Atmospheric pressure (sbcapr.F90)

!-----------------------------------------------------------------------
&namsbc_apr    !   Atmospheric pressure used as ocean forcing or in bulk
!-----------------------------------------------------------------------
!              !  file name  ! frequency (hours) ! variable  ! time interp. !  clim  ! 'yearly'/ ! weights  ! rotation ! land/sea mask !
!              !             !  (if <0  months)  !   name    !   (logical)  !  (T/F) ! 'monthly' ! filename ! pairing  ! filename      !
   sn_apr      = 'patm'      ,         -1        ,'somslpre',    .true.     , .true. , 'yearly'  ,  ''      ,   ''     , ''

   cn_dir      = './'       !  root directory for the location of the bulk files
   rn_pref     = 101000.    !  reference atmospheric pressure   [N/m2]/
   ln_ref_apr  = .false.    !  ref. pressure: global mean Patm (T) or a constant (F)
   ln_apr_obc  = .false.    !  inverse barometer added to OBC ssh data
/

The optional atmospheric pressure can be used to force ocean and ice dynamics (ln_apr_dyn = true, namsbc namelist ). The input atmospheric forcing defined via sn_apr structure (namsbc_apr namelist) can be interpolated in time to the model time step, and even in space when the interpolation on-the-fly is used. When used to force the dynamics, the atmospheric pressure is further transformed into an equivalent inverse barometer sea surface height, $\eta_{ib}$, using:

$$\eta_{ib} = - \frac{1}{g\,\rho_o} \left( P_{atm} - P_o \right)$$ (7.2)

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 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, $i.e.$ the mean value of $\eta_{ib}$ is kept to zero at all time step.

The gradient of $\eta_{ib}$ is added to the RHS of the ocean momentum equation (see dynspg.F90 for the ocean). For sea-ice, the sea surface height, $\eta_m$, which is provided to the sea ice model is set to $\eta - \eta_{ib}$ (see sbcssr.F90 module). $\eta_{ib}$ can be set in the output. This can simplify altimetry data and model comparison as inverse barometer sea surface height is usually removed from these date prior to their distribution.

When using time-splitting and BDY package for open boundaries conditions, the equivalent inverse barometer sea surface height $\eta_{ib}$ can be added to BDY ssh data: ln_apr_obc might be set to true.