The Decadal Shift Of Pre-monsoon Season Tropical Cyclone Activities Over The North Indian Ocean And The Western North Pacific In Late1990s

Based on the tropical cyclone (TC) data from IBTrACS (International Best TrackArchive for Climate Stewardship), the ERA-interim reanalysis data from European Centrefor Medium-Range Weather Forecasts (ECMWF), the reanalysis HadISST from HadleyCentre for Climate Prediction and Research and the CMAP precipitation data from ClimatePrediction Center (CPC), National Centers for Environmental Prediction (NCEP), thisthesis analysis the responses of the TC activities over the North Indian Ocean and thewestern North Pacific to the decadal shift of the Asian summer monsoon circulation. Therelationship between the TC activities and large-scale circulations has been investigatedand the mechanism by which the monsoon circulation effect on TC activities is explained.The roles of landmass-Indian Ocean heat contrast and Pacific SST decadal change in thedecadal shift of the monsoon circulation also have been investigated through bothreanalysis data and ECHAM model data. The main results of these analysis are as following:(1) The enhanced summer monsoon circulation in May after the year1999provide morefavorable low level vorticity and humidity conditions for TC activities, induced the earlierpeak season of the genesis potential and the earlier genesis time of tropical cyclones overthe Arabian Sea. The mean TC genesis date change from June5thto May20th，so the TCambient maximum potential intensity (MPI) and vertical wind shear (VWS) are morefavorable for TC intensification after1999, as the result TCs has been stronger after1999.（2）the change in monsoon circulation did not change peak season of GPI and the meanTC genesis time over the Bay of Bengal. This is because the peak season of GPI and themean TC genesis time have already in mid-May before1999, the circulation shift in Maycannot make them earlier. Thus none of the TC ambient vertical VWS, MPI or TC intensityhas significant decadal shift.（3）the TC activity of the WNP in May also shows a decadalshift around1999. Both the TC number and TC intensity have significant increasing after1999.(4) The increased TC number is due to larger GPI over the WNP after1999, which is mainly caused by the relative humidity change in mid troposphere. The TC ambient MPIand VWS are more favorable for TC intensification after1999and induce the increasingof TC intensity. The changes of TC ambient MPI and VWS are because both the changesin background circulation and the shift of TC tracks.(5) Both of the South China Sea andthe South Asian summer monsoon circulation are important for the decadal shift of TCactivity over the WNP.(6) The land surface has been much warmer after1999than beforeand this makes the landmass-Indian Ocean heat contrast more favorable for summermonsoon onset. At the same time, the SST gradient over the tropical Pacific also shows asignificant decadal shift in1999. Both of the decadal shift in landmass-Indian Ocean heatcontrast and SST pattern in tropical Pacific are important for the enhancement of thesummer monsoon circulation in May after1999. However, the landmass-Indian Ocean heatcontrast is more important for the Arabian Sea atmosphere circulation while the tropicalPacific SST pattern is more important for the circulation over the Bay of Bengal and SouthChina Sea.