Reduced Effectiveness of Atlantic Hurricane Suppression During Central Pacific El Niño

Abstract:

The occurrence of unusually warm tropical East Pacific sea-surface temperature (SST) during El Niño is an important predictor of seasonal Atlantic tropical cyclone (TC) activity. In recent decades El Niño has been characterized more often by Central Pacific Ocean warming with weaker amplitude, and there is no consensus regarding how this shift in location of ocean warming impacts Atlantic TCs due to a short data record that is complicated by Atlantic SST variability. We attempt to resolve this issue using large ensembles of simulations with a tropical-extratropical channel model in order to isolate the El Niño influence from the confounding effects of Atlantic SST and stochastic atmospheric variability. We find that although Central Pacific El Niño significantly suppresses Atlantic tropical cyclones, it is less effective than East Pacific El Niño at doing so by about 50%, as measured by accumulated cyclone energy. Still, Atlantic TCs are suppressed regardless of maximum Pacific Ocean warming location and amplitude because both El Niño types are characterized by sufficient warming east of the Pacific warm pool, which satisfies the SST threshold for deep convection leading to TC suppression via tropical Atlantic vertical wind shear enhancements. We note the importance of stochastic atmospheric variability in contributing to variability in TC activity, as there is a considerable range between the maximum and minimum ACE in the ensemble of experiments for each El Niño type, despite having fixed SST and lateral boundary conditions. This large stochastic variability may explain some of the inconsistencies in observational analyses, as it is clear that analysis of an insufficient sample size could easily produce a misleading result that Central Pacific El Niño drives an increase or no significant change in Atlantic TC activity.