Date of Award

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Shih-Ang Hsu

Abstract

Seasonal counts of frontal-wave cyclones forming over the Gulf of Mexico and its coastal plain show more storms in the five El Nino winters and fewer storms in the eight La Nina winters, from 1960 to 1989, significant at the.01 level by a rank sum test. This is corroborated by two results. First, during the same period, the frequency of frontal-overrunning weather conditions in the region, indicative of storms, was higher in El Nino winters and lower in La Nina winters, significant at the.05 level. Second, 100 years of precipitation and temperature records show wetter, cooler El Nino winters and drier, warmer La Nina winters at gulf-region land stations and climatic divisions. A threefold explanation, based on National Meteorological Center, upper-air data, is offered for the greater frequency of gulf-region cyclogenesis during El Nino winters between 1960 and 1989. 1. Jet stream. The winter, mean, 250-mb jet over the southern United States is intensified by 5 to 10 ms$\sp{-1}$ and displaced southward between 110$\sp\circ$ and 75$\sp\circ$W by an average of 200 to 285 km during the five El Nino winters between 1966 and 1989. This implies stronger and more-frequent episodes of jet-associated, upper-level troughing and divergence over the region, reinforcing surface, frontal-wave cyclones. 2. Upper- and intermediate-level troughs. In the five El Nino winters between 1963 and 1989, seasonal average heights and temperatures of the 850-, 700-, 500-, and 200-mb surfaces are lower over the region than they are in non-El Nino winters. This implies more-common presence of cold, low-pressure troughs at upper levels, reinforcing surface cyclones. 3. 850-mb level winds. A 10$\sp\circ$ eastward shift, at sea level, of the western edge of the Bermuda high during the eight El Nino winters between 1947 and 1989, changes normally due-easterly trades in the northwestern Caribbean Sea to slightly south of east, allowing greater advection of moisture and heat into the gulf from the tropics, preconditioning the area for development of surface cyclones. Only winter season shows all three conditions and an increase in cyclogenesis.

Pages

806

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