The different impacts of two kinds of Pacific Ocean warming on tropical cyclone frequency over the western North Pacific

The differences in modulation of El Niño Modoki and canonical El Niño on TC frequency over the western North Pacific (WNP) during the period June-October for 1960-2008 are examined. Based on the ENSO Modoki index and Niño-3 index, the partial correlation and composite technique are employed to reveal that TC frequency over the WNP is significantly positively correlated with ENSO Modoki index during the seasons of TC peak, especially the boreal summertime, which can be attributed to a large-scale cyclonic circulation anomaly in response to a wide range of WNP heating source related to El Niño Modoki. In contrast, during the canonical El Niño years, enhanced heating source and sink are zonally elongated and form a meridional dipole pattern over the tropical WNP, which induces an anticyclonic anomaly in the subtropics and a cyclonic anomaly near the equatorial central Pacific in the lower troposphere. As a result, a markedly negative (positive) correlation between TC frequency in the northern (southeastern) portion of the WNP and Niño-3 index exists, while a weak correlation appears in the context of the whole WNP basin. Using the realistic 3D mean state and heating profiles, the numerical experiments with a simplified baroclinic model validate that the anomalous circulation responses to heating forcing play essential roles in different modulations on TC frequency between El Niño Modoki and canonical El Niño events.