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.
Highlights
Climate has changed significantly all over the world during the past century
Temperature anomalies are overall higher than 1◦C with the temperature anomaly higher than 2◦C occurring in the Midwest of Inner Mongolia
Positive temperature anomalies of extremely warm years are above 0.5◦C in most areas of North China, where the temperature anomalies are above 1◦C in the northeast and central region of Hebei
Summary
Climate has changed significantly all over the world during the past century. Against the background of global warming, as one of the worldwide major meteorological disasters, extreme high temperature and drought events occur with increasing frequency in summers. The positive anomalies controlling area extends, covering most regions of the middle and high latitudes of Eurasia (90–120◦E, 40–60◦N), and negative anomalies controlling the Sea of Japan (Figure 8B) This pattern is in favor of higher than normal climate early summer temperatures in North China. Previous studies point out that the warming trend of the Kuroshio area SST mainly leads to positive geopotential height anomalies over most regions of the middle and high latitudes of Eurasia and significant heating in the lower troposphere, stimulating upward and northward wave activity flux, resulting in the abnormally intensified WPSH over the East Asia coast (Wang, 2004). These results are consistent with the conclusions obtained in the present study
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