Strengthened Combined Impact of the Pacific and Atlantic Meridional Modes on Eastern North Pacific Tropical Cyclones since the 1990s

Abstract There has been increased focus in recent years on the impact of the Pacific Meridional Mode (PMM) and the Atlantic Meridional Mode (AMM) on weather and climate events. This study shows an increased synergistic impact of both the PMM and AMM on eastern North Pacific (ENP) extended boreal summer (June-November) tropical cyclone frequency (TCF) since the 1990s. This increase in the combined impact of both the PMM and AMM on ENP TCF is mainly due to a stronger modulation of the AMM on TCF since the early 1990s and of a stronger modulation of the PMM on TCF since the late 1990s. A budget analysis of the genesis potential index highlights the important contribution of changes in vertical wind shear to the recent strengthened AMM-TCF relationship, while potential intensity and vertical wind shear are the two most important drivers of the recent increase in the PMM-TCF relationship. This intensified association is largely explained by changes in the mean state of sea surface temperatures in the tropical Atlantic associated with the Atlantic Muit-decadal Oscillation (AMO) and trade wind magnitude in the subtropical Pacific Ocean associated with the Pacific Decadal Oscillation (PDO). This study highlights an asymmetric effect of the AMO and PDO on these two meridional modes and ENP TC genesis frequency and provides a better understanding of ENP TC activity on interannual-to-decadal time scales.