Contribution of East Indian Ocean SSTA to Western North Pacific tropical cyclone activity under El Nino/La Nina conditions

This study investigates, under El Nino/La Nina conditions, the effects of sea surface temperature anomaly (SSTA) in the East Indian Ocean (EIO) on tropical cyclone (TC) genesis over the western North Pacific (WNP), and a possible mechanism is proposed. It is found that the EIO SSTA exerts a secondary modulation to cyclogenesis relative to the El Nino-Southern Oscillation (ENSO), and the negative correlation between the EIO SSTA and WNP TC frequency is significantly weakened by the ENSO events. Under El Nino (La Nina) conditions, no matter what phases of EIO SSTA, TC genesis generally shows similar features with low (high) frequency in the western-central WNP (15 degrees-25 degrees N, 120 degrees-150 degrees E) and high (low) frequency in the southeastern WNP (5 degrees-15 degrees N, 150 degrees-170 degrees E). In particular, the effects of warm EIO SSTA on the WNP TC activity represent an asymmetry between El Nino and La Nina. That is, the warm EIO SSTA and El Nino share combined effects to decrease TC genesis over the western-central WNP. By contrast, under La Nina conditions, the influence of warm EIO SSTA on TC activity over the western-central WNP is partially drown out by La Nina. Meanwhile, weaker monsoon trough with intense anomaly of easterly jointly induced by the two factors suppresses TC genesis over the southeastern WNP.