Influence of large-scale initial oceanic mixed layer depth on tropical cyclones

The influence of spatial variations of the oceanic mixed layer depth (OMLD) on tropical cyclones (TCs) is investigated using a coupled atmosphere-ocea n model. The model consists of a version of the Naval Research Laboratory l imited area weather prediction model coupled to a simple 2 1/2-layer ocean model. Interactions between the TC and the ocean are represented by wind-in duced turbulent mixing in the upper ocean and latent and sensible heat flux es across the air-sea interface. Four numerical experiments are conducted with different spatial variations of the unperturbed OMLD representing idealizations of broad-scale patterns observed in the North Atlantic and North Pacific Oceans during the tropical cyclone season. In each, the coupled model is integrated for 96 h with an atmospheric vortex initially of tropical storm intensity embedded in an eas terly mean flow of 5 m s(-1) and located over an oceanic mixed layer that i s locally 40 m deep. The numerical solutions reveal that the rate of intens ification and final intensity of the TC are sensitive to the initial OMLD d istribution, but that the tracks and the gross features of the wind and pre ssure patterns of the disturbances are not. In every experiment, the sea surface temperature exhibits a maximum induced cooling to the right of the path of the disturbance, as found in previous studies, with magnitudes ranging from 1.6 degrees to 4.1 degreesC, dependin g on the initial distribution of the mixed layer depth. Consistent with ear lier studies, storm-induced near-inertial oscillations of the mixed layer c urrent are found in the wake of the storm. In addition, numerical experiments are conducted to examine sensitivity of a coupled-model simulation to variations of horizontal resolution. Results indicate that the intensity and track of tropical cyclones are quantitative ly sensitive to such changes.