chap_SBC.tex

(observed, climatological or an atmospheric model product),
\textit{SSS}$_{Obs}$ is a sea surface salinity
(usually a time interpolation of the monthly mean Polar Hydrographic Climatology \citep{steele.morley.ea_JC01}),
$\left.S\right|_{k=1}$ is the model surface layer salinity and
$\gamma_s$ is a negative feedback coefficient which is provided as a namelist parameter.
Unlike heat flux, there is no physical justification for the feedback term in \autoref{eq:SBC_dmp_emp} as
the atmosphere does not care about ocean surface salinity \citep{madec.delecluse_IWN97}.
The SSS restoring term should be viewed as a flux correction on freshwater fluxes to
reduce the uncertainties we have on the observed freshwater budget.

%% =================================================================================================
\subsection{Handling of ice-covered area  (\textit{sbcice\_...})}
\label{subsec:SBC_ice-cover}

The presence at the sea surface of an ice covered area modifies all the fluxes transmitted to the ocean.
There are several way to handle sea-ice in the system depending on
the value of the \np{nn_ice}{nn\_ice} namelist parameter found in \nam{sbc}{sbc} namelist.
\begin{description}
\item [nn\_ice = 0] there will never be sea-ice in the computational domain.
  This is a typical namelist value used for tropical ocean domain.
  The surface fluxes are simply specified for an ice-free ocean.
  No specific things is done for sea-ice.
\item [nn\_ice = 1] sea-ice can exist in the computational domain, but no sea-ice model is used.
  An observed ice covered area is read in a file.
  Below this area, the SST is restored to the freezing point and
  the heat fluxes are set to $-4~W/m^2$ ($-2~W/m^2$) in the northern (southern) hemisphere.
  The associated modification of the freshwater fluxes are done in such a way that
  the change in buoyancy fluxes remains zero.
  This prevents deep convection to occur when trying to reach the freezing point
  (and so ice covered area condition) while the SSS is too large.
  This manner of managing sea-ice area, just by using a IF case,
  is usually referred as the \textit{ice-if} model.
  It can be found in the \mdl{sbcice\_if} module.
\item [nn\_ice = 2 or more] A full sea ice model is used.
  This model computes the ice-ocean fluxes,
  that are combined with the air-sea fluxes using the ice fraction of each model cell to
  provide the surface averaged ocean fluxes.
  Note that the activation of a sea-ice model is done by defining a CPP key (\key{si3}).
  The activation automatically overwrites the read value of nn\_ice to its appropriate value
  (\ie\ $2$ for SI3).
\end{description}

%% =================================================================================================
\subsection[Freshwater budget control (\textit{sbcfwb.F90})]{Freshwater budget control (\protect\mdl{sbcfwb})}
\label{subsec:SBC_fwb}

\begin{listing}
  \nlst{namsbc_fwb}
  \caption{\forcode{&namsbc_fwb}}
  \label{lst:namsbc_fwb}
\end{listing}

For global ocean simulations, it can be useful to introduce a control of the
mean sea level in order to prevent unrealistic drifting of the sea surface
height due to unbalanced freshwater fluxes. In \NEMO, two options for
controlling the freshwater budget are proposed.

\begin{description}
\item [{\np[=0]{nn_fwb}{nn\_fwb}}:] No control at all; the mean sea level is
  free to drift, and will certainly do so.
\item [{\np[=1]{nn_fwb}{nn\_fwb}}:] The global mean \textit{emp} is set to zero at each model time step.
  %GS: comment below still relevant ?
  %Note that with a sea-ice model, this technique only controls the mean sea level with linear free surface and no mass flux between ocean and ice (as it is implemented in the current ice-ocean coupling).
\item [{\np[=2]{nn_fwb}{nn\_fwb}}:] \textit{emp} is adjusted by adding a
  spatially uniform, annual-mean freshwater flux that balances the freshwater
  budget at the end of the previous year; as the model uses the Boussinesq
  approximation, the freshwater budget can be evaluated from the change in the
  mean sea level and in the ice and snow mass after the end of each simulation
  year; at the start of the model run, an initial adjustment flux can be set
  using parameter \np{rn_rwb0}{rn\_fwb0} in namelist \nam{sbc_fwb}{sbc\_fwb}.
\end{description}

% Griffies doc:
% When running ocean-ice simulations, we are not explicitly representing land processes,
% such as rivers, catchment areas, snow accumulation, etc. However, to reduce model drift,
% it is important to balance the hydrological cycle in ocean-ice models.
% We thus need to prescribe some form of global normalization to the precipitation minus evaporation plus river runoff.
% The result of the normalization should be a global integrated zero net water input to the ocean-ice system over
% a chosen time scale.
% How often the normalization is done is a matter of choice. In mom4p1, we choose to do so at each model time step,
% so that there is always a zero net input of water to the ocean-ice system.
% Others choose to normalize over an annual cycle, in which case the net imbalance over an annual cycle is used
% to alter the subsequent year�s water budget in an attempt to damp the annual water imbalance.
% Note that the annual budget approach may be inappropriate with interannually varying precipitation forcing.
% When running ocean-ice coupled models, it is incorrect to include the water transport between the ocean
% and ice models when aiming to balance the hydrological cycle.
% The reason is that it is the sum of the water in the ocean plus ice that should be balanced when running ocean-ice models,
% not the water in any one sub-component. As an extreme example to illustrate the issue,
% consider an ocean-ice model with zero initial sea ice. As the ocean-ice model spins up,
% there should be a net accumulation of water in the growing sea ice, and thus a net loss of water from the ocean.
% The total water contained in the ocean plus ice system is constant, but there is an exchange of water between
% the subcomponents. This exchange should not be part of the normalization used to balance the hydrological cycle
% in ocean-ice models.

\subinc{\input{../../global/epilogue}}

\end{document}