Origins and Mechanisms of Eastern Pacific Tropical Cyclogenesis: A Case Study

The genesis of Hurricane Hernan (1996) in the eastern Pacific was investigated using gridded analyses from the European Centre for Medium-Range Weather Forecasts and gridded outgoing longwave radiation. Hernan developed in association with a wave in the easterlies that could be tracked back to Africa in longitude–time plots of the filtered y component of the wind (2–6-day period) at 700 mb. The wave crossed Central America near Lake Nicaragua with little change in its southwest–northeast tilt, but the most intense convection shifted from near the wave axis in the Caribbean to west of the wave axis in the Pacific. The wave intensified as it moved through a barotropically unstable background state (defined by a low-pass filter with a 20-day cutoff ) in the western Caribbean and eastern Pacific. A surge in the southwesterly monsoons and enhanced convection along 108N occurred to the west of the 700-mb wave in the Pacific and traveled with the wave. This had the effect of enhancing low-level vorticity over a wide region ahead of the 700-mb wave. Available evidence suggests that additional low-level vorticity was produced by enhanced flow from the north through the Isthmus of Tehuantepec as the 700-mb wave approached. Depression formation did not occur until 6–12 h after the 700mb wave reached this region of large low-level vorticity in the Gulf of Tehuantepec. Eastern Pacific SST and vertical wind shear magnitude are typically favorable for tropical cyclone development in Northern Hemisphere summer and early fall. Because the favorable mountain interaction and the surge in the low-level monsoons appear to relate directly to the wave in the easterlies, it is argued that the strength of such waves reaching Central America from the east is the single most important factor in whether subsequent eastern Pacific cyclogenesis occurs. Possible parallels with western Pacific cyclogenesis are discussed.