8.2.2 Ocean Processes
Recently, isopycnic or hybrid vertical coordinates have been adopted in some ocean models (GISS-EH and BCCR-BCM2.0). Tests show that such models can produce solutions for complex regional flows that are as realistic as those obtained with the more common depth coordinate (e.g., Drange et al., 2005). Issues remain over the proper treatment of thermobaricity (nonlinear relationship of temperature, salinity and pressure to density), which means that in some isopycnic coordinate models the relative densities of, for example, Mediterranean and Antarctic Bottom Water masses are distorted. The merits of these vertical coordinate systems are still being established.
An explicit representation of the sea surface height is being used in many models, and real freshwater flux is used to force those models instead of a ‘virtual’ salt flux. The virtual salt flux method induces a systematic error in sea surface salinity prediction and causes a serious problem at large river basin mouths (Hasumi, 2002a,b; Griffies, 2004).
Generalised curvilinear horizontal coordinates with bipolar or tripolar grids (Murray, 1996) have become widely used in the oceanic component of AOGCMs. These are strategies used to deal with the North Pole coordinate singularity, as alternatives to the previously common polar filter or spherical coordinate rotation. The newer grids have the advantage that the singular points can be shifted onto land while keeping grid points aligned on the equator. The older methods of representing the ocean surface, surface water flux and North Pole are still in use in several AOGCMs.