Return Flow in the Gulf of Mexico. Part I: A Classificatory Approach with a Global Historical Perspective

Abstract

Return-flow events have been examined with the aid of a classification scheme that identifies each event with cold air masses that invade the Gulf during the cool season (February-March). These air masses were classified as either continental polar (cP), maritime polar (mP), or a mix of two or more of these basic types (MIX in future reference). Each event was viewed as a cycle in which the first phase represented an offshore flow typifying the cold-air outbreak over the Gulf and the second phase was associated with the return of modified air to the continent. Surface data for a 12-yr period, 1978?89, were used to make a statistical analysis of the event and each of its phases. The principal results of the study are 1) a total of 127 events occurred in this cool season over the 12-yr period. The relative percentages of mP, cP, and MIX air masses are 28%, 20%, and 52%, respectively. A median of 10.5 return-flow events occurred in the cool season where the MIX category was the dominant regime. The median duration for a return-flow cycle is 3,3, 5.2, and 6.2 days for mP, cP, and MIX, respectively, for the cool season. 2) The median duration of the offshore-flow phase for the cool season shows a wide range depending on airmass type with 30, 55, and 49 h as median times for mP, cP, and MIX, respectively. 3) The median duration of the return-flow phase for the cool season was significantly longer than the offshore-flow phase when all cases were examined en masse; but when the cases were segregated according to airmass type, the duration of the return flow for the cool season exhibited a wide range with 47, 57, and 62 h as median times for mP, cP and MIX, respectively. In order to view the return-flow events in the Gulf of Mexico from a wider perspective, a historical summary of research on this event and similar events around the world is included.