Hycom long-term mean sea level

[ivan soares]

Dear Hycom forum manager,

Where can I get the long-term mean SSH which is used by HYCOM as MSL when it assimilates SLA ?

I use HYCOM-NCODA as OBCs in my regional modelling with ROMS. I am having a problem with differences in SSH used in the open boundaries and SSH computed by ROMS in the interior, so I would like to correct the SSH in the borders.

thank you

Ivan Soares

dsoare...@gmail.com

Oceanografia e Tecnologia do Meio Ambiente

Florianopolis, SC, Brasil

[Michael McDonald]

Ivan, You should be able to find the "meanstd" (monthly/yearly mean fields) files for the GOFS 3.1 reanalysis covering 1994 to 2015 here,

https://www.hycom.org/dataserver/gofs-3pt1/reanalysis

see the "meanstd NetCDF (monthly/yearly mean fields)" link under "Data Formats" near the top. 

https://data.hycom.org/datasets/GLBv0.08/expt_53.X/meanstd/netcdf/

[ivan soares]

Dear Michael, thank you very much. After all those years using HYCOM I had never seen this link in the website.

Can I ask a few questions? I understand that SSH is not a prognostic variable in HYCOM, but I see in the study by Cummings (2007) and also in other papers found in the link to Data Assimilation, that altimeter SSH data are assimilated in HNCODA. I think that the correlation between SSH and layer thickness is used somehow. In the process of assimilating SSH you actually use SLA, right ? SLA = SSH - MSL.

Would you say that the mean GLBv0.08 SSH (above) can be used as MSL to obtain SLA from a given SSH ?

I am asking it because I am using HYCOM U/V/T/S/SSh in the open boundaries of a regional ROMS domain. After a few years of model integration, the annual mean SSh is higher near the boundaries than in the middle of the domain. This is causing higher errors near the boundaries in my simulations with Ensemble Optimal Interpolation.  So, I want to correct HYCOM SSh before I impose it in the open boundaries. I want to decrease the hycom MSL and add the satellite MDT to correct SSH.

Well, please let me know what are your insights about it.

Best,

Ivan

Ivan Dias Soares

dsoare...@gmail.com

Oceanografia e Tecnologia do Meio Ambiente

Florianopolis, SC, Brasil

[Alan Wallcraft]

GOFS uses synthetic profiles to assimilate SLA, and in GOFS 3.1 this is via SOP  It does not explicitly use a MSL.

For an overview of ISOP see:

T.L. Townsend, C.N. Barron and R.W. Helber, 2015:
Ocean Prediction with Improved Synthetic Ocean Profiles (ISOP)
NRL Review 105-112

and for more details see:

R. W. Helber, T. L. Townsend, C. N. Barron, J. M. Dastugue, and M. R. Carnes, 2013:
Validation Test Report for the Improved Synthetic Ocean Profile (ISOP) System, Part 1: Synthetic Profile Methods and Algorithm
NRL Report NRL/MR/7320--13-9364

If you want to recover SLA, I suggest using http://data.hycom.org/datasets/GLBv0.08/expt_53.X/meanstd/netcdf/GLBv0.08_53X_archMN.1994_01_2015_12_ssh.nc as the MSL.

An alternative might be to calculate the regional mean SSH (one number) from GLBv0.08_53X_archMN.1994_01_2015_12_ssh.nc for your region of interest and simply subtract this from every point of the GOFS 3.1 SSH before using it.  The point is that adding a constant to SSH has no dynamical effect and what you are describing might be that your regional model has a different region-wide mean SSH than you get from the Global GOFS SSH.

Alan.

[ivan soares]

Dear Alan,

I will try GLBv0.08_53X_archMN.1994_01_2015_12_ssh.nc as MSL. Subtracting the regional mean value may also work. I will let you know when I get my new results. 

Thank you for the info and thank you for the references about ISOP.

Best,

Ivan

[John Osborne]

I think Alan has a good approach here. For perspective on a related problem, Iermano et al. 2016 [1] describe assimilating absolute dynamic topography data into a regional ROMS model. They needed to ensure the regional model mean SSH and the ADT mean were equal. They did not detail any issues with SSH offsets between their regional model and parent model.

[1] https://www.sciencedirect.com/science/article/pii/S092479631500158X, section 2.1.3