Multi-model Projection of Global Warming Impact on Tropical Cyclone Genesis Frequency over the Western North Pacific

This study examines the global warming impact on tropical cyclone (TC) genesis frequency over the western North Pacific basin (0 degrees-40 degrees N, 100 degrees E-180 degrees; WNP) projected by five atmosphere-ocean coupled general circulation models that participate in the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3), and exhibit high performances in simulating horizontal distribution of annual-mean frequency under the current climate condition. TC-like disturbances are detected and counted in simulations for the 20th-century climate experiment and global warming experiments. It is revealed that all of the five models project an increasing trend of the frequency in the eastern part of the analysis domain, especially over the central North Pacific (5 degrees-20 degrees N, 150 degrees E-180 degrees; CNP), and a decreasing trend in the western part, with a maximum decrease over the South China Sea (10 degrees-25 degrees N, 110 degrees-120 degrees E; SCS). The former increasing trend can be interpreted by analogy with interannual variability related to El Nino and Southern Oscillation (ENSO). This is because projected changes of sea surface temperature and large-scale circulation field exhibit an El Nino-like pattern, and on the other hand, more TCs are observed in the CNP during the El Nino phases. Relative vorticity in the lower troposphere and vertical wind shear would become more favorable for TC genesis, as in El Nino situation. The authors conclude that these two dynamic factors are major contributors to the projected increase of the frequency in the CNP. Over the SCS, projected environmental conditions are diagnosed as more favorable for TC genesis than the current ones, in spite of the decrease projection of the frequency. The authors discuss that the projected decrease may be associated with a projected weakening of the activity of tropical depression-type disturbance that can later be developed into TC.