Two dominant factors governing the decadal cooling anomalies in winter in East China during the global hiatus period

摘要

Observational analyses reveal substantial cooling anomalies in East China, during the global hiatus period (1998-2013), particularly in winter. The factors contributing to the winter cooling anomalies and the underlying mechanisms are studied. The results highlight the substantial roles played by two factors: Arctic sea ice reduction (i.e., sea ice area in Barents-Kara Seas, SIA) and the sea surface temperature (SST) cooling anomalies in the Kuroshio extension region (KSST). Together, these factors contribute to approximate 90% of the trend anomaly of East China's area averaged surface air temperature (ECT). The individual contributions from SIA and KSST are investigated based on observational diagnostic analyses and sensitivity experiments. The results show that the contributions from the two factors are similar. Both SIA and KSST induce intensified geopotential height over the mid-to-high latitudes of the Eurasian continent, with the maximum over the Ural Mountains extending toward Lake Baikal. These anomalies intensify the Ural blocking activities and Mongolian cold high. Meanwhile, both forcings cause negative 500-hPa geopotential height (Z500) anomalies over the central North Pacific that extends westward to East Asia, intensifying the climatological East Asian grand trough. However, KSST causes positive Z500 anomalies over the Bering Sea, but SIA does not. The enhanced Ural blocking activities and the deepened East Asian grand trough jointly favour the intensification of the East Asian winter monsoon, thus causing cooling anomalies in East China.