AUTUMNAL RETURN OF TROPICAL AIR TO THE GULF-OF-MEXICO COASTAL-PLAIN

The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (theta(e)) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level theta(e) that flowed into the storm region from the Gulf of Mexico. The primary results of the study are the following: 1) The climatological distribution of equilibrium theta(e) over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study. 2) Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period. 3) The high-valued theta(e) that streamed into the severe storm region on 15 November 1988 tracked from the Yucatan straits and the northwestern Caribbean over a three-day period. 4) This air was able to maintain its high-theta(e) property because of an anomalously warm gulf. 5) Significant increases in available energy for deep convection could have been anticipated by means of the upper bounds on coastal theta(e) predicted by the Betts and Ridgway formulation, which was supported by observations along the Texas coast.