On the relationship between ENSO and tropical cyclones in the western North Pacific during the boreal summer

The present paper uses the satellite era data from 1979 to 2015 to examine the relationship between El Niño-Southern Oscillation (ENSO) and tropical cyclones (TCs) in the western North Pacific (WNP) during the boreal summer from June to August. It is found that WNP TC variability is characterized by two major feature changes: (1) a significant reduction of the TC number since 1998 and (2) a stronger interannual relationship between ENSO and TCs since 1998. Results suggested that such changes are largely due to the synergy effects of a shifting ENSO and the Pacific climate regime shift. Since 1998 with a cool Pacific decadal oscillation phase switching from a warm phase, more La Niña and central Pacific (CP) El Niño events occur. The decreased low-level relative vorticity and increased vertical wind shear during 1998–2015 compared to 1979–1997 are responsible for the TC reduction. The stronger interannual relationship between ENSO and TCs since 1998 is closely associated the change of CP sea surface temperature. It enhances the associations of environmental factors including vertical wind shear and mid-level relative humidity with TCs and thus increases the interannual relationship between ENSO and TCs. These two feature changes also manifest in the mean TC genesis location, with a northwestward shift of the TC genesis location during 1998–2015 and an increased relation to El Niño Modoki index since 1998. This study has an important implication for TC outlooks in the WNP based on climate predictions and projections.