[ecco-support] [EXTERNAL] How to interpret adjoint sensitivities under sea ice in ECCOv4r4

[Wang, Ou (US 329B)]

Hi Emma,

It is correct that in the Release 4 adjoint, seaice_model_ad  and seaice_fake_ad are entirely skipped.

Since sensitivities to atmospheric forcings are not directly altered by the presence of sea ice, I agree with your interpretation that adxx_taux and adxx_tauy represent sensitivities to ocean TAUX/Y, the stress beneath the sea ice.

Best,
Ou

From: ecco-support <ecco-support-bounces at mit.edu> on behalf of Emma Boland <emmomp at bas.ac.uk>
Date: Wednesday, November 6, 2024 at 5:53 AM
To: ecco-support at mit.edu <ecco-support at mit.edu>
Subject: [EXTERNAL] [ecco-support] How to interpret adjoint sensitivities under sea ice in ECCOv4r4

Hi ECCO team,

I'm looking for some guidance in how to interpret what release 4 is doing with the adjoint under sea ice, especially the wind stress sensitivities.

As far as I can tell, the behaviour is controlled by useSEAICEinAdMode (set to false in data.autodiff) and SEAICEapproxLevInAd (appears to be left at default value of 0).

autodiff_params.h says:

"   SEAICEapproxLevInAd :: level of approximation in seaice adjoint
C       -1 (and .NOT.useSEAICEinAdMode) : use seaice_fake adjoint
C       0 (and .NOT.useSEAICEinAdMode)  : omit all of seaice thermo adjoint
C       0 (and useSEAICEinAdMode)       : use all of seaice thermo adjoint
C       >= 1 (and useSEAICEinAdMode)    : omit pieces of seaice thermo adjoint "

So release 4 falls in the second category, "omit all of seaice thermo adjoint". In the adjoint, in do_oceanic_phys_ad, this skips entirely seaice_model_ad  and skips seaice_fake_ad.

Is this correct? I.e. surface sensitivities are not directly altered by the presence of sea ice (although indirectly influenced by the ocean-ice state)? 'omit all of thermo adjoint' seems to imply there is some 'seaice dynamic adjoint' applied, but I can't obviously see where this is. I'd appreciate any guidance, especially as to whether adxx_taux/y should be therefore interpreted as sensitivities to oceTAUX/Y under ice, rather than EXFtaux/y.

Thanks,

Emma

--
Dr Emma Boland | Physical Oceanographer | British Antarctic Survey
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