Analysis on the cause of climatic drift of tropical cyclone rapid intensification area

Based on the tropical cyclone data from Shanghai Typhoon Institute of China Meteorological Administration (CMA-STI) and the NCEP reanalysis monthly data for the period of 1949-2015, the influence of climatic drift of rapid intensification (RI) concentrated area in tropical cyclone (TC) activity in the Northwestern Pacific Ocean has been analyzed statistically. The results are shown as follows. (1) The frequencies of RI process and TC with RI process occurring both declined gradually. Additionally, the proportions of RI process duration to TC lifetime and TC with RI process occurring to total TC decreased. (2) The northernmost edge moved southward, the southernmost edge moved northward and the whole RI concentrated area moved northward and shrunk. The westernmost edge moved eastward, the easternmost edge moved westward and the whole RI concentrated area moved westward and shrunk. (3) The domain of weak vertical wind shear drifted westward and southward, and the domain of positive anomalies of sea surface temperature (SST) extended southward. These are the two possible reasons for the southward movement of the northernmost latitudes when RI process occurs in TC activity in the Northwestern Pacific Ocean. Meanwhile, the northward movement of southernmost latitudes may be closely related to the northward expansion of the high SST. (4) The combined effects from both the enhanced air divergence at 200 hPa and the intensive transport of water vapor at 850 hPa in the concentrated area also have significant influence on the climate drift in RI concentrated area.