Impacts of zonal SST gradients on subtropical highs and implications for early season tropical cyclone landfall frequency

Zachary Freitag Johnson , Daniel Chavas, Jhordanne Jones, Yoshimitsu Chikamoto, Hamish Ramsay

Tropical cyclone (TC) seasonal landfall probability is challenging to forecast because of the limited seasonal predictability of steering flow patterns. Past studies mainly focus on the large-scale ocean and atmospheric conditions that lead to changes in seasonal tropical cyclone genesis frequency in a given basin, but less attention has focused on seasonal landfall probability inherent to changes in steering flow patterns linked to subtropical highs (STHs). Here, we examine SST anomaly patterns that control variability in summertime STH cells in the northern and southern hemispheres. We link those ocean impacts to changes in early-season TC landfall probability in the Western North Pacific, North Atlantic, and South Indian Ocean basins. STHs in the North Pacific, North Atlantic, and South Indian Ocean exhibit increased variability on their western peripheries linked to anomalous zonal SST gradients. In the northern hemisphere, an inter-basin zonal contrast in SST anomalies fosters a westward extension in both North Pacific and North Atlantic STHs. As a result, TCs curve around STHs 6 degrees in longitude farther west in the Western North Pacific basin. In contrast, the Atlantic basin had the opposite effect due to minimal TC activity over the tropical Atlantic from inhibiting SST anomalies. The South Indian Ocean had a 9% increase in landfall probability for TCs that formed in the western half of the Southern Indian Ocean during a positive localized SST dipole. The seasonal persistence of southern hemispheric STHs resembles aquaplanet simulations of STHs, in contrast to the seasonal evolution observed in their northern hemispheric counterparts.