This implements a poor man's open boundary condition along the northernmost and southernmost artificial walls in a limited domain basin. A Newtonian damping term is added to the tracer equations which damps the solution back to data within a specified width from the walls. The form is given by Equation 22.60 and the data is generated by script run_sponge28.2 in PREP_DATA.
If option equatorial_thermocline is enabled, then the profile from Equation (28.1) is used instead of data prepared in PREP_DATA. The meridional width of the sponge layer spng_width is hard wired to 3 degrees and the reciprocal of the damping time scale spng_damp is hard wired to 1/5 days. Their purpose is to damp Kelvin waves in idealzed equatorial models. As indicated in Section 3.1, use UNIX grep to find their location if changes are to be made.
The current implementation uses data defined at the latitude of the northern and southern walls as the data to which the solution is damped. This data varies monthly, but the annual mean values can be used instead by setting variable annlev in the namelist. Refer to Section 14.4 for information on namelist variables. The width of the sponge layers is determined by a Newtonain damping time scale that is a function of latitude and set in subroutine sponge which is executed by script run_sponge.
If a more realistic sponge layer is desired, data from latitude rows within the sponge layers needs to be saved instead of just the data at the latitude of the walls. This is a bit more I/O intensive and is not an option as of this writing. Refer to Sections 3.2 and 22.8.5 for further details.