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Traditional ocean models use a single coordinate type to represent the vertical, but no single approach is optimal for the global ocean (Chassignet et al., 2000; Willebrand et al., 2001). Isopycnal (density tracking) layers are best in the deep stratified ocean, z-levels (constant fixed depths) provide high vertical resolution in the mixed layer, and terrain-following levels are often the best choice in coastal regions. The HYbrid Coordinate Ocean Model (HYCOM) (Bleck, 2002) combines all three approaches by dynamically choosing the optimal distribution at every time step via the layered continuity equation. This has lead to HYCOM being chosen for the next generation of ocean prediction systems both by NAVOCEANO and by NOAA.