No abstract

With global warming, the probability of summer compound hot and dry extreme (CHDE) days, which are higher risk compared with single-factor extreme events, increases in some regions. However, there have been few studies on the winter precursor signals of such events. In this study, we found that summer CHDEs have generally increased in the last 20 years, with the increases in the middle and lower reaches of the Yangtze River region and Southwest China being more than double those in other regions of China. The dominant mode of summer CHDEs in China is characterized by more hot–dry days in the Yangtze–Huaihe River Basin (YHRB). Importantly, we found that there is an obvious cross-seasonal relationship between the first mode of winter snow cover in the Northern Hemisphere (NH) and summer CHDEs in China. When the mode of winter snow cover in the NH is in a positive phase with a negative-phase Arctic Oscillation (AO), i.e., more snow cover in Europe, Northeast China, and the northern United States, and less snow cover in central Asia and the midlatitudes in winter, more CHDEs in China in the following summer. Compared with the signals from the AO, these signals from winter snow can be better stored and transmitted into summer through the snow, soil and ocean, inducing a northward shift of the upper-level westerly jet and strengthening of South Asia high. Through the strong dynamic forcing of negative vorticity advection with the change of westerly jet, the subsidence movement in the western Pacific subtropical high (WPSH) region is strengthened, resulting in the stable maintenance of the WPSH in the YHRB. Under the synergy of a remote mid- and high-latitude wave train in summer, which also relates closely to winter snow cover, more CHDEs ultimately occur in the YHRB of China.

Understanding intra-seasonal variation in extreme cold events (ECEs) has important implications for climate prediction and climate adaptation. However, the ECEs difference between early (from December 1 to January 15) and late (from January 16 to February 28) winters is a lack of sufficient understanding. Herein, we investigated the trends of ECEs over eastern China in early and late winters. Results showed that the number of days with ECEs had a faster and uniformly decreasing trend in late winter over eastern China, whereas the decreasing trend in early winter was not significant because of the dipole pattern with an increase of ECEs in northeast China and a decrease of ECEs in southeast China during the time period 1980–2021. This denoted that China was presenting a pattern of “cold early winter–warm late winter”. The feature of cold early winter was related to a significant increase in high-latitude blocking highs extending poleward and reaching the Arctic Circle in early winter during the last 20 years. In particular, there was a large-scale tilted high ridge from the Ural Mountains to northern Asia, which favored the negative phase of the Arctic oscillation. This, in turn, led to a strong Siberian high and East Asian winter monsoon. Strong cold advection related to the circulation anomalies caused an ECEs increase in northeast China and dominated the change in temperature over eastern China in early winter. By contrast, the decrease in ECEs in late winter in the last 20 years was more related to the interdecadal enhancement of the anticyclonic anomaly over the north Pacific (NPAC). The strong NPAC extended to East Asia in a zonal direction, causing strong warm anomalies in eastern China through warm advection and diabatic heating, which weakened the northerly and prevented the East Asian trough from moving south, resulting in a warmer East Asia and a uniform decrease in late winter.