Possible Causes of Extremely Warm Early Summer in North China During Cold and Warm Periods

Abstract

The abnormal characteristics of extremely warm early summer (EWES) in North China under different decadal backgrounds were contrastively analyzed. Their relationships with upper- and lower-level atmospheric circulation and global sea surface temperature anomalies (SSTAs) are also discussed. Results show that temperature anomalies of EWES in North China are overall higher than normal in both cold (1961–1993) and warm (1994–2019) periods, but the anomalies of the latter are much higher than that of the former. EWES in North China is directly related to the circulation lying between 40° and 50°N in the middle troposphere, which leads to positive temperature anomalies occurring from the bottom to the upper level of the troposphere together with a high anomaly trend tilting northward. The persistent and strong Eurasian continental high (ECH) and weak Northeast China cold vortex (NECV) activity, together with the strong western Pacific subtropical high (WPSH) are major factors that directly lead to EWES in North China. ECH and WPSH are stronger and larger, and NECV are weaker and more northward in the warm period than in the cold period. In addition, the positive SSTAs in the tropical Indian Ocean and the Kuroshio area are favorable for the stronger and larger ECH and WPSH as well as the weaker and more northward NECV, causing strong anticyclonic and downward circulation system controlling North China, resulting in the extremely warm temperatures there. The joint impact of the positive tropical Indian Ocean SSTAs and the Kuroshio region SSTAs is more significant during warm than cold periods, resulting in much stronger EWES in North China during warm periods.