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The leading pattern of boreal spring 250-hPa meridional wind anomalies over the North Atlantic and mid-high latitude Eurasia displays an obvious wave train. The present study documents the structure, energy source, relation to the North Atlantic sea surface temperature (SST), and impacts on Eurasian climate of this wave train during 1948–2018. This atmospheric wave train has a barotropic vertical structure with five major centers of action lying over subtropics and mid-latitudes of the North Atlantic, northern Europe, central Eurasia, and East Asia, respectively. This spring wave train can efficiently extract available potential energy from the basic mean flow. The baroclinic energy conversion process and positive interaction between synoptic-scale eddies and the mean flow both play important roles in generating and maintaining this wave train. The North Atlantic horseshoe-like (NAH) SST anomaly contributes to the persistence of the wave train via a positive air–sea interaction. Specifically, the NAH SST anomaly induces a Rossby wave-type atmospheric response, which in turn maintains the NAH SST anomaly pattern via modulating surface heat fluxes. This spring atmospheric wave train has significant impacts on Eurasian surface air temperature (SAT) and rainfall. During the positive phase of the wave train, pronounced SAT warming appears over central Eurasia and cooling occurs over west Europe and eastern Eurasia. In addition, above-normal rainfall appears over most parts of Europe and around the Lake Baikal, accompanied by below-normal rainfall to east of the Caspian Sea and over central Asia.

Analysis of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) data for the period 1998-2007 reveals large subseasonal fluctuations in sea surface temperature (SST) of the South China Sea during the summer monsoon onset. These subseasonal SST changes are closely related to surface heat flux anomalies induced by surface wind and cloud changes in association with the summer monsoon onset. The SST changes feed back on the atmosphere by modifying the atmospheric instability. The results suggest that the South China Sea summer monsoon onset involves ocean-atmosphere coupling on subseasonal timescales. While the SST response to surface heat flux changes is quick and dramatic, the time lag between the SST anomalies and the atmospheric convection response varies largely from year to year. The spatial-temporal evolution of subseasonal anomalies indicates that the subseasonal variability affecting the South China Sea summer monsoon onset starts over the equatorial western Pacific, propagates northward to the Philippine Sea, and then moves westward to the South China Sea. The propagation of these subseasonal anomalies is related to the ocean-atmosphere interaction, involving the wind-evaporation and cloud-radiation effects on SST as well as SST impacts on lower-level convergence over the equatorial western Pacific and atmospheric instability over the Philippine Sea and the South China Sea.

This study investigates the spatial scale dependence of relationship between turbulent surface heat flux (SHF) and sea surface temperature (SST) variations in the mid-latitude frontal zones, subtropical gyres, and tropical Indo-western Pacific region in winter and summer with daily observational data. A comparison of the SHF and SST/SST tendency correlation between 1° and 4° spatial scale displays a decrease of the positive SHF–SST correlation and an increase of the negative SHF–SST tendency correlation as the spatial scale increases in all the above regions. The lead–lag SHF and SST/SST tendency correlation at different spatial scales illustrates an obvious transition from the oceanic forcing to the atmospheric forcing in the western boundary currents (WBCs) and the Agulhas Return Current (ARC) in both winter and summer. The transition length scale is smaller in summer than in winter, around 2.6°–4.5° in winter and around 0.8°–1.3° in summer based on the OAFlux data. In the subtropical gyres and tropical Indo-western Pacific region, atmospheric forcing dominates up to 10° spatial scale with the magnitude of forcing increasing with the spatial scale in both winter and summer except for the Arabian Sea in summer. The Arabian Sea distinguishes from the other tropical regions in that the SST forcing dominates up to more than 10° spatial scale in summer with the magnitude of forcing decreasing slowly with the spatial scale increase.

A strong cold event hit eastern China around 24 January 2016 with surface air temperature reaching more than 10°C below the climatological mean in most regions of eastern China south of 40°N. A total of 37 strong cold events similar to the January 2016 event with temperature anomalies over eastern China exceeding −5°C have been identified during the winters from 1979/80 to 2015/16. A comparative analysis of events with surface temperature anomalies of the same intensity but limited to north of 40°N indicates that the southward invasion of cold air to eastern China south of 40°N is related to two factors. One is the latitudinal location of the upper-level wave train, the surface Siberian high, and the midtropospheric East Asian trough over the mid- to high-latitude Eurasian continent. The other is a subtropical upper-level wave train emanating from the midlatitude North Atlantic. The emergence of the subtropical wave train is related to the positive phase of the North Atlantic Oscillation (NAO). When the mid- to high-latitude wave train is located too far northward and the subtropical wave train induces an anomalous midtropospheric high over southern China, the East Asian trough does not extend southwestward and the Siberian high does not expand southeastward. In such a case, the cold air mainly affects northeastern China and northern Japan.

Surface air temperature (SAT) anomalies tend to persist from winter to the following spring over the mid- to high latitudes of Eurasia. The present study compares two distinct cases of Eurasian SAT anomaly evolution and investigates the reasons for the persistence of continental-scale mid- to high-latitude Eurasian SAT anomalies from winter to following spring (termed persistent cases). The persisting SAT anomalies are closely associated with the sustenance of large-scale atmospheric circulation anomaly pattern over the North Atlantic and Eurasia, featuring a combination of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and the Scandinavian pattern, from winter to spring. The combined circulation anomalies result in SAT warming over most of mid- to high-latitude Eurasia via anomalous wind-induced temperature advection. The sustenance of atmospheric circulation anomaly pattern is related to the maintenance of the North Atlantic triple sea surface temperature (SST) anomaly pattern due to air–sea interaction processes. The Barents Sea ice anomalies, which form in winter and increase in spring, also partly contribute to the sustenance of atmospheric circulation anomalies via modulating thermal state of the lower troposphere. In the cases that notable SAT warming (cooling) inwinter is replaced by pronounced SAT cooling (warming) in the subsequent spring—termed reverse cases—the North Atlantic SST anomalies become small and the Greenland Sea ice change is a response to atmospheric change in spring. Without the support of lower boundary forcing, the atmospheric circulation anomaly pattern experiences a reverse in the spatial distribution from winter to spring likely due to internal atmospheric processes.

Previous studies have indicated the modulation of the Madden–Julian oscillation (MJO) by the Quasi-Biennial Oscillation (QBO) and the influence of the MJO on surface temperature over Eurasia during boreal winter. The present study reveals that the MJO-related circulation anomalies are different in easterly and westerly QBO years, leading to distinct surface temperature anomaly patterns over Eurasia. During the easterly QBO years, the surface air temperature anomalies over Eurasia display a meridional dipole pattern in MJO phase 2 associated with the mid-latitude surface anticyclonic anomalies. The development of surface anomalous anticyclone is attributed to a combined effect of negative North Atlantic Oscillation (NAO)-related mid-latitude wave train and stronger MJO convection triggered poleward propagation of Rossby wave train. The negative NAO is related to the easterly QBO through the Holton–Tan relationship. The anomalous overturning circulation excited by the stronger MJO convection in easterly QBO years also contributes to the development and eastward extension of anomalous anticyclone. The anticyclonic anomalies induce the meridional temperature anomaly pattern by horizontal advection. During the westerly QBO years, the surface air temperature anomalies over Eurasia show a zonal alternating pattern in MJO phase 3, which corresponds to the development of mid-latitude Rossby wave train associated with positive NAO with a stronger MJO–NAO connection in westerly QBO years. The MJO convection induces upper-level divergent wind anomalies, contributing partially to the development of the Rossby wave source and helping the building of the mid-latitude wave train. The zonal temperature anomalies over Eurasia are also contributed by the horizontal advection associated with surface cyclonic anomalies.

Previous studies indicated that boreal spring North Atlantic horseshoe-like (NAH) sea surface temperature (SST) anomaly pattern has a significant impact on the surface air temperature (SAT) variation over the mid-high latitudes of Eurasia via an atmospheric wave train. This study investigates the connection of springtime NAH SST anomaly and the Eurasian SAT variation in thirty-two coupled climate models that participated in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Most of the CMIP5 models simulate well the spatial pattern of SST anomalies in the North Atlantic related to the spring NAH SST, but overestimate the spatial standard deviation. There exists large spreads in the connection of the spring NAH SST anomaly pattern and the Eurasian SAT variation among the CMIP5 models. The models that capture the observed spring NAH SST-Eurasian SAT connection can well reproduce the observed atmospheric wave train, in particular, with a marked anticyclonic anomaly over north Europe and a cyclonic anomaly over central Eurasia. This wave train results in above-normal SAT over west Europe and eastern Eurasia and below-normal SAT over central Eurasia. By contrast, the models that fail to reproduce the spring NAH SST-related Eurasian SAT anomalies show limitations in simulating the atmospheric wave train. Further analysis suggests that the models with larger climatological westerly winds over the mid-latitude North Atlantic and high-latitude Eurasia as well as lower climatological SST in the tropical northern Atlantic have a better ability in simulating the observed atmospheric wave train over the North Atlantic and Eurasia and thus perform better in simulating the spring NAH SST-Eurasian SAT connection.

A region of low sea surface temperature (SST) extends southward in the central part of southern South China Sea during boreal winter, which is called the South China Sea cold tongue (SCS CT). The present study investigates the factors of interannual variation of SST in the SCS CT region and explores the individual and combined impacts of El Niño-Southern Oscillation (ENSO) and East Asian winter monsoon (EAWM) on the SCS CT intensity. During years with ENSO alone or with co-existing ENSO and anomalous EAWM, shortwave radiation and ocean horizontal advection play major roles in the interannual variation of the SCS CT intensity. Ocean advection contributes largely to the SST change in the region southeast of Vietnam. In strong CT years with anomalous EAWM alone, surface wind-related latent heat flux has a major role and shortwave radiation is secondary to the EAWM-induced change of the SCS CT intensity, whereas the role of ocean horizontal advection is relatively small. The above differences in the roles of ocean advection and latent heat flux are associated with the distribution of low level wind anomalies. In anomalous CT years with ENSO, low level anomalous cyclone/anticyclone-related wind speed change leads to latent heat flux anomalies with effects opposite to shortwave radiation. In strong CT years with anomalous EAWM alone, surface wind-related latent heat flux anomalies are large as anomalous winds are aligned with climatological winds.

The present study examined the relationship between turbulent surface heat flux (SHF) and sea surface temperature (SST) variations using daily observational data. The SHF and SST relationship displays notable differences between winter and summer and prominent time-scale dependence in both seasons. In the mid-latitude SST frontal regions, SST has a larger role in driving SHF in winter than in summer. In the subtropical gyre regions, SHF plays a larger role in the SST change in summer than in winter. In winter, SHF has a larger effect on the SST change in the South China Sea than in the Arabian Sea and Bay of Bengal. In summer, the SST effect on SHF is dominant in the Arabian Sea, whereas the SHF impact on SST is dominant in the Philippine Sea. In the Gulf Stream, Kuroshio Extension and Agulhas Return Current, the SST effect extends up to 90-day time scales in winter, the SHF impact is limited to time scales below 20 days and the SST effect is dominant on time scales above 20 days in summer. In winter, the SHF effect extends up to 90-day time scales in the Bay of Bengal, South China Sea, and Philippine Sea, but is limited to time scales below 40 days in the Arabian Sea. In summer, the SST effect extends up to 90-day time scales in the Arabian Sea, whereas the SHF and SST effect is large on time scales shorter and longer than 40 days, respectively, in the Philippine Sea.

Using observational data from 1979–2015 and a linear baroclinic model (LBM), the present study examined the impact of autumn snow cover (ASC) over the western Tibetan Plateau (WTP) on the subsequent spring precipitation over southern China (SPSC). Focus was placed on the interannual time scale of the snow-precipitation relationship. The ASC over the WTP is positively correlated with the SPSC, and this snow-precipitation relationship is independent of sea surface temperature anomalies over the tropical Indo-Pacific regions. When the ASC over the WTP is more extensive than normal, in the following spring, anomalous southerly winds over eastern China occur and transport water vapor northward from the tropical ocean to the interior of the continent, which is favorable for more SPSC. Further analysis of the energy budget shows that the anomalous ASC over the WTP persists to the following spring because of local positive snow-air feedback. Higher-than-normal ASC over the WTP can cool the above atmosphere and is associated with pronounced negative geopotential height anomalies that dominate in the East Asian-western North Pacific region. These negative height anomalies lead to a weakened and northward-shifted spring East Asian subtropical jet (EASJ). The changes in the spring EASJ lead to divergent anomalies at the upper level over the upstream as well as the central areas of the EASJ; therefore, anomalous low-level convergence and ascent motion dominate over coastal East Asia and in nearby regions, providing a favorable environment for positive SPSC anomalies.