Global Ocean Prediction Using HYCOM

One important aspect of ocean model design is the choice of the vertical coordinate system. Traditional ocean models use a single coordinate type to represent the vertical, but model comparison exercises performed in Europe (DYnamics of North Atlantic MOdels - DYNAMO) (Willebrand et al., 2001) and in the United States (Data Assimilation and Model Evaluation Experiment - DAMEE) (Chassignet et al., 2000) have shown that none of the three main vertical coordinates presently in use (depth [z-levels], density [isopycnal layers], or terrain-following [sigma-levels]) can by itself, be optimal everywhere in the ocean. The HYbrid Coordinate Ocean Model (HYCOM) (Bleck, 2002) is configured to combine all three of these vertical coordinate types. It is isopycnal in the open, stratified ocean, but uses the layered continuity equation to make a dynamically smooth transition to a terrain-following coordinate in shallow coastal regions, and to z-level coordinates in the mixed layer and/or unstratified seas. The hybrid coordinate extends the geographic range of applicability of traditional isopycnic coordinate circulation models toward shallow coastal seas and unstratified parts of the world ocean. It maintains the significant advantages of an isopycnal model in stratified regions while allowing more vertical resolution near the surface and in shallow coastal areas, hence providing a better representation of the upper ocean physics