22.11 barotropic (computes external mode velocities)

After the baroclinic velocities have been computed for all latitude rows in the domain, the barotropic or external mode velocity can be computed. Subroutine barotropic^22.28 computes the barotropic velocity $\bar{u}_{i,k,j,n,\tau+1}$for n=1 and 2 by one of three options: stream_function described in Section 29.2,  prognostic_surface_pressure described in Section 29.3, or implicit_free_surface described in Section 29.4. Refer to the above sections for details.