The coupling between the potential temperature and salinity equations and the momentum equations is due to an algebraic equation of state:
with being the hydrostatic reference pressure. In order to obtain potential density from the equation of state, needs to be set to zero, which is the default in GETM.
Currently the equation of state by Fofonoff and Millard (1983) is implemented into GETM, but the more recent and more consistent equation of state by Jackett et al. (2005) which is already contained in GOTM will be added as an option in the near future.
For the equation of state, also linearised version are implemented into GETM, for details, see section 8.9 on page .
For convinient use in other subroutines the buoyancy as defined in (4) is calculated and stored in the GETM variable buoy.