[MITgcm-support] Small timestep - rapid overadjustment

[Chris Horvat]

Hi,

I'm running a reentrant ocean channel configuration (120km by 40km), and
OBCS is setting temperatures on the north and sound boundaries in order to
force a temperature gradient. Initial conditions interpolate between the
boundaries. It is an Arctic simulation, so the deep water is warmer than
the top water. (Temp. profile at http://imgur.com/4RZ0X)


Here's the problem. If I set my timestep to be too large (~3600 s),
numerical instability kills me in a few steps. If I set smaller (~600 s), I
can get much further before I run into a similar numerical error. (maybe 2
mo.s), and the first thing that happens, as expected, is the interior
begins to relax to match the imposed gradient. (seen at
http://imgur.com/Ofo5Q)

So my thought is, setting deltaT = 30s would help... but instead what I see
is an incredibly unrealistic change in the temperature profile. The
gradient is immediately wiped out in favor of extreme gradients on either
side of the channel with weak interior gradient. (Seen at
http://imgur.com/Od1lM)

Could someone give me some guidance as to what might be causing this?

Thanks!

Chris
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