Representation of Unresolved Straits

In climate modeling, it often occurs that a crucial connections between water masses is broken as the grid mesh is too coarse to resolve narrow straits. For example, coarse grid spacing typically closes off the Mediterranean from the Atlantic at the Strait of Gibraltar. In this case, it is important for climate models to include the effects of salty water entering the Atlantic from the Mediterranean. Likewise, it is important for the Mediterranean to replenish its supply of water from the Atlantic to balance the net evaporation occurring over the Mediterranean region. This problem occurs even in eddy permitting simulations. For example, in ORCA 1/4several straits of the Indonesian archipelago (Ombai, Lombok...) are much narrow than even a single ocean grid-point.

We describe briefly here the three methods that can be used in NEMO to handle such improperly resolved straits. The first two consist of opening the strait by hand while ensuring that the mass exchanges through the strait are not too large by either artificially reducing the surface of the strait grid-cells or, locally increasing the lateral friction. In the third one, the strait is closed but exchanges of mass, heat and salt across the land are allowed. Note that such modifications are so specific to a given configuration that no attempt has been made to set them in a generic way. However, examples of how they can be set up is given in the ORCA 2and 0.5configurations. For example, for details of implementation in ORCA2, search: IF( cp_cfg == "orca" .AND. jp_cfg == 2 )

Hand made geometry changes

$\bullet$ reduced scale factor in the cross-strait direction to a value in better agreement with the true mean width of the strait. (Fig. 15.1). This technique is sometime called "partially open face" or "partially closed cells". The key issue here is only to reduce the faces of $T$-cell ($i.e.$ change the value of the horizontal scale factors at $u$- or $v$-point) but not the volume of the $T$-cell. Indeed, reducing the volume of strait $T$-cell can easily produce a numerical instability at that grid point that would require a reduction of the model time step. The changes associated with strait management are done in domhgr.F90, just after the definition or reading of the horizontal scale factors.

$\bullet$ increase of the viscous boundary layer thickness by local increase of the fmask value at the coast (Fig. 15.1). This is done in dommsk.F90 together with the setting of the coastal value of fmask (see Section 8.1)

Figure 15.1: Example of the Gibraltar strait defined in a $1^{\circ} \times 1^{\circ}$ mesh. Top: using partially open cells. The meridional scale factor at $v$-point is reduced on both sides of the strait to account for the real width of the strait (about 20 km). Note that the scale factors of the strait $T$-point remains unchanged. Bottom: using viscous boundary layers. The four fmask parameters along the strait coastlines are set to a value larger than 4, $i.e.$ "strong" no-slip case (see Fig.8.2) creating a large viscous boundary layer that allows a reduced transport through the strait.

Cross Land Advection (tracla.F90)

!-----------------------------------------------------------------------
&namcla        !   cross land advection
!-----------------------------------------------------------------------
   nn_cla      =    0      !  advection between 2 ocean pts separates by land
/

Options are defined through the namcla namelist variables. This option is an obsolescent feature that will be removed in version 3.7 and followings.