The Influence of Gap Wind Events on the Monsoon Trough and Implications for Eastern North Pacific Cyclogenesis

The Tropical Analysis and Forecast Branch (TAFB) of the National Hurricane Center is responsible for the portion of the Unified Surface Analysis (USA) south of 31 North from Africa to 140 West in the Eastern North Pacific (EPAC). Starting 1 June 2011, TAFB has included on its part of the USA a depiction of the monsoon trough portion of the Intertropical Convergence Zone (monsoon trough) in addition to the more traditional “tradewind” Intertropical Convergence Zone (ITCZ). This decision was made to more accurately depict the synoptic scale environment and discriminate areas which are favorable for tropical cyclogenesis. The convergence of southwest monsoonal flow with northeast tradewinds creates a background cyclonic flow within the monsoon trough that is favorable for tropical cyclogenesis. On the other hand, the confluence of northeast and southeast tradewinds associated with the ITCZ does not readily create cyclonic vorticity. Depicting the monsoon trough enabled forecasters at TAFB to observe in real time the influence of this feature on EPAC cyclogenesis.

There are two prevailing schools of thought for tropical cyclogenesis: cyclogenesis within the monsoon trough (most typical in the Western Pacific basin), and cyclogenesis related to lower tropospheric tropical waves (most typical in the Atlantic basin). Which school of thought is more appropriate to describe EPAC cyclogenesis? Preliminary analysis indicates that every EPAC system during the 2011 hurricane season developed within the monsoon trough, while a majority can also be attributed to easterly waves. This suggests (at least this season) it may not be correct to attribute cyclogenesis to just one school of thought (i.e. easterly waves) for every system in the EPAC basin. Rather, it may be more appropriate to view tropical cyclogenesis in the EPAC basin as a merger of the two schools of thought. It also suggests that systems developing without an easterly wave may develop within the monsoon trough in a similar way to systems in the Western Pacific (WPAC) basin.

Previous research examined the influence of the interaction of easterly waves (typically formed over Africa) with the mountainous terrain of Central America on EPAC cyclogenesis. It was suggested that an important aspect of this interaction was gap winds into the Gulf of Tehuantepec. The gap winds created a perturbation in the monsoon trough which further enhanced low level relative vorticity. Is an African easterly wave necessary for this process? It has been shown that easterly waves can form and grow in the favorable background flow in the Caribbean. Furthermore, tropical cyclogenesis is possible within a monsoon trough in the absence of an easterly wave, as frequently occurs in the WPAC. If gap winds into the Gulf of Tehuantepec are not the result of easterly waves, what are other sources for gap winds to interact with the monsoon trough? This presentation will address these questions by contrasting gap wind events and its influence on the precursor disturbance for three EPAC storms from 2011: Hurricanes Calvin, Hilary, and Jova.