Impacts of the Pacific SST Anomaly on the Subseasonal to Seasonal Tropical Cyclone Activity: Case Study of the 2018 Boreal Summer

The tropical cyclone (TC) activity in western North Pacific (WNP) is modulated by intraseasonal oscillation (ISO) under the background conditions associated with the sea surface temperature (SST) anomalies. Previous studies demonstrated the significant impacts of the SST anomalies in the subtropical central Pacific associated with the Pacific Meridional Mode (PMM) on the WNP TC activity. The ISO generally facilitates the TC genesis through the moistening in its active phases. The 2018 boreal summer is characterized by warm SST anomalies in the subtropical Pacific, as well as in the tropical eastern Pacific and midlatitude WNP, which is accompanied with strong monsoon westerlies prevailing in the warm pool region. Under this condition, TC activity is enhanced in the broad domain over the WNP. We focus on how the regional Pacific SST anomalies modulate the TC activity in the subseasonal time scale, as well as in the seasonal-mean. Based on sensitivity simulations varying the SST anomalies in the subtroipical, tropical, and midlatitude WNP subdomains, we found that the SST anomalies affect the zonal distribution of the TC activity through modification in the large-scale vertical circulations. In the active period of the ISO, overall enhancement of TC activity is found in all the sensitivity simulations, and specifically, the eastward shift of the peak TC activity as a response to the warm SST anomalies in the subtropical Pacific is relaxed by the ISO related circulations and moistening.