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Analyses of the standardized precipitation evaporation index (SPEI), using the season-reliant empirical orthogonal function (S-EOF) method, indicate that the second leading mode of drought over Northeast China features an in-phase variation from spring to summer. Such an in-phase change is closely connected to the persistence of geopotential height anomalies around Lake Baikal. The positive height anomalies around Lake Baikal, with an equivalent barotropic structure in the troposphere, can decrease water vapor transport into Northeast China and induce anomalous descending motion over Northeast China during both seasons, favoring precipitation deficit and high temperature in situ and hence resulting in the synchronous variations of spring and summer droughts. Further investigation reveals that the spring North Atlantic Oscillation (NAO) plays a notable role in the in-phase change of spring–summer droughts over Northeast China. The positive phase of spring NAO could induce spring drought over Northeast China directly through its influence on the above atmospheric circulations via a zonal wave train emanating from the North Atlantic. Meanwhile, it can also increase the soil moisture in central Siberia by enhancing the local snow depth. The wetter soil moisture in the following summer, in turn, increases the meridional temperature gradient between the middle and high latitudes and then forces westerly anomalies around 60°N, consequently yielding positive height anomalies around Lake Baikal that favor the occurrence of summer drought over Northeast China. Therefore, the spring NAO is hypothesized to contribute to the in-phase variations of spring–summer droughts over Northeast China through the combined roles of zonal wave train and central Siberian soil moisture. Significance Statement: Northeast China suffers from frequent droughts severely in recent decades. Thus, it is urgent to understand the physical mechanisms of drought in Northeast China. Using the season-reliant empirical orthogonal function (S-EOF) analysis, this study indicates that the second mode of S-EOF shows an in-phase change of drought from spring to summer over Northeast China, which is associated with the persistence of geopotential height anomalies around Lake Baikal. Our study further reveals that the spring NAO plays a pronounced role in the in-phase change of spring–summer droughts over Northeast China, through the combined roles of zonal wave train and the soil moisture in the central Siberian plateau. These findings are encouraging for better understanding of drought in Northeast China and are also important for disaster prevention and mitigation. [ABSTRACT FROM AUTHOR]

This study investigates the first two leading modes of the interannual variability of frequency of snowfall events (FSE) over China in the winter during 1986–2018. The positive phase of the first leading mode (EOF1) is mainly characterized by positive FSE anomalies in northeastern–northwestern China and negative FSE anomalies in the three-river-source region. In contrast, the positive phase of the second leading mode (EOF2) is mainly characterized by positive FSE anomalies in central-eastern China (CEC). EOF1 is affected by the synoptic-scale wave activity over the midlatitudes of the East Asian continent, where active synoptic-scale wave activity over the midlatitudes may cause increased FSE over northeastern–northwestern China, and vice versa. In a winter of a negative phase of the North Atlantic Oscillation, an anomalous deep cold low may occur over Siberia, which may induce increased meridional air temperature gradient, increased atmospheric baroclinicity, and hence increased FSE over the midlatitudes of the East Asian continent. The EOF2 is affected by the interaction between anomalous northerly cold advection and anomalous southerly water vapor transport over CEC. The positive phase of EOF2 is associated with negative sea ice anomalies in the Barents Sea–Kara Sea region and negative sea surface temperature anomalies in the central-eastern tropical Pacific. Reduced sea ice in the Barents Sea–Kara Sea during January–February may cause increased northerly cold advection over CEC, while a La Niña–like condition during January may induce southerly water vapor transport anomalies over CEC. [ABSTRACT FROM AUTHOR]

Based on the long-term reanalysis datasets and the multivariate copula method, this study reveals that the frequency of summer hot drought events (SHDEs) over northeastern China (NEC) shows interdecadal variations during 1925–2010. It is revealed that the summer sea surface temperature (SST) over the North Atlantic has a significant positive correlation with the frequency of SHDEs over NEC on the decadal time scale, indicating a potential influence of the Atlantic multidecadal oscillation (AMO). Further analyses indicate that during the positive phases of the AMO, the warming SST over the North Atlantic can trigger a stationary Rossby wave originating from the North Atlantic, which splits into two wave trains propagating along two different routes. One is a zonally orientated wave train that resembles the Silk Road pattern, whereas the other is an arching wave train that resembles the polar–Eurasian pattern. A negative (positive) phase of the Silk Road pattern (polar–Eurasian pattern) may result in the weakened westerly wind along the jet stream, the downward vertical motion, and the anomalous positive geopotential center over NEC, providing favorable conditions for precipitation deficiency and high temperature and resulting in increased SHDEs. Thus, the Silk Road pattern and the polar–Eurasian pattern serve as linkages between the AMO and SHDEs over northeastern China in summer on the interdecadal time scale. Model simulations from CAM4 perturbed with warmer SST in the North Atlantic show precipitation deficiency and high temperature conditions over northeastern China in summer, supporting the potential impacts of the North Atlantic SST on SHDEs over northeastern China. The results suggest that the phase of the AMO should be taken into account in the decadal prediction of SHDEs over northeastern China in summer. [ABSTRACT FROM AUTHOR]