Present Simulation and Future Projection of Tropical Storm Activity Over the Western North Pacific in 20-km HiRAM and MRI Climate Models

Tropical storm (TS) activity at the present time (1979-2003) and future climate (2075-99) are simulated and projected from High Resolution Atmospheric Model (HiRAM) and Meteorological Research Institute (MRI) at 20-km resolution. The results show that TS activities except intensity over the western North Pacific (WNP) region are well simulated by HiRAM and MRI at 20-km resolution. Simulated TS numbers and genesis potential index (GPI) are closely related to each other. The GPI result in this study is dramatically different from the findings in CMIP5 low resolution models which show poor relationship between GPI and total TS numbers in the models.

     There are better agreements in the projection of TS activity change in the WNP between HiRAM and MRI high resolution models. During 2075 to 2099, both TS genesis numbers and TS frequency over the WNP are projected to decrease consistent with the IPCC AR5 report. However, the rate of decrease simulated by HiRAM and MRI at 20 km is much greater than that projected in IPCC AR5. TS intensity and the maximum precipitation rate are projected to increase under global warming.

    The changes in GPI contributed from individual large-scale variables are investigated to assess the mechanisms responsible for the decrease of TS genesis frequency during 2075-2099. The ensemble GPI results show that the decrease in GPI under global warming is primarily attributed to the reduction of mid-level relative humidity and large-scale ascending motion, despite that warm SST provides favorable conditions for TS formation.