Seasonal Modulation of Tropical Cyclone Occurrence Associated with Coherent Indo-Pacific Variability during Decaying Phase of El Nino

In this paper, the response of tropical cyclone (TC) activity to El Nino-Southern Oscillation (ENSO) and coherent sea surface temperature anomaly (SSTA) in the Indian Ocean (IO) is investigated, with a particular focus on the decaying phase of El Nino. The TC anomalies are obtained from the database for Policy Decision making for Future climate change (d4PDF). This dataset is based on 100-member ensemble simulations for the period of 1951-2010 using the state-of-the-art atmospheric general circulation model (AGCM) forced with the observed SST as well as the historical radiative forcing. The AGCM utilized in the d4PDF is the Meteorological Research Institute Atmospheric General Circulation Model (MRI-AGCM) with about 60 km horizontal resolution. Our analysis revealed a prolonged decrease in TC frequency (TCF) over the tropical Western Pacific during the post-El Nino years until the boreal fall. Dominance of anomalous anticyclone (AAC) over the Western Pacific induced by the delayed warming in the tropical IO is the main factor for the suppressed TC activity rather than the local SST change. In contrast, the TC number over the South China Sea tends to increase during the post-El Nino fall (September to November). The physical reason can be ascribed to the weakening of the AAC associated with the termination of IO warming. Thus, we demonstrate that the effect of the IO warming should be taken into account when the ENSO is considered as an environmental factor for predicting TC activity.