Climatological characteristics of the East Asian summer monsoon retreat based on observational analysis

Few studies have investigated the retreat of the East Asian summer monsoon (EASM) in comparison to its seasonal advances. This study examined the climatological characteristics of the EASM retreat based on the Japanese 55-year Reanalysis data and multiple observation data. First, the retreat date of EASM is defined based on the reversal of 850 hPa meridional winds over coastal East Asia. Then, the climatological characteristics of the EASM retreat are investigated based on the retreat date of the EASM. At lower troposphere, the differences of the 850-hPa winds between after and before the EASM retreat display an anticyclone (cyclone) over the East Asian continent (the subtropical western Pacific), resulting in the strengthening of northerly winds over the coastal region of East Asia. Such northerly winds lead to decrease of moisture transport over East Asia and bring colder air from higher latitudes to the coastal East Asia, both favoring the retreat of the EASM. At middle troposphere, the WPSH moves eastward, and a mid-latitude trough appears over northeast Asia. In addition, the difference fields show descending (ascending) motion and cold (warm) advection in the rear (front) of the mid-latitude trough. At upper troposphere, a divergent center exhibits a southeastward movement, which shifts from the Philippine Islands to the western North Pacific. As such, a convergence center appears over the coastal East Asia in the difference map. The retreat of the EASM is also associated with the decreased rainfall, the demise of the wet season, and the broadscale cooling over East Asia.