OMCT - Model Physics
The Ocean Model for Circulation and Tides
The Ocean Model for Circulation and Tides (OMCT; Thomas, 2002) has been developed by adjusting the originally climatological Hamburg Ocean Primitive Equation model (HOPE; Wolff et al., 1997; Drijfhout et al., 1996) to the weather time-scale and coupling with an ephemeral tidal model. It is based on nonlinear balance equations for momentum, the continuity equation, and conservation equations for heat and salt. The hydrostatic and the Boussinesq approximations are applied. Water elevations, three-dimensional horizontal velocities, potential temperature as well as salinity are calculated prognostically, vertical velocities are determined diagnostically from the incompressibility condition. Implemented is a prognostic thermodynamic sea-ice model (Hibler, 1979) that predicts ice-thickness, compactness and drift. Dynamic effects of loading and self-attraction of the water column are accounted for by means of a secondary potential proportional to the mass of the local water column (Thomas et al., 2001). Tidal dynamics are optionally considered by the complete lunisolar tidal potential of second degree calculated from analytical ephemerides, i.e., distance, right ascension, and declination of the Moon and the Sun according to VSOP87 theory (Bretagnon and Francou, 1988).
Heat exchange between atmosphere and ocean is parameterized by Newtonian coupling of atmospheric 2m-temperatures and sea surface temperatures assuming a flux constant of 40 W m-2 K-1, omitting any relaxation to climatological sea suface temperatures. Sea surface salinity is relaxed towards the World Ocean Atlas 2001 climatology (Conkright, 2002) with a relaxation time-scale of 38 days.
Atmospheric forcing includes wind stress, surface pressure as well as heat and freshwater fluxes. Continental freshwater fluxes provided by, e.g., a hydrological discharge model, can be additionally taken into account in order to nearly close the global hydrological cycle. Several simulations with various forcing conditions are carried out in order to identify the effects of atmospheric pressure forcing, loading and self-attraction, as well as the impact of ocean tides on the ocean's general circulation.
As most state-of-the-art ocean general circulation models, the OMCT applies the Boussinesq approximation, which causes the model to rather conserve volume than mass. In order to make the simulated data suitable for geodetic applications, the total ocean mass is held artificially constant by adding or removing a (small) homogeneous layer of mass at each time-step (Greatbatch, 1994). However, freshwater fluxes due to precipitation, evaporation and continental runoff represent real mass fluxes altering the total ocean mass and cause the eustatic component of sea-level to change. The model can be configured to allow the total ocean mass to vary in accordance with these fluxes.
In its original configuration, the OMCT uses a time-step of 30 minutes, a constant horizontal resolution of 1.875° in longitude and latitude, and 13 layers in the vertical. A quasi steady-state circulation has been obtained from an initial model spin-up for 265 years using climatological wind stresses (Hellerman and Rosenstein, 1983) and mean sea surface temperatures and salinities according to the climatology provided by Levitus (1982).
Information obtained from this page can be referenced as:
Thomas, M. (2002), Ocean induced variations of Earth's rotation - Results from a simultaneous model of global circulation and tides, PhD dissertation, University of Hamburg, Germany, 129pp.
References and further reading:
- Bretagnon, P., Francou, G. (1988), Planetary Theories in rectangular and spherical variables: VSOP87 solution, Astron. Astrophys. 202, 309.
- Conkright, M.E., Locarnini, R.A., Garcia, H.E., O Brien, T.D., Boyer, T.P., Stephens, C., Antonov, J.I. (2002), World Ocean Atlas 2001: Objective Analyses, Data Statistics, and Figures, National Oceanographic Data Center, Silver Spring, Maryland, 17pp.
- Dobslaw, H. (2007), Modellierung der allgemeinen ozeanischen Dynamik zur Korrektur und Interpretation von Satellitendaten, Scientific Technical Report 07/10, Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Germany, 113pp.
- Drijfhout, S., Heinze, C., Latif, M., Maier-Reimer, E. (1996), Mean circulation and internal variability in an ocean primitive equation model, J. Phys. Oceanogr., 26, 559-580.
- Greatbatch, R.J. (1994), A note on the representation of steric sea level in models that conserve volume rather than mass, J. Geophys. Res., 99, C6, 12767-12771.
- Hellerman, S., Rosenstein, M. (1983), Normal monthly wind stress over the world ocean with error estimates, J. Phys. Oceanogr., 13, 1093-1104.
- Hibler III, W.D. (1979), A dynamic thermodynamic sea ice model, J. Phys. Oceanogr., 9, 815-846.
- Levitus, S. (1982), Climatological atlas of the world ocean, NOAA professional paper, 13, U.S. Department of Commerce, 173pp.
- Thomas, M., Sündermann, J., Maier-Reimer, E. (2001), Consideration of ocean tides in an OGCM and impacts on subseasonal to decadal polar motion excitation, Geophys. Res. Lett., 28, 12, 2457-2460.
- Wolff, J.O., Maier-Reimer, E., Legutke, S. (1997), The Hamburg Ocean Primitive Equation Model HOPE, Technical Report 13, Deutsches Klimarechenzentrum, Hamburg, Germany, 103pp.