Coupled ocean-atmosphere general circulation models have become increasingly important for tropical cyclone (TC) forecast guidance at operational prediction centers. Implementation and advancement of coupled TC forecast models such as the Geophysical Fluid Dynamics Laboratory (GFDL) model and more recently the Hurricane Weather Research and Forecast (HWRF) model have significantly improved track forecasts by the National Hurricane Center. However, this effort has realized little improvement in intensity forecasts, and this skill may be limited in part by errors and biases in the ocean response predicted by these coupled models.

When atmospheric conditions are favorable, TC intensification often occurs over regions with high upper-ocean heat content. This is particularly true for potentially dangerous rapid intensification. The impact of the Loop Current (LC) and warm-core anticyclones in the Gulf of Mexico (GOM) is documented for hurricanes Gilbert (1988) and Opal (1995) (Jacob et al., 2000; Hong et al. 2000; Shay et al. 2000; Jacob and Shay, 2003), and also for hurricanes Katrina and Rita (2005) (Scharroo et al. 2005; Sun et al. 2006, Shay, 2008). In contrast, low ocean heat content can inhibit intensification; possibly contributing to the weakening of both Ivan (Walker et al. 2005) and Rita (Sun et al., 2006; Shay 2008) as they passed over cold-core cyclones in the GOM.

To correctly forecast intensity evolution, the ocean component of coupled forecast models must accurately predict the rate and pattern of SST cooling relative to the storm center. However, ocean models have not been thoroughly evaluated for this purpose. The present study evaluates an ocean model response to Hurricane Ivan (2004) over the northwest Caribbean Sea and GOM. The overarching goal of this analysis is to determine how we need to invest our greatest efforts toward improving ocean model performance.

The HYbrid Coordinate Ocean Model (HYCOM) is a primitive equation ocean model that uses a hybrid vertical coordinate designed to quasi-optimally resolve vertical structure throughout the ocean. This coordinate is isopycnic in the stratified ocean interior, but dynamically transitions to level coordinates near the surface to provide resolution in the surface mixed layer and to either level or terrain-following (σ) coordinates in the coastal ocean. This strategy enables HYCOM to use advanced turbulence closures for vertical mixing and also to be used as both a coastal and open-ocean model while retaining the advantages of isopycnic coordinates in the stratified ocean interior. Model equations and initial evaluation of the hybrid vertical grid generator is presented in Bleck (2002). Subsequent evolution and further evaluation of the model is summarized in Chassignet et al. (2003; 2007) and Halliwell (2004).


Figure 5. Map of sea surface height (top panel) on 17 Sept. 2004 shortly after Ivan made landfall (track in black line), illustrating the locations of the LC, the detached warm ring, and the two cyclonic eddies (red arrows) near the time of maximum cooling in the eastern GoM. A white dot marks the location of SEED ADCP mooring 9 (Teague et al., 2007) while the other two unmarked white dots represent the locations sampled by synthetic instruments and described in Figure 6. The remaining panels present SST maps for 10 Sept. 2004 (left) and 17 Sept. 2004 (right) for the Reynolds blended analysis of in-situ observations plus infrared and microwave satellite data (top) and from the control experiment GOM1 (bottom).