Influence Of The Cold Air Activity Over Australia On The Tropical Cyclone Genesis In The Western North Pacific

Based on the NCEP/NCAR reanalysis products, NOAA Extended Reconstructed SST and the best track data of the typhoons provided by China Meteorological Administration, the key area of the influence of the cold air activity over Australia on the low latitudes was determined, and a cold air intensity index of Australia cold air was defined. The influence of the Australia cold air activity on the tropical cyclone genesis over the western North Pacific was investigated by using composite, correlation coefficient analyses and case diagnose. The major conclusions are summarized as follows:(1) The Solomon Sea is the key area where the Australia cold air intrudes into low latitudes. The meridional wind over the Solomon Sea is the strongest meridional wind, moreover, a high value center of interannual and daily standard deviation of meridional wind over the tropical western North Pacific (WNP) at 925hPa in summer. The Solomon Sea V-wind (SSV) is closely related to the cross-equatorial flows and westerlies over the Equator as well as the SOI. As SOI is weaker (stronger), The maritime continent and the tropical Pacific Ocean has a High-East Low-West (Low-East High-West) pressure distribution pattern. The meridional wind over the Solomon Sea (SSV) is stronger (weaker). Although SSV is mainly composed by divergent wind, the rotation wind accounts for the majority of its interannual variance.(2) The location of the tropical cyclone genesis over the western North Pacific coincides with the variation of the monsoon trough during the years of strong and weak SSV. The monsoon trough over WNP is stronger and located eastward than normal, the location of the tropical cyclone genesis is eastward and southward than normal during years of strong cold air activity, while the monsoon trough is weaker and located westward than normal, the location of the tropical cyclone genesis is westward and northward than normal during years of weak cold air activity. The distribution of vorticity in lower troposphere, the moisture transport, the vertical shear of horizontal wind, the convections in low latitudes and the barotropic perturbation energy tendency indicates that strong SSV leads to strong WNP summer monsoon, vice versa. The locations of the tropical cyclone over WNP are eastward and southward (westward and northward) than normal during strong (weak) years of cold air activities over Australia. Anomaly cold air flux and divergence caused by strong SSV maybe the reason that there is a strong(3) A case study shows that the cold air outbreak makes changes of mid to low latitude circulation almost at the same time. At the eastern coast of Australia the formation of southerly conveying belt causes cold air advection from south and divergence flows. Therefore SSV is strengthened, and an equatorial anticyclone forms to its east. At the north of the equator, low level convergence, cyclonic vorticity, potential pseudo-equivalent temperature are enhanced, so as to facilitate the tropical cyclone genesis.