The Role of Air–Sea Coupling in the Superstorm of 1993

A coupled ocean-atmosphere model will be used to understand the sensitivity of air-sea coupling to the simulation of the Superstorm of 1993. During the period of 12-13 March 1993, an intense extra-tropical cyclone formed in western Gulf of Mexico, which was comparable in strength to a Category 1 hurricane and was dubbed as the “Storm of the Century”. There are two potential sources by which a winter cyclone originating in the Gulf of Mexico acquires more strength: 1) by gaining more heat and moisture from the ocean surface of the Gulf of Mexico, and or 2) by the destabilizing effect of the presence of strong horizontal temperature gradients between the warm ocean surface of the Gulf of Mexico and the surrounding cold land surface. Coupled ocean-atmosphere models offer insight into this intricate relationship where oceanic forcing mechanisms prompt an atmospheric response. It is apparent in the simulated winter cyclone that ocean-to-atmosphere fluxes are at a maximum near a warm core eddy, Eddy Vasquez, that separated itself from its parent Loop Current. The position of this warm core eddy, in the northwestern Gulf of Mexico, results in the initial intensification and rapid deepening of the cyclone. As the Superstorm of 1993 tracks eastward across the warm Gulf of Mexico waters, heat flux maximums shift into the region south of New Orleans, Louisiana and later off the mid-Atlantic coast, where it increases local wind speeds and rainfall totals.

A number of model integrations have been conducted to simulate this case of the Superstorm of 1993 where the air-sea coupling interval has been changed and even turned off. The inter-comparisons of these model simulations indicate that the air-sea coupling interval in the model strongly modulates the translation speed of the cyclone and the associated front. This is a result of the changes to the heat flux, with the uncoupled model exhibiting the slowest eastward movement of the frontal system compared to either observations or the model simulation with the highest coupling interval.