Revisiting the East Asian summer monsoon structure: a combined effect of condensational heating and synoptic eddy activities

The East Asian summer monsoon (EASM) is a unique regional monsoon in the extratropics involving not only tropical but midlatitude processes. Most of the previous studies identified the role of condensational heating which is largely relevant to the tropical moisture transport in determining the dynamical structure of EASM. However, how midlatitude synoptic eddy activities can affect the EASM structure has not been well recognized. With dynamical diagnoses, this study revisits the EASM structure by emphasizing the roles of feedbacks of condensational heating versus synoptic eddy activities. As EASM is characterized by a grand low-level low with strong humid southerly flows extending from the tropics to the northeastern Asia, its vertical structure is found to have a dramatic meridional difference bounded at around 35.5°N. In the southern domain, EASM features a meridional overturning cell and a baroclinic structure with an upper-level high versus a lower-level low in geopotential height, which are primarily controlled by substantial condensational heating due to abundant monsoon precipitation. However, in the northern domain, EASM exhibits an equivalent barotropic structure with an upper-level low versus a lower-level low. Such a unique structure results from a combined effect of feedbacks of condensational heating and synoptic eddy activities, in which the upper-level low is dominated by the synoptic eddy dynamical feedback while the lower-level low is induced by the both feedbacks. The role of the midlatitude synoptic eddy activities in shaping the EASM structure proposed in this study provides a new perspective for understanding the formation and variation of EASM.