Analysis of Global Atmospheric Circulation Adjustments before and after the Onset of Asia Summer Monsoon Using Climatic Wind Variation

The climate wind-field variation characteristics in time and space during and around the Asian summer monsoon onset (1 April to 30 June) were examined and discussed. According to the diagnostic analysis of variability, on 10 April, 21 April, 31 May, 11 June, and 28 June, the Northern Hemisphere experienced six evident circulation adjustments. The variation can reflect and analyze the adjustment. The generalized monsoon region has the highest value areas of variation. The variation is an objective quantitative tool for analyzing atmospheric circulation adjustments and studying generalized monsoons. This analysis resulted in the following conclusions: The six major adjustments correspond to the onset of spring (the collapse of stratospheric polar vortex), the complete collapse of winter circulation (mid-spring), the establishment of the South China Sea summer monsoon, the establishment of South Asian summer monsoon, the entering Meiyu season in Yangtze-Huaihe River basin (the establishment of East Asian summer monsoon), and the intermission of Meiyu. The summer monsoon onset in the South China Sea is the strongest circulation adjustment (mutation) in the seasonal transition of the Northern Hemisphere. Following the adjustment, the circulation in the Northern Hemisphere turns to its summer state. The onset of summer monsoon in the South China Sea and South Asia is related to global circulation adjustment on each isobaric surface rather than a local phenomenon. The circulation adjustment of summer monsoon in the South China Sea and South Asia is most evident at 700 hPa. The summer monsoon in the South China Sea first occurs to the south of the South China Sea at 850 hPa. The summer monsoon in South Asia is the first onset in the eastern Arabian Sea west of the Indian Peninsula. The collapse of winter circulation and the establishment of the South China Sea, South Asia, and East Asia summer monsoon are related to the northward movement of the ridgelines of the Mid tropospheric subtropical high in the Northern Hemisphere, the South Asian high in the upper troposphere, and the Qinghai Tibet high in the stratosphere. The northward movement of the direct solar point is the main reason for the northward movement of the ridgeline.