that of a positive EU-like wave train, favoring the early formation of the SH. In addition, the reduced HCESCE warms the overlying atmosphere, thus weakening the atmospheric baroclinicity and transient eddy activities over the SH region. These weakened eddy activities can intensify the EU-like pattern via vorticity forcing and strengthen the subtropical westerly jet via the eddy–mean flow interaction. The processes for the impacts of the NASST and HCESCE anomalies on the formation of the SH can be verified by a series of numerical simulations.

The Asia–Pacific region suffered record rainfall in summer 2020, which was accompanied by the strongest Asian subtropical westerly jet (ASWJ) of the past four decades. Meanwhile, the COVID-19 pandemic spread rapidly around the world, resulting in an abrupt reduction in emissions in East Asia. Here, we investigate whether the enhanced ASWJ induced by plummeting aerosols contributed to the record-breaking rainfall. The results show that tropospheric warming in Southeast Asia, in particular southern China, due to local aerosol reduction, acted to increase the meridional temperature gradients in the mid–lower troposphere, which supported a strong ASWJ in the upper troposphere via the thermal wind balance. The latter enhanced divergence in the upper troposphere over the Asia–Pacific region, which provided a favorable ascending motion for the record rainfall that took place there. Therefore, against a background of carbon neutrality (i.e. the reduction in aerosols), our results imply more strong summer rainfall in the Asia–Pacific region.

The East Asian winter monsoon (EAWM) is the most important climate system for transporting Arctic cold air to the tropics in boreal winter. Rapid intensification of the EAWM, such as a cold surge, can lead to increased tropical convection over the western Pacific, but the possible effects from the intraseasonal variation of EAWM is unclear. Using high temporal and spatial resolution satellite data, including Outgoing Longwave Radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) precipitation, we show that strong intraseasonal EAWM events are associated with increased tropical convection over the western Pacific for about 6–8 days. Our statistical analysis shows that the lifetime of a strong intraseasonal EAWM event is about 2 weeks, with the beginning, peak, and ending phases occurring at days −6, 0, and 6, respectively. During days 0 to 8, increased convection is observed over the western tropical Pacific, due to the anomalous convergence associated with the strengthened northerly winds over the South China Sea. Over land, increased precipitation is observed over Vietnam, northwestern Kalimantan, and the southern Philippines. In addition, the East Asian local Hadley circulation is strengthened during these days, in association with the enhanced tropical convection.

The impacts of Arctic sea ice on the interannual variability of winter extreme low temperature (WELT) in Northeast China (NEC) and the associated atmospheric circulation patterns are explored in this study based on meteorological observation and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. Results show that WELT in NEC has prominent interannual variability. We further use ±0.8 standard deviation as the threshold to select the years of frequent and rare extreme low temperature anomalies. Using composite analysis, we find that there are significant negative geopotential height anomalies at 500 hPa over NEC and positive geopotential height anomalies along the Arctic region, which represent the intensification of the East Asian trough (EAT) and the negative Arctic Oscillation (AO) phase in the years of more frequent WELT. The opposite characteristics are detected in the years of rare WELT. Furthermore, we determine that the Barents-Kara Seas are key sea ice regions in Arctic area. In the years of frequent WELT, the decrease of autumn Barents-Kara Seas sea ice and the positive sea surface temperature anomaly can last until the following winter, which is conducive to the intensification of anticyclonic anomalies in Ural regions and the northward extension of Ural ridge (UR). The northerly flow in front of UR guides the cold air penetrating southward from polar regions. Moreover, the anomalous cyclone over East Asia deepens the EAT. The northerly wind behind EAT guides the cold air to the NEC region, causing the wintertime low temperature there. The almost opposite situation occurs in the years of rare WELT.