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7 Ocean Currents
7.5 Modelling of Oceanic General Circulation
First we will define the meaning of 'oceanic general circulation'. According to
McWilliams (1996), the oceanic general circulation is a system which includes
currents at horizontal space and time scales larger than the mesoscale (which
is of the order of 100 km and three months), the associated pressure, density,
temperature and salinity fields, plus all other elements involved in establishing
the dynamical balance for these fields. Sometimes the general circulation also
includes the biochemical processes. Various oceanic general circulation models
(OGCM) have been developed relatively recently. A detailed description of the
OGCM is beyond the scope of this book. Here we will only provide some basic
characteristics of these models from the point of view of fluid mechanics and
illustrate some model results.
The OGCM's are based on the Navier-Stokes equations (see Appendix C) for
the rotating Earth and for sea water which comprises water and dissolved salts.
In the vertical direction, the hydrostatic approximation is assumed when the
vertical pressure gradient is totally determined by the vertical density profile.
Until recently the common practice has been to assume the ocean has a 'rigid
lid' at the surface. Thus no fluid is transferred across the surface, although
momentum, heat, and chemical components are. Under such an assumption,
surface waves are excluded and the dynamics of tides are distorted. The set of
the momentum equations and the mass-conservation equation, together with
equations for heat and salt conservation, plus equations of state used to determine the velocities u, v, w, the pressure p, etc. are known as the primitive
equations (Mellor, 1991).
The most widely used ocean model is that developed in the 1960s, at the
Geophysical Fluid Dynamics Laboratory of Princeton University, formerly a
part of the Institute for Advanced Study (Bryan, 1969; McWilliams, 1996).
Since then, a lot of improvements and modifications have been added to the
model. In the high-resolution version, the grid spacing is 1° longitude and 0.3°
latitude between lOON and 10 0 S, and 10 m in the vertical in the upper 100 m of
the ocean. The spacing increases poleward of 10° latitude and below the depth
of 100 m. The initial conditions for the model are climatological oceanographic
data (Levitus, 1982).
The major currents in the oceans which are at least qualitatively reproduced
by the OGCM are (McWilliams, 1996):
• the eastward Antarctic Circumpolar Current,
• the westward, horizontally divergent surface currents in the equatorial
Atlantic and Pacific, lying just above equatorial undercurrents,
• the Indonesian throughflow,
• the Somali gyre in the western tropical Indian Ocean (occurring mainly
during northern-hemisphere summer),
.
• the subtropical gyres with western boundary currents in all basins,
7 Ocean Currents
7.5 Modelling of Oceanic General Circulation
First we will define the meaning of 'oceanic general circulation'. According to
McWilliams (1996), the oceanic general circulation is a system which includes
currents at horizontal space and time scales larger than the mesoscale (which
is of the order of 100 km and three months), the associated pressure, density,
temperature and salinity fields, plus all other elements involved in establishing
the dynamical balance for these fields. Sometimes the general circulation also
includes the biochemical processes. Various oceanic general circulation models
(OGCM) have been developed relatively recently. A detailed description of the
OGCM is beyond the scope of this book. Here we will only provide some basic
characteristics of these models from the point of view of fluid mechanics and
illustrate some model results.
The OGCM's are based on the Navier-Stokes equations (see Appendix C) for
the rotating Earth and for sea water which comprises water and dissolved salts.
In the vertical direction, the hydrostatic approximation is assumed when the
vertical pressure gradient is totally determined by the vertical density profile.
Until recently the common practice has been to assume the ocean has a 'rigid
lid' at the surface. Thus no fluid is transferred across the surface, although
momentum, heat, and chemical components are. Under such an assumption,
surface waves are excluded and the dynamics of tides are distorted. The set of
the momentum equations and the mass-conservation equation, together with
equations for heat and salt conservation, plus equations of state used to determine the velocities u, v, w, the pressure p, etc. are known as the primitive
equations (Mellor, 1991).
The most widely used ocean model is that developed in the 1960s, at the
Geophysical Fluid Dynamics Laboratory of Princeton University, formerly a
part of the Institute for Advanced Study (Bryan, 1969; McWilliams, 1996).
Since then, a lot of improvements and modifications have been added to the
model. In the high-resolution version, the grid spacing is 1° longitude and 0.3°
latitude between lOON and 10 0 S, and 10 m in the vertical in the upper 100 m of
the ocean. The spacing increases poleward of 10° latitude and below the depth
of 100 m. The initial conditions for the model are climatological oceanographic
data (Levitus, 1982).
The major currents in the oceans which are at least qualitatively reproduced
by the OGCM are (McWilliams, 1996):
• the eastward Antarctic Circumpolar Current,
• the westward, horizontally divergent surface currents in the equatorial
Atlantic and Pacific, lying just above equatorial undercurrents,
• the Indonesian throughflow,
• the Somali gyre in the western tropical Indian Ocean (occurring mainly
during northern-hemisphere summer),
.
• the subtropical gyres with western boundary currents in all basins,