7 
Monthly climatological fields are used to supply the model with salinity and temperature 
inflow data at its open boundary. To lessen their influence and allow more real evolution, 
a sponge layer is placed along the boundary cross-section. Its purpose is to act as a 
buffer between the inner model domain and a ’’climatological reservoir” outside. The 
sponge layer literally acts as a filter located between the interior and exterior. It is flushed 
by the normal flow component on the boundary line. The flow either transports a 
climatological signal from the reservoir or an outflow signal from the interior into the 
sponge. When the flow direction is reversed from outward to inward, the sponge 
gradually releases the stored signal into the interior until its capacity is exhausted, and 
more and more of the reservoir signal passes through it. Conversely, with outward flow, 
the sponge empties into the reservoir which is thought to have an infinite capacity and 
thus does not respond to the input. The performance of the sponge depends on its 
capacity, which should be sufficiently large to avoid any effect of the reservoir on the tidal 
reversals, but sufficiently small to give a delayed input effect for sustained inflow. 
3 Model numerics 
3.1 Discretisation 
3.1.1 Spatial scheme 
According to the layout of the model equations in spherical co-ordinates, the 
computational grid consists of regular rectangles in longitude and latitude. Concerning 
the vertical representation we have a z-coordinate model. The vertical grid is composed 
of horizontal subsurfaces (interfaces) at fixed depth levels which are thought to run 
through the domain, cutting it into slices. In this way the whole domain is divided into 
elementary units: the grid cells. Cells at the same depth have the same constant 
thickness, except for the cells constituting the surface and bottom layers. 
The upper side of the cells forming the surface layer represents the movable air-sea 
interface. The first fixed z-level, i.e. the interface between the first and second cells in a 
column, is placed so low that it is never touched by the moving sea surface. Flooding and 
falling dry thus affect only the top layer. 
Cell thickness at the bottom varies from column to column, depending on the water 
depth. The bottom cell contains the volume that is left after fixing grid cells from top to 
bottom. It usually does not have the maximum thickness allowed in this particular layer. 
As in most other models, the intervals between the z-level subsurfaces increase from top 
to bottom.