where $P_{atm}$ is the atmospheric pressure and $P_o$ a reference atmospheric pressure.
A value of $101,000~N/m^2$ is used unless \np{ln_ref_apr}{ln\_ref\_apr} is set to true.
In this case, $P_o$ is set to the value of $P_{atm}$ averaged over the ocean domain,
\ie\ the mean value of $\eta_{ib}$ is kept to zero at all time steps.
3 different options are available to define this reference atmospheric pressure using \np{nn_ref_apr}{nn\_ref\_apr}:
\begin{description}
\item$\bullet$ 0 sets $P_o$ to a constant value of $101,000~N/m^2$
\item$\bullet$ 1 sets $P_o$ to the value of $P_{atm}$ averaged over the ocean domain (the mean value of $\eta_{ib}$ is kept to zero at all time steps)
\item$\bullet$ 2 reads a time-dependant $P_o$ value from a 1D file defined via \np{sn_apref}{sn\_apref} structure (usefull for regional configurations)
\end{description}
The gradient of $\eta_{ib}$ is added to the RHS of the ocean momentum equation (see \mdl{dynspg} for the ocean).
For sea-ice, the sea surface height, $\eta_m$, which is provided to the sea ice model is set to $\eta-\eta_{ib}$
