Climatological features of the Western Pacific subtropical high southward retreat process in late-spring and early-summer

Based on the climatological daily mean NCEP/NCAR reanalysis data, NOAA outgoing longwave radiation (OLR) data, and weekly NOAA CAMP precipitation from 1979 to 2006, the variation of the Western Pacific subtropical high (WPSH) ridge during late-spring and early-summer and its relationship with the onset of the Asian summer monsoon is discussed from an climatological view. It is found that there is a remarkable southward retreat process (SRP) of the WPSH during late-spring and early-summer, mainly at both low and high levels of the troposphere, with an approximate two week lifespan. Afterwards, the first northward jump of the WPSH occurs. The end date of the WPSH SRP in the upper troposphere is about 10 days earlier than the beginning of the WPSH SRP in the lower troposphere, showing a meaningful leading signal for predicting the WPSH SRP in the lower troposphere and the subsequent northward jump of the WPSH. The WPSH SRP at tropospheric low levels happens simultaneously with a notable eastward shift of the WPSH. After the WPSH SRP at tropospheric low levels concludes around the end of May, the incline of the WPSH ridge axis turns over due to the change of the meridional gradient of air temperature. The Asian summer monsoon onset and associated variations of strong convection and rainfall in Asia are closely related to the WPSH SRP during late-spring and early-summer. In the mid and late period of the WPSH SRP at tropospheric high levels around the end of April, the summer monsoon onset takes place in the Andaman Sea and the Bay of Bengal (BOB)., Following the start of the WPSH SRP at tropospheric low levels, the South China Sea summer monsoon breaks out (May 14~15). By the end of the WPSH SRP, in the beginning of June, the Indian summer monsoon kicks off. Upon the end of the WPSH return stage, the East Asian summer monsoon begins. The commencement of each component system of the summer monsoon corresponds nicely to a particular stage of the WPSH SRP. This offers valuable information for monsoon onset prediction in different sectors of Asia. In addition, it is found that there is a typical wet-dry-wet sandwich precipitation pattern, with two rainfall belts in the regions south and north to the WPSH main body, and a dry belt under it. The variation of this rainfall pattern is related to the shift of WPSH ridge.