URI


Motivation


The substantial impact of sea-surface temperature (SST) on the genesis and intensification of tropical cyclones has long been recognized. It is well known that tropical cyclones climatologically develop only over warm oceans with SSTs of 26°C or higher. Both theoretical and numerical models also indicate a great sensitivity of maximum (or potential) storm intensity to SST.

Tropical cyclone modeling efforts over the past decades had been primarily concerned with perplexing problems of convective parameterization, vortex movement and vortex-flow interaction. One of the potentially significant constraints on dynamical predictions of tropical cyclones is the lack of knowledge about the ocean response to the storm forcing. For the vast majority of research and operational dynamical models, conditions of fixed SST in time are assumed. Yet numerous observational and numerical studies have shown that tropical cyclones produce significant changes in the underlying ocean thermodynamic structure, which also involves SST changes. SST may decrease by up to 6°C as a result of strong wind forcing. Vertical turbulent mixing within the upper oceanic layer, accompanied by the mixed layer deepening and entrainment of cooler thermocline water to the warm mixed layer, is the primary mechanism of SST decrease during the tropical cyclone passage. The heat fluxes to the atmosphere account for less than 20% of the total SST decrease.

Because of the sea-surface cooling effect associated with storm forcing, the tropical cyclone-ocean system has both positive and negative feedbacks ( Positive and Negative feedback diagram ). During the genesis and development stages, a positive feedback in the tropical cyclone-ocean system exists. As the tropical cyclone strengthens, the evaporation rate grows due to the increase in the surface wind speed. The enhancement of the moisture supply from the ocean leads to an increase in the latent heat energy that drives the circulation of the tropical cyclone. As the storm continues to intensify, the increasing surface wind stress generates strong turbulent mixing that deepens the ocean mixed layer. The associated SST decrease may then result in a reduction of the total heat flux (latent plus sensible) into the atmosphere and lead to a decrease in storm intensity. This process represents a negative feedback mechanism.




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