Interannual variability of tropical cyclone activity over the eastern North Pacific

On average, 16 tropical cyclones form over the Eastern North Pacific each year, with a standard deviation above 4. For the period of 1966-2003, the annual frequency ranges from 8 to 27. The mechanism behind the interannual variability of tropical cyclone activity is of interest. Here we represent tropical cyclone activity in terms of tropical cyclone frequency and the accumulated cyclone energy of each hurricane season (July, August and September). Two extreme events are selected from the time series, with 1992 being an active event and 1977 as an inactive.

While sea surface temperatures are consistently warm and favorable, most other environmental parameters, such as vertical wind shear, low-level relative vorticity and mid-troposphere relative humidity, experience great change between these two years, favoring more cyclogenesis in 1992. In addition, stronger meridional PV gradient reversal is observed over the Caribbean in 1992, producing a more unstable basic state upstream. A westward extension of the Atlantic subtropical ridge over the Gulf of Mexico and Central America is also observed during 1992 hurricane season, producing anticyclonic circulation and stronger easterly winds extending over the Eastern North Pacific, contributing to the formation of a well-established monsoon trough. The extension of subtropical ridge in 1992 also reduces precipitation over the Caribbean and Central America, while increased precipitation over the Eastern North Pacific is likely to be brought by stronger tropical cyclone activity in 1992.