Impact of Central American Orography on Simulation of Eastern North Pacific Hurricanes

The eastern North Pacific (ENP) basin (0-30°N, 140°W-80°W) has the highest density of tropical cyclone (TC) activity in the globe. On average, about 16 TCs form in this region during each hurricane season, accounting for 19% of global total number, 9 of which can further intensify into hurricanes. Understanding ENP TC genesis mechanisms is still a great challenge, as it involves complex interactions among tropical easterly waves, background cyclonic flows within the monsoon trough and gap winds due to the complexity of local topography. Using the Weather Research and Forecasting model (WRF) in a tropical channel configuration, we attempt to quantify the direct impact of Central American orography on ENP TCs. We performed two sets of simulations: 1) control simulations with realistic topography (CTL), and 2) no gap-wind simulations with artificial topography that closes mountain gaps at Gulf of Tehuantepec and Papagayo (NOGAP). The CTL captures the realistic gap winds variability and other regional circulations related to tropical cyclogenesis, and produces an average number of 11 simulated TCs per season. By simply closing the mountainous gap, NOGAP generates 26% less TCs than CTL. We note that this suppression comes primarily from the eastern development region (EDR), where the number of TCs decreases from about 6 TCs per season in CTL to about 4 TCs per season in NOGAP. Analyzing the environmental favorability for tropical cyclogenesis indicates that the decrease in low-level vorticity due to the absence of gap winds and decrease in mid-level humidity due to the change in topography-induced regional circulations are mainly responsible for the reduction in ENP TC numbers. This study sheds new light on the dynamical linkage between ENP TC activity and unique features of Central American orography and highlights the importance of resolving fine-scale topography in models to make skillful ENP TC simulations.