Surface boundary condition for the ocean

The surface ocean stress is the stress exerted by the wind and the sea-ice on the ocean. It is applied in dynzdf.F90 module as a surface boundary condition of the computation of the momentum vertical mixing trend (see (6.29) in §6.7). As such, it has to be provided as a 2D vector interpolated onto the horizontal velocity ocean mesh, $i.e.$ resolved onto the model (i,j) direction at $u$- and $v$-points.

The surface heat flux is decomposed into two parts, a non solar and a solar heat flux, $Q_{ns}$ and $Q_{sr}$, respectively. The former is the non penetrative part of the heat flux ($i.e.$ the sum of sensible, latent and long wave heat fluxes plus the heat content of the mass exchange with the atmosphere and sea-ice). It is applied in trasbc.F90 module as a surface boundary condition trend of the first level temperature time evolution equation (see (5.11) and (5.12) in §5.4.1). The latter is the penetrative part of the heat flux. It is applied as a 3D trends of the temperature equation (traqsr.F90 module) when ln_traqsr=true. The way the light penetrates inside the water column is generally a sum of decreasing exponentials (see §5.4.2).

The surface freshwater budget is provided by the emp field. It represents the mass flux exchanged with the atmosphere (evaporation minus precipitation) and possibly with the sea-ice and ice shelves (freezing minus melting of ice). It affects both the ocean in two different ways: $(i)$ it changes the volume of the ocean and therefore appears in the sea surface height equation as a volume flux, and $(ii)$ it changes the surface temperature and salinity through the heat and salt contents of the mass exchanged with the atmosphere, the sea-ice and the ice shelves.

The ocean model provides, at each time step, to the surface module (sbcmod.F90) the surface currents, temperature and salinity. These variables are averaged over nn_fsbc time-step (7.1), and it is these averaged fields which are used to computes the surface fluxes at a frequency of nn_fsbc time-step.

Table 7.1: Ocean variables provided by the ocean to the surface module (SBC). The variable are averaged over nn_fsbc time step, $i.e.$ the frequency of computation of surface fluxes.

Variable description	Model variable	Units	point
i-component of the surface current	ssu_m	$m.s^{-1}$	U
j-component of the surface current	ssv_m	$m.s^{-1}$	V
Sea surface temperature	sst_m	$K$	T
Sea surface salinty	sss_m	$psu$	T