Ocean general circulation model

[citation needed] They depict oceans using a three-dimensional grid that include active thermodynamics and hence are most directly applicable to climate studies.

They are the most advanced tools currently available for simulating the response of the global ocean system to increasing greenhouse gas concentrations.

[1] A hierarchy of OGCMs have been developed that include varying degrees of spatial coverage, resolution, geographical realism, process detail, etc.

The first generation of OGCMs assumed “rigid lid” to eliminate high-speed external gravity waves.

However, in order to analyze those eddies and currents in numerical models, we need grid spacing to be approximately 20 km in middle latitudes.

Thanks to those faster computers and further filtering the equations in advance to remove internal gravity waves, those major currents and low-frequency eddies then can be resolved, one example is the three-layer quasi-geostrophic models designed by Holland.

[5] In the late 1980s, simulations could finally be undertaken using the GFDL formulation with eddies marginally resolved over extensive domains and with observed winds and some atmospheric influence on density.

Early in the 1990s, for those large-scale and eddies resolvable models, the computer requirement for the 2D ancillary problem associated with the rigid lid approximation was becoming excessive.

OGCMs have many important applications: dynamical coupling with the atmosphere, sea ice, and land run-off that in reality jointly determine the oceanic boundary fluxes; transpire of biogeochemical materials; interpretation of the paleoclimate record;climate prediction for both natural variability and anthropogenic chafes; data assimilation and fisheries and other biospheric management.

[14] Oceans are a kind of undersampled nature fluid system, so by using OGCMs we can fill in those data blank and improve understanding of basic processes and their interconnectedness, as well as to help interpret sparse observations.

The big advantage of finite element grids is that it allows flexible resolution throughout the domain of the model.

The vertical grids used for ocean general circulation models are often different from their atmospheric counterparts.

Z-coordinate systems have difficulties representing the bottom boundary layer and downslope flow due to odd diabatic mixing.

[20] In a sigma coordinate system the bottom topography determines the thickness of the vertical layer at each horizontal grid point.

Sigma coordinates allow the boundary layer to be better represented but have difficulties with pressure gradient errors when sharp bottom topography features are not smoothed out.

[16][21] Molecular friction rarely upsets the dominant balances (geostrophic and hydrostatic) in the ocean.

With kinematic viscosities of v=10−6m 2 s−1 the Ekman number is several orders of magnitude smaller than unity; therefore, molecular frictional forces are certainly negligible for large-scale oceanic motions.

A similar argument holds for the tracer equations, where the molecular thermodiffusivity and salt diffusivity lead to Reynolds number of negligible magnitude, which means the molecular diffusive time scales are much longer than advective time scale.

Although there is a considerable degree of overlap and interrelatedness among the huge variety of schemes in use today, several branch points maybe defined.

Those special dynamical parameterizations (topographic stress, eddy thickness diffusion and convection) are becoming available for certain processes.

In the vertical, the surface mixed layer (sml) has historically received special attention because of its important role in air-sea exchange.

In the horizontal, parameterizations dependent on the rates of stress and strain (Smagroinsky), grid spacing and Reynolds number (Re) have been advocated.

Therefore, the principle direction of mixing is neither strictly vertical nor purely horizontal, but a spatially variable mixture of the two.

This equilibrium is often defined as a statistical parameter at which the change over time of a range of variables gets below a set threshold for a certain number of simulation timesteps.

In this method, the temperature and salinity fields are repeatedly extrapolated with the assumption that they exponentially decay towards their equilibrium value.

These boundary conditions on ocean flows are difficult to define and to parameterize, which results in a high computationally demand.

The closure of the different passages in the ocean can then be simulated by simply blocking them with a thin line in the bathymetry.