Case Study on the Impact of the Vortex in the Easterlies in the Tropical Upper Troposphere on the Western Pacific Subtropical Anticyclone

By employing the NCEP/NCAR reanalysis data (1000–10 hPa, 2.5°× 2.5°), the impact of the vortex in the easterlies (EV) over the tropical upper troposphere on the zonal movement of the western Pacific subtropical anticyclone (WPSA) during 19–25 June 2003 is analyzed in this paper. It is shown that the EV can extend from middle troposphere to the height of 50 hPa, reaching a maximum at 200 hPa. The vertical thermal distribution appears to be "warmer in the upper layer and colder in the lower layer". The WPSA retreats eastward abnormally when the EV and the vortex in the westerlies (WV) encounter around the same longitude while they move toward each other. It is also shown that the vorticity variation extends from the troposphere to the height of 50 hPa, with the most prominent change occurring at 200 hPa by the diagnostic analyses of the vertical vorticity equation. The WPSA appears to retreat abnormally eastward while the negative/positive vorticity change becomes stronger near the east/west side of the EV, and the areas with positive vorticity tendency both in the EV and WV join together into one belt along 130°E during the process of the EV and the WV moving toward each other. In the vorticity equation, the positive contribution caused by the hotizontal advection term is the maximum, and the minimum is caused by the β effect. It is also found that enhanced horizontal vorticity advection and β effect, as well as the "barotropic development" resulted from the in-phase superposition of the southerly and the northerly winds in the easterlies and westerlies near 130°E, are in agreement with the WPSA eastward retreat.