[MITgcm-devel] tidal forcing

[Jean-Michel Campin]

Hi Dimitris and other,

Until now, when you run some global ocean with tides, the tidal forcing
was specified as an equivalent atmospheric pressure, presumably through pkg/exf.

There are few limitations with this approach:
1) the exf atmospheric pressure field cannot be used for something else,
 such as a more precise estimation of air density (#define EXF_CALC_ATMRHO).
2) the tide geopotential is added to the ocean pressure field (totPhiHyd) 
 which prevent to use it inside the EOS (selectP_inEOS_Zc=2, default
 with EOS: JMD95P, UNESCO, MDJWF and TEOS10). 
3) if one want to specify both an atmospheric loading and a tidal forcing,
 the forcing input field become a mix, not practical nor very natural.
 And this is also the case when coupled to atmospheric GCM.

Therefore, I propose to add an other 2-D forcing field to account for
tidal forcing. The most natural way would be a tidal geopotential (or
any time-dependent geopotential anomaly), units: m^2/s^2.

An other advantage is that it would be available in the same form for
Ocean in p-coords set-up as well as for atmospheric set-up.

The gradient of it would be added to horizontal momentum tendency as 
part of the forcing (in S/R EXTERNAL_FORCING_U & EXTERNAL_FORCING_V)
which is different form current atmospheric loading, for instance regarding
the Adams-Bashforth (pressure gradient in or out AB depending on staggerTimeStep, 
whereas forcing has it's own parameter: momForcingOutAB).

Any suggestion or comments ?

Cheers,
Jean-Michel

On Fri, Sep 29, 2017 at 04:41:26PM +0000, Menemenlis, Dimitris (329C) wrote:
> Jean-Michel and Atanas, two small coupling details that worry me:
> 
> 1. How do we include tides?  Right now for ocean only simulations
> we add hourly tidal potential forcing to atmospheric pressure.
> What is best way to do this in coupled model?
>