The Impact of Canonical and Non-canonical El Niño and the Atlantic Meridional Mode on Atlantic Tropical Cyclones
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This project examines the roles of sea surface temperature (SST) variations in the tropical Atlantic and Pacific on Atlantic Tropical Cyclones (TCs, including hurricanes). The SST variations considered are those associated with two types of El Nino events, the traditional East Pacific (EP) events in which the largest SST warming occurs in the equatorial EP, and the non-traditional Central Pacific (CP) El Nino events in which the maximum warming occurs along the equator near the dateline. In addition, the work will consider the impact of SST anomalies associated with the Atlantic Meridional Mode (AMM), a meridional dipole pattern with warming north of the equator accompanied by cooling south of the equator and vice versa. Previous research associates La Nina events with enhanced Atlantic TC activity while El Nino events are associated with suppression of TCs in the Atlantic, and in particular the PIs estimate that the probability of one or more major hurricanes making landfall on the U.S. coast is 23% during El Nino compared to 63% during La Nina. However, the extent to which these relationships hold for both CP and EP El Nino events has not been determined, and the few studies that have been performed show conflicting results. The AMM is also believed to have an impact on Atlantic TCs, but the mechanism of this influence is not well understood, in part because the AMM modulates several environmental factors that cooperate in their influence on Atlantic TCs. The research tests three specific hypotheses, the first of which posits that the primary influence of the AMM on Atlantic TCs is through the cross-equatorial SST gradient associated with the SST anomalies, which modulates the circulation and thermodynamics of the overlying atmosphere. The second hypothesis is that the geographic difference between CP and EP El Nino events is of less importance than the strength of the SST anomalies, thus CP El Ninos are expected to have a smaller influence than EP events simply because the CP events tend to be weaker. the third hypothesis is that there is constructive interference between La Nina events and positive AMM excursions, so that the combination of two is extremely supportive of Atlantic TCs, while a similar destructive interference between El Nino and negative AMM results in near-average Atlantic TC activity. The bulk of the work for the project consists of numerical experiments using the Weather Research and Forecasting (WRF) model, configured as a Tropical Channel Model (TCM), meaning a re-entrant domain in the zonal dimension with meridional boundaries at 30S and 50N. The work has broader societal impacts due to the potential value of better understanding of relationships between Atlantic TCs and SST variability associated with El Nino and the AMM, which could be used to anticipate how active the Atlantic hurricane season will be in a given year. In addition, the work supports an early-career scientist from a traditionally underrepresented group.