Heat Waves in China: Definitions, Leading Patterns, and Connections to Large-Scale Atmospheric Circulation and SSTs

Based on the daily maximum temperatures (T-max) from 587 surface observation stations in China during 1959-2013, heat waves are detected using both absolute and relative definitions. The spatiotemporal variations of heat wave occurrence/duration/amplitude are compared between the two definitions. Considering the significant differences in regional climatology, relative threshold is more meaningful to detect the local extremes. By utilizing the empirical orthogonal function, the integral index heat wave total intensity is decomposed into three dominant modes: interdecadal (ID), interannual-tripole (IA-TR), and interannual-dipole (IA-DP) modes. The ID mode shows uniform anomalies over the whole China, with the maximum in north, and its corresponding time series depict notable interdecadal variations with a turning point around mid-1990s. The IA-DP mode exhibits opposite-signed anomalies over north and south China. The IA-TR mode shows an anomalous tripole pattern with negative anomalies over central China and positive anomalies over north and south China in its positive phase. Both the IA-DP and IA-TR patterns are more obvious since mid-1990s with mainly year-to-year variations before that. All the three modes are controlled by anomalous high-pressure systems, which are accompanied by local-scale dry land conditions. The diabatic heating associated with anomalous convective activities over tropical western Pacific triggers Rossby wave trains propagating northward along the East Asia, which causes abnormal heat waves through descending motion over the high-pressure nodes. In turn, the severe convections are generated by enhanced Walker circulation in the tropical Pacific due to warming and/or cooling sea surface temperature (SST) anomalies in the tropical western and eastern Pacific, respectively.