Upper ocean heat content is an important factor in the rapid intensfication of tropical cyclones. During storm passage, a large fraction of upper ocean cooling and reduction in heat content is due to entrainment of cooler less turbulent water from below the oceanic mixed layer. Analysis of high-resolution measurements acquired during the passage of hurricane Gilbert (1988) in the western Gulf of Mexico revealed that the heat and mass budgets strongly depended upon the entrainment scheme used. In this paper, the time evolution of mixed layer quantities for different entrainment closure schemes during Gilbert is investigated using high resolution numerical models with realistic forcing fields and the results are compared to observations. The main objective is to understand the upper ocean heat and mass budget variability due to the various entrainment closure schemes and to identify a scheme that compares well with observations.

The Miami Isopycnic Coordinate Ocean Model (MICOM) initialized with observational data and realistic forcing is first used in this study. This model uses an explicit mixed layer configured with four entrainment mixing mechanisms. Based on linear regression analysis, mixed layer temperatures simulated using a closure scheme that depends only on near-inertial shears at the layer base fit the data better than the other three schemes. While the rates of simulated cooling and deepening differ for the four schemes, the pattern remains qualitatively similar confirming observational findings. However, the explicit mixed layer in MICOM precludes the use of higher order entrainment closure schemes. Therefore, to evaluate these higher order schems, the Hybrid Coordinate Ocean Model (HYCOM) will be used to simulate the upper ocean response and the results compared to observations. Clearly, identifying an entrainment closure scheme is crucial for the upper ocean and coupled response studies and the resulting air-sea interaction during storm passage.