The Relationship between Continuous Rainstorms and Atmospheric Intraseasonal Oscillation during the First Rainy Season in Guangdong in 2019

In order to make better medium-term and extended-range forecast of continuous backflow rainstorms, wavelet analysis and Lanczos time filter were used to analyze the relationship between daily precipitations and atmospheric intraseasonal oscillations during the first rainy season in Guangdong in 2019. The mean circulations of two backflow continuous rainstorms over southwest of Guangdong during April and May and their low-frequency propagation characteristics were analyzed and compared with the frontal-type continuous rainstorms over north of Guangdong during June. The results show that the two continuous backflow rainstorms near Yangjiang from April to May and the precipitation during the first rainy season exhibited a quasi-23-day periodic oscillation. They were continuous backflow rainstorms with or without the influence of cold air, and their corresponding atmospheric circulation fields and low-frequency propagation were significantly different. During the continuous backflow rainstorms on 12–14 April, there were stable “west blocking” and “east blocking” at the middle and high latitudes at 500 hPa, which made cold air continuously moving southeast from the south of the high pressure that was transformed over the East China Sea. The convergence asymptote of relatively dry and cold strong southeast wind and the warm humid southerly wind from the mid-southern South China Sea by Indochina Peninsula was formed near Yangjiang at 925 hPa. During the continuous rainstorms on 23–26 May, there was a stable high-pressure ridge over the regions of eastern China, East China Sea, and Yellow Sea at 500 hPa. Because of such, Guangdong for a long time was under the southwestern airflow coming from the plateau trough, and on the ground was in the southwestern position of the degenerative high-pressure ridge from the East China Sea, in front of the trough from Beibu Bay to the southwest of China. At 925 hPa, the convergence asymptote was formed from the strong southerly winds coming from the Bay of Bengal centered in Yangjiang and the southeast winds coming from the east of the Pearl River Estuary. The low-frequency northeast wind (Southeast wind) from the south (southwest) of low frequency anticyclone in the central China (east of the East China Sea) was gradually strengthened southward, merging at the southwest of Guangdong with the enhanced low-frequency northerly (southerly) winds coming westward from the western Pacific near 140°E (eastward from the Bay of Bengal), which in turn encountered with (without) low-frequency eastern wind from the north of low frequency cyclone propagating northward from the mid-northern South China Sea. This finally resulted in the continuous backflow rainstorms with (without) obvious influence of cold air. But the frontal-type continuous rainstorm during June was affected by convergence between cold air induced by deep East Asian trough and southwesterly from Bengal Bay. The enhanced low-frequency southwesterly propagated southwards and eastwards to Guangdong from mid China and Bengal Bay. It converged with the low-frequency cold dry northeasterly winds from the periphery of the mid-latitude anticyclone at the south of Yangtze River or the north of South China Sea, which led to the happening of the continuous rainstorms during 9–13 June.