The Composite Analysis of the Relationship between the Abnormal Retreat of the Subtropical Anticyclone over the Western Pacific Ocean and the Easterlies Vortex in Meiyu Period

The process of the vortex in the easterlies (EV) affecting the short-term eastward retreat of subtropical anticyclone over the Western Pacific (WPSA) is analyzed by employing the NCEP/NCAR reanalysis data sets. The short-term eastward retreat of the WPSA is related to the EV in the upper troposphere in tropical regions. The structure and alteration of the EV before and after the eastward retreat of the WPSA are shown in this paper. The reduction of the positive vorticity and the enhancement of the north wind foreshow the withdrawal of the WPSA. The ascent motion of flow field in the west of the EV is strengthened. This may become one of the main mechanisms of the WPSA short-term eastward retreat. The diagnostic analyses of the complete vertical vorticity equation show that the WPSA retreated eastward abnormally when the two positive vorticity trend areas on the west side of the EV and on the east side of the vortex in the westerlies (WV) connected. The vorticity advection is the most important factor of the WPSA eastward retreat. The contribution to the vorticity trend from the ascent motion term was adverse to the vorticity advection term. Spatial non-uniform heating effect is opposite in the upper and lower troposphere. Therefore, the short-term eastward retreat of WPSA is influence by the EV, the mechanisms lie in dynamic effect and thermodynamic effect.