Potential Vorticity Diagnostic Analysis on the Impact of the Easterlies Vortex on the Short-term Movement of the Subtropical Anticyclone over the Western Pacific in the Mei-yu Period

By employing NCEP−NCAR 1°×1°reanalysis datasets,the mechanism of the easterlies vortex(EV)affecting the short-term movement of the subtropical anticyclone over the western Pacific(WPSA)in the mei-yu period is examined using potential vorticity(PV)theory.The results show that when the EV and the westerlies vortex(WV)travel west/east to the same longitude of 120°E,the WPSA suddenly retreats.The EV and WV manifest as the downward transport of PV in the upper troposphere,and the variation of the corresponding high-value regions of PV significantly reflects the intensity changes of the EV and WV.The meridional propagation of PV causes the intensity change of the EV.The vertical movement on both sides of the EV is related to the position of the EV relative to the WPSA and the South Asian high(SAH).When the high PV in the easterlies and westerlies arrive at the same longitude in the meridional direction,the special circulation pattern will lower the position of PV isolines at the ridge line of the WPSA.Thus,the cyclonic circulation at the lower level will be strengthened,causing the abnormally eastward retreat of the WPSA.Analysis of the PV equation at the isentropic surface indicates that when the positive PV variation west of the EV intensifies,it connects with the positive PV variation east of the WV,forming a positive PV band and making the WPSA retreat abnormally.The horizontal advection of the PV has the greatest effect.The contribution of the vertical advection of PV and the vertical differential of heating is also positive,but the values are relatively small.The contribution of the residual was negative and it becomes smaller before and after the WPSA retreats.

Medium-Range Oscillations in the Summer Tropical Easterlies at 200 hPa

By the use of space-time spectral analysis and band-pass filter, some of the features of the medium-range oscillations in the summer tropical easterlies (10 S-20 N) at 200 hPa are investigated based on a two-year (1980 and 1982) wind (u, v) data set for the period from May to September. Space-time power spectral analysis shows that the total energy of the westward moving waves was the largest and that of the standing waves and eastward mov ing waves was relatively small in the 200 hPa easterlies: the total energy of the eastward moving waves was m minimum at 10° N. Three kinds of the medium-range oscillations with about 50 day, 25 day and quasi-biweekly periods were found in the easterlies, which all show a remarkable interannual variations and latitudinal differences in these two years. The wave energy of zonal wind is mainly associated with the planetary waves (1-3). which all may make important contributions to the 50 day and 25 day oscillations in different years or different latitudes, The quasi-biweekly oscillation is mainly related to the synoptic waves (4-6). In equatorial region, the 50 day oscillation was dominant with a eastward phase propagation in 1982 while the dominant oscillation in 1980 was of 25 day period with a westward phase propagations in 1980. Both of them are of the mode of zonal wavenumber 1. Strong westward 50 day oscillation was found in 10° N-20° N in these two years. Regular propagations of the meridional wind 50 cay oscillation were also found in the easterlies.The 50 day and 25 day oscillation of zonal wind all demonstrate southward phase propagation over the region of the South Asia monsoon and northward phase propagation near international date line, where are the climatic mean position of the tropical upper-tropospheric easterly jet and the tropical upper tropospheric trough (TUTT). respectively.

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 horizontal advection term is the maximum, and the minimum is caused by theβ effect. It is also found that enhanced horizontal vorticity advection andβ 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.

Barotropic Process Contributing to the Formation and Growth of Tropical Cyclone Nargis

This study reveals the barotropic dynamics associated with the formation and growth of tropical cyclone Nargis in 2008,during its formation stage.Strong equatorial westerlies occurred over the southern Bay of Bengal in association with the arrival of an intraseasonal westerly event during the period 22-24 April 2008. The westerlies,together with strong tropical-subtropical easterlies,constituted a large-scale horizontal shear flow,creating cyclonic vorticity and thereby promoting the incipient disturbance that eventually evolved into Nargis.This basic zonal flow in the lower troposphere was barotropically unstable,with the amplified disturbance gaining more kinetic energy from the easterly jet than from the westerly jet during 25-26 April. This finding suggests that more attention should be paid to the unstable easterly jet when monitoring and predicting the development of tropical cyclones.Energetics analyses reveal that barotropic energy conversion by the meridional gradient of the basic zonal flow was indeed an important energy source for the growth of Nargis.