Open boundary conditions are based on the methodology of Stevens (1990). There are two types of open boundary conditions: `active' in which the interior is forced by data prescribed at the boundary and `passive' in which there is no forcing at the boundary and phenomena generated within the domain can propagate outward without disturbing the interior solution.
Open boundaries may be placed along the northern, southern, eastern and/or western edges of the domain. At open boundaries, baroclinic velocities are calculated using linearized horizontal momentum equations and the streamfunction is prescribed from other model results or calculated transports (e.g. directly or indirectly from the Sverdrup relation). Thus, the vertical shear of the current is free to adjust to local density gradients. Heat and salt are advected out of the domain if the normal component of the velocity at the boundary is directed outward. When the normal component of the velocity at the boundary is directed inward, heat and salt are either restored to prescribed data (`active' open boundary conditions) or not (`passive' open boundary conditions).
In contrast to the above described `active' open boundary conditions, `passive' ones are characterized by not restoring tracers at inflow points. Additionally, a simple Orlanski radiation condition (Orlanski 1976) is used for the streamfunction.
Refer to Chapter 20 for all the options and details.