Return Flow in the Gulf of Mexico. Part II: Variability in Return-Flow Thermodynamics Inferred from Trajectories over the Gulf

Abstract

A return-flow case study is examined with the benefit of an unprecedented set of observations obtained during the Gulf of Mexico Experiment (GUFMEX). This case represents the return of modified continental air to the coastal plain in mid-February, and the work is designed to complement the classificatory study of return flow that is found in the companion paper by Crisp and Lewis. Surface air trajectories are combined with land- and ocean-based upper-air data to methodically follow the airmass modification process from the exit point off the Gulf coastal plain to its subsequent entry point on land. Upper-air data from the U.S. Coast Guard (USCG) ship Salvia are especially valuable in this tracking process, but data from an oil platform at the edge of the continental shelf, as well as special onshore observations, significantly contribute to a macroscopic tracking of air involved in the return flow. Results indicate that the warming and moistening process is complicated and requires careful assessment of both air-sea interaction processes and larger-scale vertical motion. The principal results are 1) the thermodynamic character of the returning air mass exhibits significant differences along the entire Gulf coast, and 2) the mixed-layer modeling theory appears to account for the warming and moistening processes for air in the central Gulf that tracks over the Loop Current. The processes determining the character and stratification of the air mass become very complicated, however, as the air approaches neutrally stable conditions and begins its northward track back toward land. The paper concludes with a synopsis of the airmass modification process built upon a composite chart that combines analyses from the various observational platforms.