THE STRUCTURE AND EVOLUTION OF GAP OUTFLOW OVER THE GULF-OF-TEHUANTEPEC, MEXICO

Mesoscale-model simulations are used to examine the structure and dyna mics of a gap-outflow event over the Gulf of Tehuantepec, Mexico, that was associated with a surge of cold air along the eastern slopes of t he Sierra Madre. The simulated gap-outflow winds emerged from Chivela Pass, reached a maximum speed of 25 m s(-1), and turned anticyclonical ly as they fanned out over the gulf. Northerly winds were also able to ascend the mountains east, and to a lesser extent west, of Chivela Pa ss, indicating that the movement of cold air across the Sierra Madre w as not confined to the pass. A mesoscale pressure ridge was aligned al ong the axis of the gap-outflow jet, which was flanked to the west by an anticyclonic eddy, and to the east by a weaker cyclonic eddy. A mod el-derived trajectory along the axis of the outflow jet traced an iner tial path, with anticyclonic curvature produced primarily by the Corio lis acceleration, The cross-flow pressure-gradient acceleration along this trajectory was negligible because it followed the axis of the mes oscale pressure ridge. Trajectories west (east) of the jet axis experi enced stronger (weaker) anticyclonic curvature than expected from iner tial balance because the cross-flow pressure-gradient acceleration pro duced by the mesoscale pressure ridge reinforced (opposed) the anticyc lonic deflection by the Coriolis acceleration. As a result of these di rectional variations in the cross-flow pressure-gradient acceleration, a fanlike wind pattern was observed rather than a narrow jet. Because of the large changes in SST and surface roughness that are observed d uring these gap-outflow events, better representation of these effects might improve future mesoscale-model simulations. Such improvements c ould be accomplished through coupled atmosphere-ocean mesoscale modeli ng, which could also be used to advance understanding of the oceanogra phy of the gulf.