2006 LAYERED OCEAN MODEL WORKSHOP

A key factor for improving tropical cyclone intensity forecasts by coupled models is accurate initialization of the state of the ocean. This issue is explored in the northwest Caribbean and southeast Gulf of Mexico for September 2002 just prior to hurricane Isidore. Initialization from climatology is inadequate because climatologies do not accurately represent the location of variable oceanographic features associated with different heat content such as the warm Loop Current, and also because very anomalous conditions were present in the upper ocean prior to Isidore. In particular, the warm surface layer was much thicker than normal in the northwest Caribbean and in the Loop Current as documented by pre-storm Airborne Expendable Conductivity Temperature and Depth (AXCTD) profiles. We evaluate the latest HYCOM Global Ocean Data Assimilation Experiment (GODAE) nowcast product for providing the initial ocean state. Comparison of nowcast temperature and salinity fields to aircraft observations and to Navy Modular Ocean Data Assimilation System (MODAS) analysis fields demonstrates that the present HYCOM-GODAE product accurately represents the location of important features such as the Loop Current, but does not reproduce the anomalous ocean state as accurately as the MODAS analysis. Salinity is inaccurately represented in both HYCOM and MODAS due to insufficient observations. Significant improvement is expected in the next generation HYCOM-GODAE product (NCODA) because it will use a more sophisticated assimilation procedure that also assimilates subsurface ocean observations. In the meantime, initial and boundary conditions for hurricane studies are being produced by a special 1/12-degree Atlantic run with HYCOM relaxed to MODAS temperature and salinity.