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This study revealed a significant interdecadal change in the impact of spring western Tibetan Plateau (TP) snow cover (TPSC) on subsequent summer compound heat waves (CHWs) in western Europe (WE) after 1998. This interdecadal change is attributed to a change in a western Europe high–western TP low (WEH–TPL) atmospheric circulation pattern. This pattern arises due to both the inherent variability of TPSC and the phase transition of the Atlantic multidecadal oscillation (AMO) after 1998. The increased magnitude and persistence of western TPSC from spring to summer after 1998 enhanced the snow–atmosphere coupling effect, intensifying ascent and decent motion over the TP and WE, respectively, and strengthening the WEH–TPL pattern. In addition, the post-1998 positive AMO phase favors continuous and stable downstream Rossby wave propagation, enhancing the WEH–TPL pattern and the TPSC–CHWs relationship. Further analyses reveal that the interdecadal changes in the TPSC and the AMO around 1998 contribute to the presence of "double jets" over the North Atlantic–central Eurasian sectors. The TPSC–related anomalous atmospheric circulation and AMO phase shift contribute to the southern and northern branches of the intensified westerly jet, respectively. These conditions create a favorable environment for the formation and persistence of summer CHWs in WE. Numerical modeling experiments with a linear baroclinic model confirm these findings. Our findings suggest that in the context of a changing climate, TPSC plays a pivotal role in the genesis of summer CHWs in WE and may serve as a valuable predictor for CHWs. Significance Statement: This study discovered that starting from 1998 there was a significant change in how spring snow cover on the western Tibetan Plateau affects summer compound heat waves in western Europe. After 1998, the snow cover on the western Tibetan Plateau increased in size and lasted longer from spring to summer, intensifying the interaction between the snow and the atmosphere. This led to more rising and sinking air over the Tibetan Plateau and western Europe, respectively. Also, after 1998 a positive phase of the Atlantic multidecadal oscillation (AMO) was favorable for the circulation connection between western Europe and the Tibetan Plateau. Further analysis showed that these changes in snow cover and the AMO after 1998 caused "double jets" in the North Atlantic and central Eurasia that created better conditions for summer heat waves in western Europe. Numerical models are used to confirm these findings. Our research indicates that, in a changing climate, the snow cover on the western Tibetan Plateau plays a crucial role in the development of summer heat waves in western Europe and can be a useful predictor for these heat waves. [ABSTRACT FROM AUTHOR]

The current work investigated the interdecadal changes in the leading empirical orthogonal function (EOF) pattern of the interannual variation in spring [March–May (MAM)] snow-cover extent (SCE) over the Tibetan Plateau (TP) (SSC_TP). The leading EOF pattern of the SSC_TP is transformed from an east to west dipole pattern during the period 1970–89 (P1) to a monopole structure during the period 1991–2020 (P2). Observational analysis shows that during P1, the negative Antarctic Oscillation (AAO) (−AAO) is associated with low-level cross-equator southeasterly anomalies across the Bay of Bengal and transports more water vapor to the eastern TP. Moreover, at a high level, anomalous northerly winds accompanied by an anomalous sinking motion dominate the western TP, favoring an east-wet–west-dry dipole pattern of SSC_TP. Further analysis shows that the −AAO induces anomalous divergence over the Antarctic, which contributes to the formation of a Rossby wave source (RWS). This RWS is related to a northeastward-propagating atmospheric wave train that crosses the equator and contributes to the SSC_TP variation during P1. In contrast, in P2, the Arctic Oscillation (AO) is associated with a barotropic atmospheric wave train originating from southern Greenland, moving across the North Atlantic Ocean and North Africa and reaching the TP. This wave train results in significant positive vorticity and ascending airflow above the TP and favors a monopole pattern of the SSC_TP. Further analysis shows that the AO can induce divergence anomalies over southeastern Greenland and RWS anomalies there. This RWS induces an atmospheric wave train that propagates eastward and reaches the TP during P2. The above mechanisms have been supported by the results of numerical experiments performed using the linear baroclinic model. [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]

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]

This study investigates the changes in the impact of the interannual variation of autumn snow cover over the eastern Tibetan Plateau (TP; ASCETP) on winter surface air temperature (SAT) over North America (NA) in the mid‐1990s using both observational data and a linear baroclinic numerical model. The index of ASCETP shows an abrupt change with a predominant negative phase before and a predominant positive phase after 1994; thus, the analysis is performed for two subperiods: 1979–1994 (P1) and 1995–2014 (P2). An ASCETP‐related energy budget analysis shows that the ASCETP variation has a more pronounced cooling effect on the above atmosphere in P1 than in P2. The snow‐related anomalous negative geopotential height above the TP is more pronounced and extends farther northeastward in P1 than in P2. Further analysis shows that the relationship between the ASCETP and NA winter SAT is not steady. During P1, associated with an anomalous positive ASCETP, negative geopotential height anomalies emerge over the northeastern TP. These negative anomalies impose an anomalous vorticity perturbation near the East Asian westerly jet core that propagates eastward, forming a wave train‐like pattern. It crosses the North Pacific Ocean and reaches the NA region, leading to positive and negative winter SAT anomalies over western and eastern NA, respectively. In contrast, during P2, the impact of anomalous ASCETP on the variation in the winter SAT over NA is weak, probably due to the relatively weak local cooling effect of the ASCETP and the weak ASCETP‐related vorticity forcing around the East Asian westerly jet. Key Points: Anomalous autumn snow cover over the eastern Tibetan Plateau (ASCETP) affects winter surface air temperature (SAT) over North AmericaThe relationship between the ASCETP and North America winter SAT is not steadyAnomalous ASCETP induces a hemispheric‐scale wave train pattern before 1995, which establishes the snow‐SAT relationship [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]

Interannual variation of surface air temperature over eastern Eurasia (SATEA) during boreal spring was found to be highly connected with snow cover anomalies over western and eastern Eurasia in the period from 1972 to 2009. SATEA experienced a transition of phase from cold to warm during the late 1980s and the roles of snow cover anomalies played during the transition over the two regions show significant contrasts. The relationship between snow cover anomalies over western Eurasian (SCAWE) and SATEA declined from 1972-1987 (P1) to 1989-2009 (P2) and might be mainly influenced by the Arctic Oscillation (AO). In contrast to this, the relationship between snow cover anomalies over eastern Eurasian (SCAEE) and SATEA increased from P1 to P2 and does not involve the impact of AO. Surface heat flux associated with SCAEE could only explain limit parts of the risen temperature and the atmospheric circulation plays a more important role in the strengthened relationship between SCAEE and SATEA from P1 and P2 through the wind induced heat advection over eastern Eurasian areas. Numerical experiments using a linear baroclinic model indicate that the snow cover anomalies around Lake Baikal may impact the variation of SATEA through modulating atmospheric circulation over the eastern Eurasia-North Pacific areas. [ABSTRACT FROM AUTHOR]