Characteristics and Possible Causes for the Climate Anomalies over China in Summer 2020
In the summer of 2020, the weather and climate in China were extremely abnormal. The national average precipitation was 373.0 mm, with 14.7% more than normal, which is the second most since 1961. The intraseasonal variation of summer rainfall was obvious. The rainbelt from June to July was mainly located in the Yangtze-Huaihe River Valley (YHRV), but moved northward to Northeast China, North China, and Southwest China in August, making the distribution of summer rainfall in 2020 not belong to any of the four traditional patterns of rainfall. The observational analysis on atmospheric circulation and sea surface temperature indicates that the atmospheric circulation in the mid-high latitude was characterized by “two ridges and one trough” over Eurasia from June to July. The East Asian summer monsoon (EASM) was very weak, and the western Pacific subtropical high (WPSH) was much stronger and further westward than normal. The meridional movement of WPSH presented the characteristics of quasi-biweekly oscillation, accompanied with the early first northward jump and delayed second northward jump, which makes the intensified water vapor transport from the tropical Northwest Pacific. Combined with the cold air activities going southward from the mid-high latitude, the abnormal moisture convergence was located over the middle and lower reaches of the Yangtze River, which leads to excessive Meiyu rainfall in YHRV. The persistent Indian Ocean basin-wide warming played an important role in maintaining the stronger WPSH and weaker EASM than normal during June-July. By contrast, the atmospheric circulation in mid-high latitude was adjusted to “two troughs and one ridge” over Eurasia in August, with a low trough near Mongolia; the WPSH changed from the East-West zonal distribution to the “block” pattern, located further northward. The abnormal southwesterly moisture transport along the edge of WPSH extended to North-Northeast China, forming an abnormal “northeast-southwest” rainbelt, which is obviously different from that during June-July. The abnormal activity of the tropical Madden-Julian Oscillation was an important factor for the adjustment of the atmospheric circulation in mid-low latitude in August 2020.