Baroclinic vs. barotropic mode

Mode descriptions

Some description of the Baroclinic and Barotropic equations is given in the SELFE paper. A little elaboration will convey how these are implemented and used in practice.

The baroclinic and barotropic modes are toggled in the model by means of the parameter ***. They differ in that in Baroclinic mode, the contribution of salinity-induced density differences is included in the momentum equations. In contrast, in Barotropic mode salt and temperature are (at most) considered passive tracers.

In barotropic mode:

• You can choose whether to calculate salt and temperature transport by toggling the itracer parameter in the param.in file.
• This choice affects performance (see below) and also data requirements for salt and temperature. The model (****is/isn't smart) about ...
• The contribution of density to horizontal pressure gradients is dropped.
• The turbulence closure is still calculated in 3D, but the supression of turbulence due to stratification is not included.

Performance considerations

Barotropic mode calculation can be a lot faster than baroclinic. A key speedup is achieved by setting itracer=0 and neglecting salt and temperature entirely, which is particularly common in the case that subcycling of transport is a performance limiter.

Sequential barotropic-baroclinic analysis

One common usage pattern is to use a sequence of barotropic-baroclinic model runs. The barotropic simulation is used in a preliminary sweep in order to calculate 3D boundary conditions for the subsequent baroclinic analysis. Typically this is done with transport off, maximizing speed for the barotropic step.