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The spring snow cover (SC) over the western Tibetan Plateau (TP) (TPSC) (W_TPSC) and eastern TPSC (E_TPSC) have displayed remarkable decreasing and increasing trends, respectively, during 1985–2020. The current work investigates the possible mechanisms accounting for these distinct TPSC changes. Our results indicate that the decrease in W_TPSC is primarily attributed to rising temperatures, while the increase in E_TPSC is closely linked to enhanced precipitation. Local circulation analysis shows that the essential system responsible for the TPSC changes is a significant anticyclonic system centered over the northwestern TP. The anomalous descending motion and adiabatic heating linked to this anticyclone leads to warmer temperatures and consequent snowmelt over the western TP. Conversely, anomalous easterly winds along the southern flank of this anticyclone serve to transport additional moisture from the North Pacific, leading to an increase in snowfall over the eastern TP. Further analysis reveals that the anomalous anticyclone is associated with an atmospheric wave pattern that originates from upstream regions. Springtime warming of the subtropical North Atlantic (NA) sea surface temperature (SST) induces an atmospheric pattern resembling a wave train that travels eastward across the Eurasian continent before reaching the TP. Furthermore, the decline in winter sea ice (SIC) over the Barents Sea exerts a persistent warming influence on the atmosphere, inducing an anomalous atmospheric circulation that propagates southeastward and strengthens the northwest TP anticyclone in spring. Additionally, an enhancement of subtropical stationary waves has resulted in significant increases in easterly moisture fluxes over the coastal areas of East Asia, which further promotes more snowfall over eastern TP. [ABSTRACT FROM AUTHOR]

The Multivariate compound heatwaves (MCHWs) present a significant risk to human health and ecological diversity, which attract widespread attention from researchers and society. The intensity and variability of summer MCHWs over Western Europe (WE) (MCHWs_WE) have substantially increased over the last two decades. The growing chance of such events is likely related to human activity‐induced climate change. However, the possible contributions from multiple atmospheric boundary forcing remain unclear. This study investigates the combined effects of North Atlantic sea surface temperature (SST), Tibetan Plateau (TP) snow cover (SC), and Arctic sea ice (SIC) on the variability of extreme summer MCHWs_WE. Observational analysis shows that an intensified anticyclonic system prevailing over WE, one of the centers of an atmospheric wave train dominating the North Atlantic‐Europe sector and persists from late spring to summer, is the essential system contributing to the extreme MCHWs_WE. Spring North Atlantic SST anomalies in the form of a dipole pattern persist from spring to summer and contribute to the summer extreme MCHWs_WE by exciting a downward propagating Rossby wave train pattern. Additionally, excessive late spring SC over the western TP and SIC over the Arctic, which are stimulated by the North Atlantic anomalous SST‐related wave train, can intensify the MCHWs_WE‐related anticyclonic system through vertical circulation and wave energy transport. The study further quantifies the relative contributions of the aforementioned factors. The findings of this study could potentially offer valuable insights for improving the prediction skill of summer extreme MCHWs_WE. Plain Language Summary: Western Europe (WE) has faced more and more multivariate compound heatwaves (MCHWs) during summer in recent decades, which pose a greater human health risk than univariate heatwaves. In this study, Our study identifies MCHWs in WE as being characterized by high temperatures and humidity, mainly influenced by a strong anticyclonic high pressure system. Furthermore, the strengthening of the MCHWs_WE‐related anticyclone is a consequence of the combined impacts of preceding North Atlantic sea surface temperature (SST), Tibetan Plateau (TP) snow cover (SC), and Arctic sea ice (SIC). Spring positive dipole pattern of North Atlantic SST anomalies, exhibiting persistence from spring to summer, excite downward propagating Rossby waves. There wave trains from the Atlantic to Eurasia serve as a background field that enhances the linkage between SIC, TPSC, and the MCHWs_WE‐related anticyclonic system. These factors ultimately contribute to the strengthening of anticyclones over West Eurasia (WE) through vertical circulation and wave energy transport, which jointly impact MCHWs_WE. Key Points: Multivariate Compound heatwaves in Western Europe (MCHWs_WE) become more frequent in recent decades, which are linked to intensified anticyclonic systemThe development of the anticyclone over WE is associated with the dipole pattern of North Atlantic sea surface temperature, excess Tibetan Plateau snow cover and Arctic sea iceCombined effects of multiple forcing factors can account for approximately 40% of the variability in summer MCHWs_WE [ABSTRACT FROM AUTHOR]

Present study investigates the characteristics and factors of summer drought variations over Asia during 1950–2020 using 3-month Standardized Precipitation Evapotranspiration Index (SPEI03) in July. The leading mode of Asian summer drought variations exhibits a roughly north–south contrasting distribution across 30° N with dominant interannual variability. The interannual variation of the leading mode is associated with both precipitation and temperature. In comparison, the contribution of precipitation is larger in the eastern part of the mid-latitude Asia. The Asian summer drought variations subject to both individual and combined impacts of preceding winter equatorial central-eastern Pacific (ECEP) sea surface temperature (SST) anomalies and concurrent North Atlantic (NA) tripole SST anomalies. Preceding ECEP SST anomalies induce anomalous vertical motion and precipitation over the mid-latitude Asia through large-scale divergence and convergence in previous winter and spring. The resulted soil moisture anomalies persist to summer and are conducive to anomalous summer precipitation through evaporation. The accompanying changes in cloud-radiation, surface longwave radiation and sensible heat flux induce temperature and evapotranspiration anomalies. The NA tripole SST anomalies induce anomalous vorticity forcing over the western mid-latitude NA. This contributes to a wave train that propagates from the NA to East Asia and affects wind and vertical motion over the mid-latitude Asia. [ABSTRACT FROM AUTHOR]

The year‐to‐year fluctuation and total amount of spring precipitation over the Tarim Basin (TB) (SPTB) have both significantly increased after 1985. The data are then divided into two subperiods, that is, 1961–1985 (P1) and 1986–2020 (P2), and the contributions from various climate factors, including the North Atlantic and Indian Ocean sea surface temperatures (SSTs), as well as from Tibetan Plateau (TP) snow, to the interdecadal changes from P1 to P2 are examined. In P1, the SPTB‐related atmospheric circulation anomalies display a zonal wave‐like pattern dominating the middle–high North Atlantic‐Eurasian continent. In contrast, in P2, the SPTB‐related atmospheric circulation anomalies feature a meridional tripole pattern prevailing over the North Atlantic and western Europe. The observational analysis indicates that abnormally low pressure in the north and high pressure in the southwest of the TB are the key atmospheric systems that influence moisture transportation to the Tarim Basin and consequently impact the variability of the SPTB. The anomalous SSTs in the North Atlantic and Indian Ocean play a significant role in inducing changes in the SPTB by regulating the moisture transport pathway in the vicinity of the TB. In P1, the SSTs in both the North Atlantic and the tropical Indian Ocean contribute to the SPTB mainly in the periphery of the TB, while in P2, they contribute to the SPTB in most of the TB. In addition, the snow cover over TP also contributes to the SPTB changes. Distinguished from the effects of oceanic SST, the TP snow primarily influences the variability of the SPTB within the central region of the TP during P1 and around the periphery of the TP during P2. [ABSTRACT FROM AUTHOR]

A prominent change occurred in the relationship between August Indochina Peninsula (ICP) precipitation and tropical Pacific sea surface temperature (SST) around 1980. More August ICP precipitation falls in La Niña developing years during 1959–1979, but in El Niño developing years during 1983–2003. The relation change is associated with a change in the evolution of equatorial Pacific SST anomalies. During 1959–1979, negative SST anomalies develop in the equatorial eastern Pacific (EEP) in early spring and extend westward. They cause anomalous southwesterly and reduce surface latent heat flux (LHF) over the tropical western North Pacific (WNP), inducing positive SST anomalies there in early summer. Negative SST anomalies form in the tropical South Indian Ocean (SIO) due to LHF increase attributed to a local air-sea interaction process in preceding winter and a remote forcing of negative EEP SST anomalies in late spring-early summer. More August ICP precipitation is due to anomalous ascent induced by a Rossby wave response to positive WNP SST anomalies and a cross-equatorial vertical circulation due to negative SIO SST anomalies. During 1983–2003, positive SST anomalies form in the equatorial central Pacific (ECP) in late spring and extend eastward. Negative SST anomalies develop in the tropical southeastern Indian Ocean (SEIO) due to sustained LHF increase induced by both preceding winter negative SST anomalies and late spring-early summer positive SST anomalies in the EEP. More August ICP precipitation is induced by a Rossby wave response to positive ECP SST anomalies and a cross-equatorial vertical circulation due to negative SEIO SST anomalies. [ABSTRACT FROM AUTHOR]

In this study, the interannual variations of winter precipitation over central Asia (CA) are investigated over the period 1948–2018 using both observational analysis and a linear baroclinic model (LBM). The focus is on the characteristics and factors of the leading empirical orthogonal function mode (EOF1) of winter precipitation over CA. The results show that the key circulation anomalies associated with the positive phase of EOF1 feature a tripole pattern that is responsible for transporting moisture from the subtropical North Atlantic region to CA. An examination of the lower boundary conditions indicates that anomalous North Atlantic sea surface temperature (SST), Kara Sea ice in the preceding autumn, and simultaneous snow cover in central Asia can promote large-scale atmospheric waves that contribute to the EOF1-related anomalous tripole pattern. This result is verified by an analysis of the apparent heat source (Q1) in the atmosphere and the LBM experiments. Linear regression (LR) models were constructed using the precursors revealed by the above observational analyses to perform hindcasts for EOF1 over the period 1950–2018. The North Atlantic SST and the Kara Sea ice in the preceding autumn are shown to be effective predictors in the LR model that can capture the variation in EOF1 during this period. The seasonal forecasts of winter precipitation over CA for the 2011–18 period based on the LR models outperform those of the Climate Forecast System version 2 (CFSv2) over central and northern CA. [ABSTRACT FROM AUTHOR]

This study evaluates the relationship between El Niño‐Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) in 26 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. Of particular concern is the plausible factors for the model's capability of simulating the ENSO‐EAWM relationship. Results show that the model's ability of simulating the ENSO‐EAWM relationship is more dependent upon the longitudinal extension of ENSO‐related equatorial Pacific sea surface temperature (SST) anomalies than the amplitude of the equatorial central‐eastern Pacific SST anomalies. The influence of the amplitude of ENSO on the simulation of the ENSO‐EAWM relationship depends on the westward extension of ENSO‐related equatorial Pacific SST anomalies. Another factor for the model's ability of simulating the ENSO‐EAWM relationship is the SST anomalies in the tropical western North Pacific (WNP). A westward extension of the equatorial Pacific SST anomalies shifts the west branch of anomalous Walker circulation too far westward, which causes westward displaced anomalous ascending (descending) motion around the Philippines through modulating regional meridional vertical circulation in El Niño (La Niña) years. The weak SST anomalies in the tropical WNP lead to the failure of inducing anomalous lower‐level anticyclone (cyclone) over the Philippine Sea through a Rossby wave response in El Niño (La Niña) years. The accompanying weak anomalous lower‐level southwesterly (northeasterly) winds along the west flank of the anomalous anticyclone (cyclone) account for the weak ENSO‐EAWM relationship. Key Points: The El Niño‐Southern Oscillation‐East Asian winter monsoon (ENSO‐EAWM) relationship varies largely among the CMIP6 modelsThe longitudinal location of equatorial Pacific sea surface temperature (SST) anomalies is more critical than the amplitude to simulating the ENSO‐EAWM relationshipThe tropical western North Pacific SST anomalies are another important factor in the simulation of the ENSO‐EAWM relationship [ABSTRACT FROM AUTHOR]

The present study explores the changed relationship between the interannual variations in spring (April–May) precipitation over southern China (SPSC) and sea surface temperature (SST) anomalies in the tropical Pacific and south Indian Oceans during the 1960–2017 period. Observational analysis shows that the relation between SPSC and El Niño–Southern Oscillation (ENSO) was significant before the mid-1980s (P1) and after the early 2000s (P3) but insignificant in between, from the mid-1980s to the early 2000s (P2). In P2, positive anomalous SPSC was significantly correlated with negative anomalous SST in the south Indian Ocean. During this period, an anomalous anticyclone and intensified southwesterly winds tended to appear over tropical India accompanied by a negative anomalous south Indian Ocean SST, which caused anomalous low-level convergence over the western Pacific. As a result, the western Pacific subtropical high (WPSH) tended to weaken and retreat eastward. This resulted in anomalous moisture convergence in southern China, favoring enhanced SPSC. Further analysis shows that the negative south Indian Ocean SST anomalies tended to induce anomalous cross-equatorial vertical circulation where the south Indian Ocean and southern China are controlled by descending and ascending airflow. The ascending motion may also contribute to positive anomalous SPSC. The observed contribution of the south Indian Ocean SST anomalies to the SPSC variation is confirmed by numerical experiments using an atmospheric model. The intensified variance of SST in the south Indian Ocean and the eastward shift of the ENSO-related circulation anomalies over the western tropical Pacific may partly account for the changes in the SST–SPSC relationship. [ABSTRACT FROM AUTHOR]

The autumn precipitation over the Sichuan Basin (APSB) experienced a significant wet-to-dry shift around the mid-1980s during the 1961–2010 period. Thus, the dataset is divided into two subperiods, 1961–1985 (P1) and 1986–2010 (P2), which are denoted as the wet period and dry period, respectively. The possible mechanisms accounting for the wet-to-dry shift in the APSB are investigated using both observational data and a baroclinic atmospheric model. Analysis shows that the sea surface temperature (SST) in the tropical Indian-western Pacific (TIWP) regions displays continuous warming during the period under examination, which may intensify the ascent motion over the warm pool and favor the descent motion over the Sichuan Basin. Meanwhile, the Gill-type atmospheric response to anomalous TIWP warm SST is accompanied by anomalous northerly winds prevailing over central and Southwest China. Both the intensified descent motion and anomalous northerly winds over central and Southwest China favor the switch to dry conditions in the APSB. Further analysis shows that the decadal changes in the North Atlantic SST lead to an atmospheric wave pattern prevailing over the extratropical Eurasian continent. An anomalous cyclone over central-eastern Eurasia, which is one component of the wave pattern, is accompanied by significant northerly winds penetrating southward to central China. These northerly winds can cause more-than-normal snow cover around Lake Baikal, which may in turn modulate the atmospheric circulation through positive feedback and therefore result in intensified northerly winds, favorable for the switch to the anomalous dry state of the APSB. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

By decomposing the variations of daily mean variables into synoptic, intraseasonal, and low‐frequency components, the present study compares seasonal mean latent heat flux (LHF) anomalies related to the three components over the tropical Indo‐Pacific region during El Niño and La Niña years. It is found that the high‐frequency (intraseasonal and synoptic) LHF has an important contribution to total seasonal mean LHF anomalies over the tropical eastern Indo‐western Pacific region. Large seasonal mean LHF anomalies related to high‐frequency wind variations are identified in weak low‐frequency seasonal mean wind regions where high‐frequency LHF may accumulate over a season as it increases in both westerly and easterly phases of high‐frequency wind variations. In those regions, the LHF anomalies related to high‐frequency wind variations cancel part of the low‐frequency LHF anomalies, reducing total seasonal mean LHF anomalies. As El Niño–Southern Oscillation (ENSO)‐induced lower‐level wind anomaly region moves with the stage of ENSO, large seasonal mean LHF anomalies related to high‐frequency wind variations displace with the season. The intensity of high‐frequency wind fluctuations is also a factor influencing the seasonal mean LHF related to high‐frequency wind variations. Thus, the ENSO affects the high‐frequency LHF variations through modulating both the location of weak low‐frequency wind region and intensity of high‐frequency wind fluctuations. Key Points: High‐frequency wind‐related latent heat flux (LHF) has an important contribution to seasonal mean LHF anomalies in ENSO yearsThe region of high‐frequency wind‐related LHF contribution moves with the stage of ENSOThe high‐frequency LHF contribution is associated with weak seasonal mean winds and enhanced high‐frequency wind fluctuations [ABSTRACT FROM AUTHOR]

In this study, the leading empirical orthogonal function mode of the interannual variations of boreal spring (April–May) precipitation over China (SPC) is shown to shift from a mono‐sign distribution during 1964–1985 (P1), with the largest loading over southern China, to a north‐south dipole structure during 1986–2004 (P2), with the northern and southern poles centered over the Yangtze River and the Pearl River Delta, respectively. The results show that the El Niño‐Southern Oscillation(ENSO)‐related negative western tropical Pacific sea surface temperature anomalies persist longer from the previous winter into the following spring in P2 than in P1. The ENSO‐related western tropical Pacific anticyclone in spring is stronger and extends farther northeastward after the mid‐1980s. As a result, in P2, the moisture transport by anomalous southerly winds along the western flank of the western tropical Pacific anticyclone penetrates northward to central China, and moisture convergence and divergence are observed over the Yangtze River and southern China, respectively. The SPC‐related North Atlantic sea surface temperature anomalies (NAST) are stronger after the mid‐1980s than before the mid‐1980s, and the NAST‐related atmospheric wave train over the middle‐latitude Eurasian continent is more pronounced in P2 than in P1. Furthermore, to the east of 90°E, the downstream propagation pathway of the NAST‐related wave train turns southeastward, which may be caused by the weakened intensity of the meridional gradient of the potential vorticity over this region. The pronounced wave train‐related positive height anomalies over the western North Pacific favor a north‐south dipole structure in the SPC variation in P2. Key Points: The leading EOF mode of spring precipitation over China (SPC) changes from a mono‐sign pattern to a north‐south dipole mode in the mid‐1980sThe ENSO‐related western tropical Pacific SST anomalies (SSTAs) contribute to the interdecadal change of the SPCBoth the North Atlantic SSTAs and the mean flow over East Asia favor a north‐south dipole pattern of SPC after the mid‐1980s [ABSTRACT FROM AUTHOR]

This study employs two machine learning (ML) models, namely the light gradient boosting machine (LightGBM) and the extreme gradient boosting (XGBoost) model, to perform seasonal forecasts for winter precipitation over China. The seasonal forecast results derived from a dynamic model are used for the purpose of comparison. The predictors employed in the two ML models consist of climate variables from the preceding autumn. The ML models are trained using data during 1979 to 2010 without additional predictor selection process. Consequently, the ML models autonomously extract useful information and then apply it to perform seasonal forecasts of winter precipitation for 2011–2020. Results indicate that the two ML models exhibit reasonable forecast skill and demonstrate superior performance over various regions when compared to the dynamic model. Additionally, the ensemble forecasts of the two ML models and the dynamic model exhibit higher skill in China compared to the dynamic model forecast alone. The forecast skill of the ML models can be attributed to their skillful forecast of the first empirical orthogonal function mode (EOF1) of the winter precipitation. In addition, the split time count method and the Shapley additive explanation (SHAP) values analysis are performed to further understand and explain the ML models forecast results. Results show that the sea surface temperature (SST), especially the SST over the western Pacific plays an important role in the ML model seasonal forecasts of winter precipitation over China, consistent with previous studies. The outcomes of this investigation provide valuable insights that can enhance the practical implementation and understanding of ML models in the field of seasonal forecasting. This study utilizes two machine learning (ML) models, LightGBM and XGBoost, to predict winter precipitation in China. The ML models use climate variables from the preceding autumn as predictors and are trained on data from 1979 to 2010. The ML models autonomously extract useful information to forecast winter precipitation for the years 2011–2020 without a separate and prior predictor selection process. The key findings of the work include: • The two ML models autonomously extract valuable information from the previous autumn data without a separate predictor selection process and demonstrate reasonable forecast skills compared to a dynamic model in predicting winter precipitation in China. • The ML models outperform the dynamic model across various regions, and the ensemble forecasts of the ML and dynamic models show higher skill, particularly in southern China. • The ML models' forecast skill is attributed to their accurate predictions of the first EOF1 of winter precipitation, and the results highlight the importance of sea surface temperature over the western Pacific. [ABSTRACT FROM AUTHOR]

The current work investigated the impact of the interannual variation in the autumn-winter snow cover extent (SCE) over the extratropical Eurasian continent on Northeast Asian winter precipitation (NAWP) using observational data from 1972 to 2019. The leading singular value decomposition (SVD) mode of the interannual variation in the NAWP is significantly related to autumn SCE anomalies over Central Asia (SCE_CA), with decreased autumn SCE_CA corresponding to increased NAWP. The possible mechanisms responsible for this autumn SCE_CA-NAWP relationship are examined. The analytical results show that the reduced autumn SCE_CA has a significant warming effect on the overlying atmosphere and induces an anomalous anticyclone over CA. This anticyclonic response cooperates with the North Atlantic sea surface temperature anomaly (SST)-generated atmospheric wave pattern, promoting a further eastward extension of the wave pattern to the downstream regions of the Eurasian continent. The anomalous autumn SCE_CA can persist to the following winter, and its related atmospheric wave train is maintained and further strengthened by the increased winter SCE over North China (SCE_NC). In winter, the atmospheric circulation features a pronounced low-pressure system over Northeast Asia and a high-pressure system over the western North Pacific sector. The anomalous southeasterly winds between these two systems transport water vapor from the lower-latitude Pacific Ocean to the interior of the continent, accounting for more NAWP. A composite analysis further indicates that this autumn SCE_CA-NAWP relationship is primarily independent of the North Atlantic SSTs. • There is a significant negative correlation between the autumn SCE anomalies over Central Asia (SCE_CA) and Northeast Asian winter precipitation (NAWP). • The critical system linking the SCE_CA-NAWP is a continental-scale atmospheric wave pattern originating from the North Atlantic and reaching North Pacific. • The autumn SCE over CA and winter SCE over North China are essential in maintaining and propagating the continental-scale atmospheric wave pattern. [ABSTRACT FROM AUTHOR]

This study investigated the interdecadal variations in winter precipitation over the nonmonsoonal Eurasian regions (30°-70°N, 0°-80°E) (NER) and possible contributions of the Arctic sea ice for the period 1948‐2019. The dominant mode of the interdecadal variations in winter precipitation over NER (DM_NWPE) features opposite loading over the Mediterranean region and over the northern NER and Central Asia. The positive phase of DM_NWPE with enhanced precipitation over the Mediterranean region and suppressed precipitation over the northern NER and central Asia is associated with positive height anomalies over the high-latitude North Atlantic and Eurasia and negative height anomalies extending from the subtropical North Atlantic Ocean to western Eurasia. Increased sea ice in the Barents Sea may promote an atmospheric wave pattern extending to the downstream Eurasian regions, contributing to decreased precipitation over northern NER and Central Asia. Reduced sea ice in the Baffin Bay-Davis Strait-Labrador Sea (BDL) can promote a southeastward atmospheric wave pattern, contributing to enhanced precipitation over the Mediterranean region. The atmospheric response to the Arctic sea ice forcing is confirmed by experiments with a linear baroclinic model. A dipole pattern of sea surface temperature (SST) anomalies in the North Atlantic Ocean may contribute to the interdecadal variation in DM_NWPE. Further analysis showed that the Arctic sea ice-DM_NWPE relationship is primarily SST independent and the Arctic sea ice plays a more critical role in DM_NWPE variations than the North Atlantic SST anomalies. • The NWPE interdecadal variations feature the opposite variation of precipitation over the Mediterranean and the northern NER–Central Asia. • The increased sea ice in the Barents Sea and reduced sea ice in BDL contribute to the dominant mode of the NWPE interdecadal variations. • The Arctic sea ice plays a more critical role than the North Atlantic SST anomalies in the NWPE interdecadal variations. [ABSTRACT FROM AUTHOR]

The interdecadal change of the leading mode of the mean winter precipitation over China has been investigated using observational data for the period from 1960 to 2012. The leading empirical orthogonal function (EOF) mode (EOF1) of the winter precipitation over China displays a mono-sign pattern over southeastern China, accounting for 49.7 % of the total variance in the precipitation. Both the El Niño-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) can impact EOF1. A positive (negative) EOF1 is accompanied by warm (cold) ENSO events and weak (strong) EAWM, and the latters can cause anomalous southerlies (northerlies) along the coast of southeastern China, accompanied by the transportation of water vapor from the Bay of Bengal and the South China Sea favoring a wet (dry) winter over southeastern China. An abrupt transition of the EOF1 is observed around the mid-1980s. Therefore, the data are divided into two subperiods, i.e., 1960-1987 (P1) and 1988-2009 (P2). Significant differences in the large scale atmospheric circulation and sea surface temperature anomalies associated with EOF1 during these two subperiods are observed. EOF1 is closely related to the mid- to high-latitude atmospheric circulation in P1, while its relationship to the tropics obviously increases during P2. The partial regression analysis results show that the interdecadal change of EOF1 is caused by both the interdecadal changes of the EAWM and ENSO around the mid-1980s. In P1, the lower-level anomalous southerlies along the coastal southeastern China accompanied by water vapor transportation that causes above-average precipitation are related to an anti-cyclonic system centered over the mid-latitude western North Pacific associated with EAWM. In P2, the influence of the EAWM is weaker, and the southerly anomaly over the coastal southeastern China is mainly caused by the anticyclone over Philippines, which is related to the ENSO. [ABSTRACT FROM AUTHOR]

The influence of the tropical Pacific sea surface temperature (SST) on the wintertime surface air temperature (SAT) in China is investigated using both the observational data and the output of coupled ocean-atmosphere numerical models during the period from 1960 to 2006. A singular value decomposition analysis (SVD) is applied between 500-hPa geopotential height (Z500) in the Northern Hemisphere and SST in the tropical Pacific Ocean to get the tropical Pacific SST-forced atmospheric patterns. The association of the SAT over China and the tropical Pacific SST is measured by calculating the temporal correlation coefficient (TCC) between the SAT and the expansion coefficient of the atmospheric component of the leading two SVD modes. Results show that the SAT over China is significantly correlated to the second SVD mode (SVD2). The SST component of SVD2 is characterized by negative tropical Pacific SST anomalies centered over the midequatorial Pacific Ocean. The atmospheric component of SVD2 (ASVD2) shares many similarities in spatial structures to the Arctic Oscillation (AO). The time variation of ASVD2, however, is found more closely correlated to the variation of SAT over China than the AO. When SVD2 is in its positive phase, the SAT over China tends to be warmer than normal. Further analysis indicates that the TCC between the SAT in China and ASVD2 is largely decreased after the long-term climate trend is removed. The time variability of the tropical Pacific SST-forced large-scale atmospheric patterns and its relationship to SAT are reasonably captured by the multimodel ensemble (MME) seasonal forecasts. An examination of the MME forecast skill indicates that ASVD2 contributes significantly to the TCC skill of MME forecasts. [ABSTRACT FROM AUTHOR]

Multimodel ensemble (MME) seasonal forecasts are analyzed to evaluate numerical model performance in predicting the leading forced atmospheric circulation pattern over the extratropical Northern Hemisphere (NH). Results show that the time evolution of the leading tropical Pacific sea surface temperature (SST)-coupled atmospheric pattern (MCA1), which is obtained by applying a maximum covariance analysis (MCA) between 500-hPa geopotential height ( Z500) in the extratropical NH and SST in the tropical Pacific Ocean, can be predicted with a significant skill in March-May (MAM), June-August (JJA), and December-February (DJF) one month ahead. However, most models perform poorly in capturing the time variation of MCA1 in September-November (SON) with 1 August initial condition. Two possible reasons for the models' low skill in SON are identified. First, the models have the most pronounced errors in the mean state of SST and precipitation along the central equatorial Pacific. Because of the link between the divergent circulation forced by tropical heating and the midlatitude atmospheric circulation, errors in the mean state of tropical SST and precipitation may lead to a degradation of midlatitude forecast skill. Second, examination of the potential predictability of the atmosphere, estimated by the ratio of the total variance to the variance of the model forecasts due to internal dynamics, shows that the atmospheric potential predictability over the North Pacific-North American (NPNA) region is the lowest in SON compared to the other three seasons. The low ratio in SON is due to a low variance associated with external forcing and a high variance related to atmospheric internal processes over this area. [ABSTRACT FROM AUTHOR]

The seasonality of the influence of the tropical Pacific sea surface temperature (SST)-forced large-scale atmospheric patterns on the surface air temperature (SAT) over China is investigated for the period from 1969 to 2001. Both observations and output from four atmospheric general circulation models (GCMs) involved in the second phase of the Canadian Historical Forecasting Project (HFP) are used. The large-scale atmospheric patterns are obtained by applying a singular value decomposition (SVD) analysis between 500-hPa geopotential height (Z500) in the Northern Hemisphere and SST in the tropical Pacific Ocean. Temporal correlations between the SAT over China and the expansion coefficients of the leading SVD modes show that SAT over China can be significantly influenced by these large-scale atmospheric patterns, especially by the second SVD mode. The relationship between the SAT over China and the leading atmospheric patterns in the observations is partly captured by the HFP models. Furthermore, seasonal forecasts of SAT over China are postprocessed using a statistical approach. This statistical approach is designed based on the relationship between the forecast Z500 and the observed SST to calibrate the SAT forecasts. Results show that the forecast skill of the postprocessed SAT over China can be improved in all seasons to some extent, with that in fall having the most significant improvement. Possible mechanisms behind the improvement of the forecast are investigated. [ABSTRACT FROM AUTHOR]

The relationship between the autumn snow cover over the eastern Tibet Plateau (TP) (ASCETP) and the North America (NA) surface air temperature (SAT) during 1979-2014 is investigated in this work. A climate transition of ASCETP is noticed in the year 1994 and the data is divided into two subperiods correspondingly: 1979-1994 (P1) and 1995-2014 (P2). The climate impact of the ASCETP variation on the NA winter SAT in P1 and P2 are discussed and compared. During P1, more ASCETP induces negative geopotential height anomalies over the northeastern Tibet Plateau, which impose an anomalous vorticity perturbation near the East Asia westerly jet (EAWJ) core. This anomalous vorticity perturbation propagates downstream along the EAWJ in a Rossby wave train form, which penetrates the North Pacific Ocean and arrives NA and results in increasing and decreasing of SAT over west and east part of NA in winter respectively. Besides, observation analysis results show that the wave train associated sea surface temperature (SST) decline in the North Atlantic Ocean boosts the propagation of the wave train and extends it to the Eurasia continent. In contrast, during P2, the climate impact of ASCETP changes on the winter SAT over NA is not as significant as P1 probably due to the relatively weak local cooling effect and the corresponding weak vorticity forcing in the vicinity of EAWJ. The difference between P1 and P2 can be traced back to the formation of snow cover anomalies in autumn. During P2, ASCETP anomalies are related to a SST-induced Rossby wave train from North Atlantic Ocean while ASCETP anomalies in P1 tend to be caused by local processes. [ABSTRACT FROM AUTHOR]