Thanks for this questions, Shikhar,
In this case, the state toward which the model is restored is the same as the initial conditions in the Phillips configuration, although this need not be the case. To make them different, you would simply provide different target interface heights in the call to initialize_sponge, although I am not sure why one would want to do this.
The layer-mode sponge code restores the interface positions to the locations specified in a stretched Z*-coordinate, and not a z-coordinate. The sea surface height is not directly altered by the layer-mode sponges, and volume (in Boussinesq mode) or mass is conserved. The configuration on line is exactly one of the cases was used in my 2013 paper - the others differed only in their resolutions and time-steps, on in the application of the interface height diffusion, as described in that paper.
It should also be made clear that MOM6 has two completely separate sets of sponge code. The ALE_sponge code (in MOM_ALE_sponge.F90) damps the temperature, salinity and perhaps velocities toward specified values in z*-space (which are then vertically remapped to the model's vertical grid), but it does not move any mass between layers. The older layer-mode sponge code (in MOM_sponge.F90) primarily moves water between layers to drive the interface heights toward specified (stretched Z*-space) positions, although it can also damp the temperature, salinity and other tracer values toward specified values for each layer. The ALE_sponge code relies on the remapping step to adjust the layer positions, so it is not compatible with a layer mode configuration. The ALE_sponge form also has a well-developed and exercised capability to specify (depth-space) target properties that evolve with time. The targets form the layer-mode sponges do not evolve with time, but this could probably be added if anyone wanted such a capability and could make a compelling case for it; if so, please add an issue to the MOM6 code page at
Issues · mom-ocean/MOM6 describing what you are looking for with the label 'enhancement'.