Statistical characteristics of Asian blocking activity during the Yangtze-Huaihe Meiyu season

The NCEP/NCAR daily analysis data for June and July of 1960—2018 and daily precipitation data collected at national weather stations in China for the same period are used to analyze geographical distribution of blocking activities in Asia, the characteristics of blocking events in key areas, and the relationship between blockings and abnormal Meiyu during the Yangtze-Huaihe Meiyu season. The results show that: (1)In the Yangtze-Huaihe Meiyu seasons (June—July) of the last 59 years, there are a total of 363 blocking events in Asia, and the centers are mainly distributed in three key areas, i.e., the Ural Mountains (40°—80°E), the Lake Baikal (80°—120°E) and the Okhotsk Sea (120°—160°E); (2) the frequency and number of days of blocking events in the three key areas from high to low are the Okhotsk Sea, the Ural Mountains and the Lake Baikal. The double-blocking situation is dominated by the Ural Mountains-Okhotsk Sea double-blocking pattern, which accounts for 60% of the double-blocking days in Asia. The average life cycle of the blocking event is about 7 d, and the longest holding time is 13 d; (3) there are obvious interdecadal changes and increase trends in the blocking frequency and number of days in the three key blocking areas, which are more obvious in the Okhotsk Sea. These changes are consistent with the increase trend of cumulative rainfall in the Yangtze-Huaihe Meiyu seasons of the past 59 years; (4) the amount of rainfall in the Yangtze-Huaihe Meiyu season is closely related to blocking activities. There are significantly more (less) blocking events in the Okhotsk Sea and Ural-Okhotsk in years of positive (negative) Meiyu rainfall anomalies, while the blocking events in the Ural and Baikal regions are not significantly related to Meiyu; (5) in the Yangtze-Huaihe Meiyu season, the number of blocking days in the Okhotsk may be related to the early sea surface temperature anomaly signal of ENSO.