The impact of the Loop Current on the GFDL/URI coupled hurricane-ocean model intensity forecasts of hurricanes Katrina, Rita and Wilma in the Gulf of Mexico

The heat from the ocean is the power that drives hurricanes. Even when atmospheric conditions are highly favorable, the characteristics of the underlying ocean determine the amount of energy available for intensification and thus the maximum strength that any storm can attain. The GFDL/URI coupled hurricane-ocean model has been used at NCEP since 2001 and provided operational guidance for forecasters at TPC in both the Atlantic and East Pacific ocean basins.

The ocean component of the GFDL/URI coupled system, the Princeton Ocean Model, has a specially designed initialization and data assimilation procedure to initialize the ocean temperature, salinity and current velocity fields at the beginning of each forecast. This procedure has been recently improved to include the initialization of the realistic position and structure of the Loop Current in the Gulf of Mexico. The Loop Current enters the Gulf of Mexico through the Yucatan Straits, meanders northward, sometimes extending as far north as the Louisiana coast, and exits through the Florida Straits between Florida and Cuba. A key characteristic of the Loop Current for hurricanes is that the distance that it penetrates into the Gulf of Mexico varies substantively. When the Loop Current penetrates far into the north-central Gulf of Mexico, as it occurred in the summer and fall of 2005, it carries with it the deep warm surface layer typical of the Caribbean that promotes hurricane intensification.

In this study we investigate the impact of the Loop Current on the intensity prediction of the GFDL/URI coupled model for three hurricanes that cross the Gulf of Mexico in 2005: Katrina, Rita, and Wilma. The ocean initialization procedure in the GFDL/URI coupled model permits assimilation of the position of the northern boundary of the Loop Current in the Gulf of Mexico using satellite altimeter data. This position was specified based on the satellite data during Katrina, Rita and Wilma in the first set of experiments and average monthly climatology in the second set of experiments. In the first set of experiments, all simulated storms intensified over the Loop Current, especially hurricanes Rita and Katrina. As a result, the GFDL model intensity forecasts were improved by 5-15 kts by assimilating a more realistic position of the Loop Current in the Gulf of Mexico.