The empirical orthogonal function (EOF) and correlation analyses are applied to NCEP/NCAR atmospheric circulation data, HadISST data, and air temperature data collected at 160 observational stations in China to study the major spatial distribution modes of air temperature anomaly in southern China during midsummer (July-August). Critical circulation systems corresponding to higher than normal temperature and possible external forcing signals of the ocean are investigated in great detail. The results show that there are two major modes of high temperature anomaly in southern China in the midsummer, i.e., the Jianghuai type with the center of high temperature anomaly located in the middle and lower reaches of the Yangtze River valley and Huai River valley and the Jiangnan type with the center of high temperature anomaly located to the south of the Yangtze River valley. Both the atmospheric circulation and external forcing signals are quite different between the two types.
For the Jianghuai type, the critical circulation systems are positive anomalies of geopotential height at 500 hPa and 200 hPa and weakened East Asian subtropical westerly jet. Meanwhile, the oceanic forcing signals include the anomalous "-+-" sea surface temperature (SST) pattern from tropical Indian Ocean to the eastern Pacific and anomalous warm SST in the mid-latitude of North Atlantic Ocean. In the midsummer of 2016, both the atmospheric circulation anomalies and the SST anomaly pattern were consistent with the characteristics of the Jianghuai type, further confirming the statistical results. For the Jiangnan type, the critical circulation system is the westward-extending and intensified western Pacific subtropical high (WPSH). Both the anomalous warm SST in the equatorial central-eastern Pacific in the preceding winter and the basin-wide warming pattern in the tropical Indian Ocean from the spring to summer influence the strong WPSH in the midsummer. However, the effect of SST forcing in the tropical Indian Ocean has been proved to be more significant and more persistent compared to that in the equatorial central-eastern Pacific.