Interdecadal variability of Western North Pacific tropical cyclone activity

This study investigates the relationship between the interdecadal variation of the WNP annual TC activity and the springtime oceanic and atmospheric conditions of the NP. The annual TC activity shows significant interdecadal variations, with active periods during 1962-75 and 1989-97 and inactive period during 1976-88. The interdecadal variation of the annual TC activity is partly linked to the decadal variability of the NP and the overlying atmosphere, which can be reflected by the March - May SST anomalies in the area 8°-14°N, 170°E-155°W (SSTA index) and the 850-hPa geopotential height anomalies in the area 15°-30°N, 160°E-160°W (H850A index). Negative (positive) H850A index and positive (negative) SSTA index are generally associated with active (inactive) annual TC activity. The March-May SST pattern is largely related to the EOF1 (PDO pattern), EOF2 (Victoria pattern) and EOF3 of the anomalous SST field while the anomalous atmospheric circulation pattern is mainly related to the EOF1 (PDO pattern) and EOF2 (EP-NP pattern) of the anomalous 850-hPa geopotential height field. The phase and the magnitude of these patterns can be reflected by the SSTA and H850A indices respectively. These indices also serve as the proxies for the atmospheric conditions over the WNP during the TC season. Positive SST anomalies in the tropical central NP and a north-south dipole of the 850-hPa geopotential height anomalies, with a positive center in the north and a negative center in the south, over the NP are generally associated with active TC season. Such anomalies generally persist into the peak TC season and lead to an anomalous cyclone over the subtropical WNP and westerly anomalies over the tropical WNP. As a result, the stronger monsoon trough and weaker subtropical high lead to the favorable atmospheric conditions for the TC formation over the tropical WNP.