Climate Change Modeling Methodology pp 63-87 | Cite as

# Oceanic General Circulation Models

## Abstract

The purpose of this text is to provide an introduction to aspects of oceanic general circulation models (OGCMs), an important component of Climate System or Earth System Model (ESM).

## Keywords

Ocean Model Oceanic General Circulation Model Mesoscale Eddy Thermohaline Circulation Couple Climate Model## Glossary

- Bathymetry
The depth of ocean floor. Bathymetric map represents the terrain of the seafloor.

- Biogeochemical cycle
A pathway that is described by the physical, biological, and chemical processes that control the evolution of elements found in the Earth system.

- Boussinesq approximation
In Boussinesq approximation, a fluid parcel is assumed to maintain the same volume or density during its transport because of near incompressibility. This approximation was named after French physicist and mathematician, Joseph Valentin Boussinesq. By adopting this approximation, sound waves that propagate through a density change can be eliminated in numerical model.

- Diapycnal mixing
Mixing of a fluid across different density surfaces. To be contrasted with isopycnal mixing which occurs along the same density surfaces.

- El Niño-Southern Oscillation (ENSO)
A quasi-periodic change of the ocean and atmospheric conditions along the equatorial Pacific Ocean. The change in the sea surface temperature (SST) can be as large as ±2°C during its extreme phases: anomalously warm over the tropical Pacific (El Niño) and cold (La Niña). Surface air pressures measured at both ends of the tropical Pacific basin vary closely with the change of SST.

- Geostrophic approximation
The angular momentum is balanced by the Coriolis force and the horizontal pressure gradient force. It is generally true when the spatial and temporal scales are large, roughly over 100 km and a few days in the deep ocean.

- Hydrostatic approximation
The equation describing vertical motion of the ocean column is simplified to assume that the vertical pressure at any level is due to the weight of the air and water above it. Variation of density is considered only in vertical direction when gravitational acceleration term (g) exists. This is valid when the vertical scale of a feature is small compared to the horizontal scale for both the atmosphere and ocean.

- Isopycnal coordinate
Vertical coordinate that follows a constant density surface.

- Meridional overturning circulation (MOC)
This has often been assumed to be the same as the thermohaline circulation. However, the MOC explicitly describes the ocean circulation system with the upwelling/downwelling and associates the northward/southward transport.

- Ocean gyre
A large-scale rotating circulation in the ocean primarily forced by the atmospheric wind and the Coriolis force. These include the North Atlantic Gyre, South Atlantic Gyre, Indian Ocean Gyre, North Pacific Gyre, and South Pacific which tend to be more elongated in the east-west direction. There are also other types of Gyre forced by different mechanisms such as baroclinicity.

- (Oceanic) Mesoscale eddy
A vigorous rotational circulation or vortex at spatial scales roughly 100 km and smaller, existing for weeks to months.

- Rossby radius (of deformation)
The horizontal scale at which rotational effect becomes as important as buoyancy or gravity wave effects. Mathematically, this can be computed in terms of potential temperature, temperature, wind speed, or the depth of the boundary layer. This radius is important in determining the phase speed and wavelength of Rossby waves.

- Salinity
Dissolved content of the salt in the ocean. Traditionally, salinity is represented in the unit of either g/Kg or PSU (Practical Salinity Unit).

- Shallowness approximation
This approximation can be applied when the vertical-to-horizontal aspect ratio is very small.

- Structured (regular)/ unstructured (irregular) grid
A structured (unstructured) grid has regular (irregular) connectivity with neighboring points. In structured (unstructured) grid, its connectivity can (cannot) be easily represented with a two- or three-dimensional array.

- Thermocline
A distinct ocean layer where the temperature changes greatly with its depth compared to the layers above and below. It is often thought of as a boundary separating the well-mixed upper ocean and the deep ocean.

- Thermohaline circulation
The global oceanic circulation driven by the density gradients, primarily determined by salinity and temperature.

- (Atmospheric) Wind stress
The horizontal force exerted by the atmospheric wind on the ocean surface. This can also be interpreted as the vertical transfer of horizontal momentum from the atmosphere to the ocean surface. Wind stress is a function of the square of the wind speed.

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