[MITgcm-support] SSH drift and obcs again

[Thomas Haine]

OK, I've made a little more progress. The obc corners are the problem:

i) I can get zero cg2d_b and eta drift if I have obcs on opposite
boundaries (N&S or E&W). This requires me to put a solid wall on the
boundaries where I don't specify obcs. (Incidentally, I have zero eta
drift with N&S obcs and periodic E/W bcs but *not* vice versa. This I
don't understand but presumably it depends on my specific
configuration). So, obcs with no obc corners works OK - meaning I can
correct for net total convergence across the obcs accurately.

ii) I can get zero cg2d_b and eta drift with an obc corner (I've tried 1
and 3 corners) only if I put zero depth at each corner. I.e. adding a
single-cell island to block the obc corner works.

I've checked that: my obc forcing files have consistent speeds at the
corners (as Martin's message of 24 October) and that my bathy file has
no gradient at the corners.

So, how should I calculate the total convergence of fluid across my obcs
when the obcs have a corner?  If I can calculate this exactly then I can
correct for it and guarantee zero eta drift.

Thanks, Tom.
-- 
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 Thomas W. N. Haine
                                                                                                                                                              
 Associate Professor of Physical Oceanography,
 Department of Earth & Planetary Sciences,
 329 Olin Hall, The Johns Hopkins University,
 Baltimore, MD 21218, USA.
 Tel : 410 516 7048,   Fax : 410 516 7933
 Thomas.Haine at jhu.edu
 http://www.jhu.edu/~eps/faculty/haine
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