29.2.6 zero_island_flow

Specifying the value of the stream function $\psi$ on two or more distinct land masses amounts to an over-specification of conditions for solving the stream function equation. This over-specification is strictly not correct. Nevertheless, it is sometimes useful to be able to specify a zero net transport between two land masses. Option zero_island_flow effectively combines the coastal perimeter cells from two arbitrary land masses into one perimeter which implies that the stream function will have the same value on both land masses; even though the land masses are not physically connected. Therfore, there is no net transport between the land masses.

Option zero_island_flow requires ``land_mass_a'' and ``land_mass_b'' to be specified through namelist. Values for ``land_mass_a'' and ``land_mass_b'' can be taken from the island map which is printed out when MOM 2 executes. Refer to Section 14.4 for information on namelist variables. It must always be kept in mind that option zero_island_flow is an over-specification which may have side effects. To judge whether these side effects are significant or not, a companion experiment should be run without specifying the flow between land masses. Although this option is only written to specify zero net transport between two land masses, it can be easily extended to handle more land masses.

Note: Specifying a non-zero net transport between arbitrary land masses has been tried by re-setting $\psi$ after it has been predicted by the conjugate gradient solver. This method seems to work for land masses with relatively short perimeters but fails for land masses with long perimeters. The right way to specify flow is to modify the conjugate gradient solver by specifying the net flow between land masses within the ``scan'' loop. In principle, a modification of this kind should work but, so far, attempts have failed.