Impact of Different ENSO Regimes on Southwest Pacific Tropical Cyclones

The influence of different types of ENSO on tropical cyclone (TC) interannual variability in the central southwest Pacific region (5 degrees-25 degrees S, 170 degrees E-170 degrees W) is investigated. Using empirical orthogonal function analysis and an agglomerative hierarchical clustering of early tropical cyclone season Pacific sea surface temperature, years are classified into four separate regimes (i.e., canonical El Nino, canonical La Nina, positive-neutral, and negative-neutral) for the period between 1970 and 2009. These regimes are found to have a large impact on TC genesis over the central southwest Pacific region. Both the canonical El Nino and the positive-neutral years have increased numbers of cyclones, with an average of 4.3 yr(-1) for positive-neutral and 4 yr(-1) for canonical El Nino. In contrast, during a La Nina and negative-neutral events, substantially fewer TCs (averages of similar to 2.2 and 2.4 yr(-1), respectively) are observed in the central southwest Pacific. The enhancement of TC numbers in both canonical El Nino and positive-neutral years is associated with the extension of favorable low-level cyclonic relative vorticity, and low vertical wind shear eastward across the date line. Relative humidity and SST are also very conducive for genesis in this region during canonical El Nino and positive-neutral events. The patterns are quite different, however, with the favorable conditions concentrated in the date line region for the positive-neutral, as compared with conditions farther eastward for the canonical El Nino regime. A significant result of the study is the demonstration that ENSO-neutral events can be objectively clustered into two separate regimes, each with very different impacts on TC genesis.