Study On The Mechanism Of Tropical Cyclone Activity Change Over The Western North Pacific Under The Backgrourld Of Global Warming

Considering uncertainty in relationship between the tropical cyclone (TC) activity and global warming, a TC downscaling model is developed in this study associated with the TC best track datasets and large-scale environmental fields for understanding the physical mechanism of the interannual and decadal variations of TC activity, examining the key factors affecting the variability of TC track and intensity and assessing the relative reliability of TC intensity records among different warning centers over the western North Pacific (WNP). The main results obtained are as follows:1. The TC theoretical model system including the TC formation,TC track and TC intensity models is developed and the ability of the model system is examined on the interannual, decadal and climate timescales, which provides a useful way for understanding the possible mechanism of TC activity over the WNP basin under the background of climate change, assessing the quality in TC records, forecasting seasonal TC activity and examining the ability of climate models in simulating environmental fields affecting TC activity.2. Using the developed TC theoretical model system, the relative reliability of TC intensity records among the three best track datasets (Joint Warning Typhoon Center-JTWC, the Regional Specialized Meteorological Center of Tokyo-RSMC and Shanghai Typhoon Institute of China Meteorological Administration-STI) is assessed. Results show the TC intensity record from the JTWC best track dataset is relative reliable among the three TC datasets. The model can capture the annual variations and long-term trend in the JTWC dataset, while these variations in the RSMC and SIT best track dataset can not simulate.3. Using the developed TC theoretical model system, the key factors affecting the interannual variations of TC track and intensity is examined. Results show that:1) The combined change of TC formation locations and large-scale steering flows significantly affect westward TC prevailing TC tracks, while the change of the west-northward prevailing TC tracks is mainly affected by the change in large-scale flows,2) The change of TC tracks appears to be the most important factor in TC intensity change associated with the variations of SST and shear under the different ENSO conditions.4. Using the developed TC theoretical model system, the key factors affecting the decadal variations of TC tracks and intensity is further investigated. Results show that:1) The contributions of TC formation locations and large-scale steering flows to the TC track change show a consistent pattern, where the contributions of large-scale flows appears to be dominant, 2) Similiar to that on the interannual time scale, the TC track changes remain to be the most important factor for the decadal variations of TC intensity, which is closely associated with the significant SST warming (cooling) over the tropical central/eastern Pacific that induces, to the west of the main warming (cooling) region, a localized intense cyclonic (anticyclonic) area in the low level and a anticyclonic (cyclonic) area in the upper level. These are favorable (unfavorable) for TC formations in the southeast quadrant of the WNP and then more(less) TCs have longer (shorter) time for intensification and develop into Cat45 TCs.