Characteristics Of Potential Vorticity And Thermodynamic Mechanism Of Impact Of Easterlies Vortex On The Abnormally Retreat Of The Western Pacific Subtropical Anticyclone

By employing the NCEP/NCAR reanalysis data sets and the precipitation data, using cases analysis, dynamic and thermodynamic diagnostic analysis, the characteristics of structure and evolution of the easterlies vortex (shortened as EV) over the tropical upper troposphere relating to the east-west direction shift of the subtropical anticyclone over the Western Pacific Ocean (shortened as WPSA) are analyzed in this project. The relationship and regulation between the east-west direction movement of the WPSA and the EV over the tropical upper troposphere are shown. By using the complete vertical vortex equation and the potential vorticity thinking, the thermal forcing impact and the relevant mechanisms of EV on the quasi-horizontal movement of WPSA are analyzed. The potential vorticity forcing anomaly from the high troposphere of EV relating to the short term movement of the WPSA are discussed as well.The characteristics of distribution and evolution of the potential vorticity (PV) located in the easterlies vortex (EV) and in the westerlies vortex during the east-west direction movement of the subtropical anticyclone over the Western Pacific (WPSA) are composite and diagnostic discussed. In addition, based on the non-adiabatic heating effect, the relation between the PV anomaly and the eastward retreat of WPSA is analyzed; by PV equation, the impact factors of the PV change are diagnosed as well. Results show that:the movement and development of EV are related to the positive PV anomaly in the troposphere. EV trends in direction of the increasing PV. The increasing intensity in the center of EV and vertical expansion are due to the enhancement of PV in the upper and lower level. Strong cooling in the east side of EV in the upper level cause the positive PV anomaly, in the meantime, weak heating under EV accelerate PV beneath it, which, joining with upper level downward PV anomaly, is a cause of expanding EV. The WPSA retreats eastwards when EV in the south side of WPSA and the vortex in the westerlies in the north side encounter in the vicinity of the same longitude while they move in the opposite directions, and then high PV meridionally connect at500hPa～600hPa level, as well as PV nearly EV thrusts into the ridge of WPSA. The PV advection is the main contribution to the positive PV anomaly, the non-adiabatic heating effect is also important.Case analysis shows as follows. The atmosphere non-adiabatic heating effect generated by the vertical transport and local change discussed above is mainly connected to the retreat of WPSA. It can be found that the distribution and intensity of the non-adiabatic heating effect nearby the easterlies vortex can trigger to the short-term retreat eastward of the WPSA. The WPSA prefers to extending to the colder region, that is the WPSA moves toward the region in which the heating is weakening or the cooling is strengthening. During this period of the WPSA retreat, the non-adiabatic heating effect changes largely within the east and west sides of the EV; the thermal character is the enhancement of cooling in the east side of the EV, meanwhile, the effect in the west side exhibits to the opposite trend, with the most prominent effect at300hPa in the troposphere. The evidence on the factors causing to the change of thermal condition show that the most important contribution to the heating rate trend is the vertical motion effect, the second is the effect of the local change of the heating rate, and the horizontal advection effect is the third.In a word, the mechanism of the atmospheric inner and outer forcing triggered by the EV to the short term movement of the WPSA is described. A possible conceptual model is concluded revealing the thermodynamic relationship between the EV and the eastward retreat of the WPSA. The modification of thermodynamic structure always causes the change of meridional wind. In the upper troposphere, the enhancements of the north wind and the ascent motion in the west side of the EV, and the development of the south wind in the east side, benefit the abnormal eastward retreat of WPSA. Therefore, the east-west direction shift of the WPSA has a close relation with thermodynamic effect of the EV over the tropical region.