The Effect of the Ocean Eddy on Tropical Cyclone Intensity

It is recognized that understanding the interaction of ocean eddies with tropical cyclone is useful for the prediction of the tropical intensity. The recent devastating hit of Hurricane Katrina to the southern states of the U.S. is a case in point to highlight the critical role the oceanic thermal structure (such as the ocean eddy or Loop Current) plays in affecting the intensity of tropical cyclones. From earlier studies, the role of the warm ocean eddies play on TC intensity has been well addressed. However, there is no systematically discussion and this issue still remains unresolved. Recently, Lin et al. (2005) suggest that a great proportions of the TCs in the North western Pacific ocean have encountered the ocean eddies during their life time and therefore the warm eddies are acting as supertyphoon boosters. To have a better understanding of the above processes, a systematic evaluation of the roles the ocean eddies play in affecting the TC intensity is conducted in this study using a simple hurricane-ocean coupled model Several numerical experiments with different oceanic thermal structure are designed to elucidate the impact of the ocean eddy on tropical cyclone intensity. It is shown that in this simple model a storm encountering the eddy can result in rapid intensification. To highlight the role of the eddy on tropical cyclone intensity, the "eddy feedback factor" is defined as FEDDY i.e. , where ƒxp is the pressure depression in the eye of a storm and the index °¥ctrl' indicates the case without ocean eddy, while the index °¥eddy' represents the case that the TC encounters an ocean eddy. A typical value of the eddy feedback factor of 0.5 indicates that the model's steady-state tropical intensity can be 50% deeper in the cases with warm ocean eddy as compared to those cases without the eddies. The efficiency of the eddy feedback not only depends on the oceanic structure but also on other environment parameters including the properties of the tropical cyclone. According to a set of numerical experiments conducted in this study, the eddy feedback factor is a function of the unperturbed sea surface temperature and mixed-layer depth, the relative humidity, the translation speed, the stratification below the mixed layer, the size of the tropical, and the thermal structure in eddies. The feedback of the ocean features, including warm and cold eddies on TC intensity evolution, can be systematic identified and evaluated from this study.