Hurricane Patricia in 2015 was the second-strongest tropical cyclone on record worldwide, with a maximum surface wind speed of 215 mph (~ 96m s-1). Although Patricia fortuitously spared major cities, it reminded us of the threat tropical cyclones (TCs) pose in the eastern North Pacific (ENP) and the importance of improving our understanding and prediction of ENP TCs. Patricia's intensity and the hyperactive 2015 ENP hurricane season have been partially attributed to the strong El Nino; however, there is still a lack of fundamental understanding of the relationship between the El Nino-Southern Oscillation (ENSO) and ENP TCs. In addition to being an active TC region, the ENP is also characterized by a unique topographically-locked feature, the Central American gap winds (CAGW). Here, we demonstrate that ENSO modulates ENP TCs in distinct spatial patterns, with enhanced (reduced) TC occurrence in the western portion of the ENP during El Nino (La Nina), but reduced (enhanced) TC occurrence in the eastern nearshore area, where landfalling TCs preferentially form. This intrabasin difference is primarily driven by ENSO-induced anomalous CAGW variability, which intensify (weaken) during El Nino (La Nina), producing low-level anticyclonic (cyclonic) relative vorticity anomalies and thus an unfavorable (favorable) environment for TC genesis. Besides ENSO-related CAGW's modulation, the synoptic CAGW events also have great influences on ENP TCs. Many satellite observations reveal that CAGW-induced cyclonic vorticity can trigger cyclogenesis especially when it interacts with the monsoon trough and easterly waves. Therefore understanding the distinct roles that the synoptic versus seasonal timescale CAGW play in modulating ENP TCs is of particular importance. To estimate CAGW's contributions on ENP TCs, we performed numerical simulations with a high-resolution (27km) TC-permitting tropical channel model (TCM). Two sets of experiments were performed for the 1990-2016 hurricane seasons. By closing mountainous gaps at Tehuantepec (TT) and Papagayo (PP), ENP TC activity significantly decreases about 25%, and ENSO-driven intrabasin variability is greatly eliminated. Analysis shows that CAGW-induced vorticity primarily contributes to this reduction, while reduced vertical wind shear offsets this unfavorable environmental condition by some degree. Furthermore, the CAGW-affiliated mid-level aridification is also responsible for this TC suppression.
