Distinct Impacts of Two Types of Spring North Pacific Meridional Mode on North Pacific Tropical Cyclone Genesis Frequency during the Following Summer and Autumn

Abstract The Pacific Meridional Mode (PMM) is a dominant air-sea interaction pattern in the subtropical northeastern Pacific. It exerts a significant influence on the tropical cyclone (TC) activity in the North Pacific. This study reveals that the spring PMM can be classified into two types: the subtropical North PMM (SNPMM) and the central tropical PMM (CTPMM). The key difference between SNPMM and CTPMM lies in the sea surface temperature (SST) anomalies pattern. SST anomalies associated with the SNPMM are confined to the subtropics, whereas those associated with the CTPMM can spread to the tropical Pacific. Further analysis shows that the SNPMM exerts little influence on TC genesis frequency (TCGF) in the tropical North Pacific. In contrast, the CTPMM suppresses TCGF in summer and autumn over the northwestern West Pacific, but promotes it over the southeastern West Pacific (SE) and northwestern East Pacific (ENW). The impact of the spring CTPMM on the tropical North Pacific TCGF during the following summer and autumn is mainly through inducing SST anomalies in the tropical Pacific. Eastward shift of the tropical Pacific Walker circulation and cold SST anomalies in the tropical western Pacific related to the CTPMM result in negative potential intensity anomalies over the northwestern West Pacific, and thus suppress TCGF therein. However, the CTPMM-related tropical SST anomalies favor TCGF in the SE and ENW regions by weakening vertical wind shear and enhancing low-level vorticity. The physical mechanism by which the CTPMM influences TCGF was further confirmed by numerical experiments. This study highlights that not all PMM events can influence TCGF in the North Pacific.