[MITgcm-support] definition of PHIBOT in ECCO 4

[Quinn, Katherine]

To get ocean bottom pressure from PHIBOT (i.e. phiHydLow) you need to
make the Greatbatch correction to remove spurious mass variations.  This
bottom pressure includes freshwater flux.  In theory, if atmospheric
pressure forcing is used, PHIBOT will also contain atmospheric pressure
but it isn't in the latest ECCO release.  If you want to remove the
spurious mass variations and freshwater flux, just remove a spatial mean
at each time step from PHIBOT.  Note, a time invariant term of
depth*gravity (m^2/s^2 units) is missing from PHIBOT, add this back if
you want the total ocean mass.

If you want to see the details, look at how Mload (total OBP in kg/m^2)
is calculated in the latest updates of sbo_calc.F
(http://mitgcm.org/viewvc/MITgcm/MITgcm/pkg/sbo/sbo_calc.F).  Version
1.15 uses phiHydLow, version 1.16 uses rhoInSitu and the finite volume
method.

cheers, Katy

On 08/15/2014 05:03 PM, Noel Bartlow wrote:
> Hi,
>
> I am new to oceanography and ECCO (I am a geodesist).  I am attempting to model some GPS displacements caused by ocean loading and would like to use the ocean bottom pressure, PHIBOT, from ECCO 4.  Forgive my naive questions, but I can’t seem to find anywhere an exact definition for PHIBOT.  My guess is that the actual bottom pressure is defined as:
>
> P= g*RhoConst*ETAN+PHIBOT*RhoConst
>
> But I am not sure.  Is ETAN the correct ocean height to use, or should it be RCENTER?  One varies in time and the other doesn’t.  Should I be taking into account the RhoAnomaly and integrating over the water column?  What about the fresh water flux (SIatmFW), is that taken into account or is ocean mass or volume conserved in PHIBOT?  Does PHIBOT include atmospheric pressure, or is it relative to the ocean surface?  I am interested in variations over time, especially seasonal variations, so I need to take these into account.
>
> Thanks to anyone who can help me figure this out.
>
> ____________________________________________
> Noel Bartlow
> Miles Postdoctoral Fellow
> Institute of Geophysics and Planetary Physics
> Scripps Institution of Oceanography
> University of California, San Diego
> Phone: (858) 534-4643
>
>
>
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>


--
Katherine J. Quinn
Atmospheric and Environmental Research         voice: 781-761-2234
131 Hartwell Avenue                              fax: 781-761-2299
Lexington, MA  02421-3126                     e-mail: kquinn at aer.com


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