Impacts of the combined modes of the tropical Indo-Pacific sea surface temperature anomalies on the tropical cyclone genesis over the western North Pacific

This work presents two combined modes of the summer sea surface temperature anomalies (SSTAs) in the tropical Indo-Pacific Ocean during 1979-2015 and investigates their influences on the tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP). The first mode (referred to as ENSO-IODM) features Indian Ocean dipole (IOD) SSTAs in the tropical Indian Ocean (TIO) and El Nino-like SSTAs in the tropical Pacific Ocean (TPO) and often appears during the ENSO phase persistence summers. It usually triggers an anomalous cyclone and more TCs over the southeastern WNP together with an anomalous anticyclone and fewer TCs over the northwestern WNP. These are primarily attributed to the effect of the El Nino-like SSTAs over the TPO. The second mode (referred to as ENM-IOBM) is characterized by Indian Ocean basin (IOB) positive SSTAs in the TIO and La Nina Modoki-like (ENSO Modoki; ENM) SSTAs in the TPO and often presents during the ENSO phase transition summers. The combined effects of the IOBM and ENM SSTAs induce an anomalous anticyclone over the WNP that significantly suppresses TC genesis over the central WNP. The impacts of the two combined modes on the WNP TCGF also reveal inter-decadal changes with significant weakening of the ENSO-IODM and strengthening of the ENM-IOBM around the early 1990s. These are attributed to the weakened variability of the ENSO-IODM and intensified variability of the ENM-IOBM around the early 1990s, and show largely association with the changes in the ENSO cycle. Most of the ENSO events prior to the early 1990s show a long decaying phase with persistence features but exhibit a short decaying phase even with phase transition after the early 1990s. This brings more ENSO-IODM structures in the summer TIPO SSTAs before the early 1990s and more ENM-IOBM structures during the latter period, and thereby results in such above inter-decadal changes.