Tropical cyclone genesis potential index over the western North Pacific simulated by LASG/IAP AGCM

Abstract

Tropical cyclone genesis potential index (GPI) is a useful metric for gauging the performance of global climate models in the simulation of tropical cyclone (TC) genesis. The performance of LASG/IAP AGCM GAMIL2.0 in the simulation of GPI over the western North Pacific (WNP) is assessed in this paper. Since GPI depends on large scale environmental factors including low-level vorticity at 850 hPa, relative humidity at 700 hPa, vertical wind shear between 850 and 200 hPa, maximum potential intensity (MPI), and vertical velocity, the bias of GPI simulation is discussed from the perspective of thermal and dynamical factors. The results are compared with the ECMWF reanalysis data (ERA40). The analyses show that both the climatological spatial pattern and seasonal cycle of GPI over the WNP are reasonably simulated by GAMIL2.0, but due to the overestimation of relative humidity, the simulated GPI extends to 170°E, about 10° east to that in the reanalysis data. It is demonstrated that the bias in the simulation of monsoon trough, which is about 5° north to the reanalysis, leads to an overestimation of GPI during May–June and September–October, but an underestimation during July–August. Over the WNP, the response of GPI to ENSO is well captured by GAMIL2.0, including the eastward (westward) shift of TC genesis location during El Nino (La Nina) years. However, the anomalous convective center associated with El Nino shifts westward about 20° in comparison to ERA40, which leads to the biases in both vertical velocity and relative humidity. These eventually result in the westward deflection of the boundary between the positive and negative GPI centers along 20°-30°N. The results from this study provide useful clues for the future improvement of GAMIL2.0.