Mesoscale air‐sea interactions related to tropical and extratropical storms in the Gulf of Mexico

Data indicate that the climatological vorticity of the lower atmosphere of the northwestern Gulf of Mexico is relatively strong, with monthly means exceeding a Rossby number of 3 from October through March, with a maximum of 4.6 in December. It has been hypothesized that wintertime cyclogenesis over the northwestern gulf is enhanced by this lower level atmospheric vorticity field when it occurs simultaneously with appropriate upper level venting. Observations from November 1982 to mid‐February 1983 were studied in which seven significant cyclones were generated in the northwestern gulf. It was found that all seven storms occurred when the vorticity correlate of horizontal air temperature difference was ∼3–5°C above the climatological mean difference. Although these results corroborate the postulate that such lower level vorticity is related to cyclogenesis, the data from mid‐February through March 1983 were less supportive. During those months, only two of the six significant storms developed when the horizontal air temperature difference was above the climatological mean. Surprisingly, the location of the most unstable air within the atmospheric boundary layer is not found in the northwestern Gulf of Mexico but instead along the shelf break. Relatively strong free convection in the atmospheric boundary layer is found over the Loop Current and the deep western gulf as expected. The unstable regions of the deep gulf can enhance the potential for frontogenesis and cyclogenesis, but it is likely that the linear character of the strong shelf break free convection does not enhance cyclogenesis. It is shown that a maximum in the frequency of tropical storms within the Gulf of Mexico exists some 275 km south of the Mississippi delta at 27°N, 90°W. This maximum is a result of only those storms which originate within the gulf. Two plausible effects of the Loop Current and its rings on tropical storms are discussed. One is that these ocean features are large and consolidated heat and moisture sources from which a nearby, slowly moving atmospheric disturbance can extract energy. The second is that of the cyclonic vorticity that can be generated in the lower atmosphere by such oceanographic features.