29.1 Concerning which external mode option to use

The methods available for updating the velocity field are the following:

• Option stream_function uses the Bryan (1969) rigid lid streamfunction approach.
• Option rigid_lid_surface_pressure uses the rigid lid surface pressure approach of Dukowicz, Smith, and Malone (1993).
• Option implicit_free_surface employs the implicit free surface method of Dukowicz and Smith (1994).
• Option explicit_free_surface and explicit_eb or explicit_fb employ the free surface method of Killworth, Stainforth, Webb and Paterson (1991).
• Option explicit_free_surface employs an explicit free surface method which fully incorporates the undulating surface height within the baroclinic momentum and tracer equations. It also allows for longer tracer time steps than the Killworth et al method.

Researchers at GFDL are using the last option (explicit free surface) most frequently (Summer 1999). This method is the only one which will be available in post MOM 3 releases of MOM.