Relationship between Antarctic Oscillation and the genesis activity of the yearly latest tropical cyclone in the western North Pacific

Abstract

AbstractWe find a significant negative correlation between the Antarctic Oscillation (AAO) and the genesis longitude of the last TC of the year (hereafter the last TC) in the western North Pacific (WNP). We also find that the mean longitude of the last TC genesis has experienced a significant westward shift since 1997–1998. That means both a positive AAO phase and AAO phase of the recent epoch (1998–2015) favour the last TC occurring more frequently in the western part of the WNP. Thus, we compared the differences between the positive and negative AAO phases on interannual time scale and the epochal differences between the 1998–2015 and 1983–1997 periods to find the common factors responsible for the more frequent occurrence of the last TCs in the western WNP. The common differences in the 850 hPa stream flows reveal that an anomalous cyclone occurs in the South Indian Ocean, whereas an anomalous anticyclone appears in the WNP. The anomalous northerlies from the Southern Indian Ocean anomalous cyclone move northward, enhancing the cross‐equatorial flows and associated westerlies. The enhanced anomalous westerlies extend eastward and meet the anomalous easterlies originated from the WNP anomalous anticyclone at the South China Sea (SCS). As a result, the anomalous cyclones are strengthened in the SCS and around the Philippines. Further analysis of the differences in the outgoing longwave radiation, 850 hPa relative vorticity, 200–850 hPa vertical wind shear, sea surface temperature, horizontal divergence, 500 hPa omega and velocity potential and divergent winds between the two phases and the two epochs confirm that favourable environments for generation of the last TC take place in the western WNP during the positive AAO phase and in the 1998–2015 epoch.