The wind stress curl is determined by the juxtaposition of the zonal tradewinds in the subtropics
and the mid-latitude westerlies (Fig. 1.2.2). It
forces anticyclonic subtropical and cyclonic subpolar gyres in each ocean basin (Fig. 1.2.3, see
Plate 1.2.3, p. 44, and Niiler, Fig. 4.1.4). On the
rotating earth, the wind stress curl acts to force
water into (out of) the interior of the subtropical
(subpolar) gyre. To conserve potential vorticity the
water column moves equatorward (poleward) in the
subtropical (subpolar) gyre. This Sverdrup balance
is one of the main building blocks of ocean circulation theory (Webb and Suginohara, Chapter 4.2).
In the subtropical gyres, it generates an equatorward transport of several tens of millions of cubic
metres per second.
Volume and vorticity conservation require that
water moved meridionally by the gyres circulates
back to its initial latitude within western boundary
currents. These currents are narrow (ϳ100 km),
intense (ϳ1 m s
91 ) and deep (91000 m) baroclinic
jets carrying warm water to higher latitudes (see
below). The resulting warm sea surface temperatures and the sharp horizontal gradients generate
intense atmospheric low pressure systems in
the western boundary current regions (e.g. Cione
et al., 1993).
In the absence of a continent to provide a meridional boundary in the Southern Ocean, the strong
westerlies drive the energetic eastward flow of the
Antarctic Circumpolar Current, resulting in large
interbasin transfers of heat and mass (Fig. 1.2.3,
see Plate 1.2.3, p. 44). This circulation feature is
central in the oceans’ role in climate (Rintoul et al.,
Chapter 4.6).
Near the equator, the Coriolis force reduces to
zero. The increasing contribution of non-geostrophic
dynamics in both the ocean and atmosphere
results in a complex system of zonal equatorial
currents (Godfrey et al., Chapter 4.3) which play
an important role in interhemispheric exchanges
(Fratantoni and Richardson, 1999). The only
direct low-latitude ocean basin interconnection is
between the Pacific and Indian Oceans via the
Indonesian Throughflow. This warm, westwardflowing low-salinity transport plays a central role
in determining the oceanic heat and freshwater
budgets of both the Pacific and Indian Oceans and
in closing the global ocean thermohaline circulation (Gordon, Chapter 4.7). There is evidence that
the strength of the exchange is linked to the phase
of El Niño-Southern Oscillation (ENSO); (Godfrey,
1989; Gordon, Chapter 4.7).
As the wind-driven circulation moves water
around the subtropical gyres, it also moves it vertically from the surface into the main thermocline
and then back into the surface. Surface density
(determined by the combination of ocean circulation and air–sea exchange, Nurser and Large,
Chapter 5.1) increases with increasing latitude. In
those parts of the gyre where the water is moved
equatorward, it moves into regions of lighter surface
water and hence it descends or is subducted into the
ocean interior as it flows along surfaces of constant
density (Price, Chapter 5.3). Once subducted, these
waters are transported around the gyres at depth
within the main thermocline. When they return to
their original latitude 10–50 years later (the longer
times being for the denser and deeper waters), they
will again enter the surface layer through their
participation in winter convection. Heat, salt and
other properties carried into the main thermocline
through this mechanism remain isolated from the
atmosphere for several decades. The volume of this
reservoir is five to ten times greater than the layer
that participates directly in the seasonal cycle.
Using low values of diapycnal mixing as
measured in the main thermocline (Toole and
McDougall, Chapter 5.2), and specified wind stress
and surface density fields, the subduction process, or
thermocline theory (Price, Chapter 5.3; Pedlosky,
1996), has successfully explained many of the major
features of the density field of the upper 1000 m of
the main gyres. Examples include the tongues of
low-salinity Antarctic Intermediate Water penetrating northward near the base of the subtropical gyres
at depths of 800–1000 m in the Atlantic and Pacific
Oceans (Figs 1.2.4 and 1.2.5, see Plate 1.2.4, p. 44,
and Plate 1.2.5, p. 44) and the tongue of low-salinity
North Pacific Intermediate Water penetrating southward in the North Pacific at depths of about 500 m
(Fig. 1.2.5, see Plate 1.2.5, p. 44). The subduction
process also operates in the subtropics and sets the
properties of the equatorial thermocline (Liu and
Philander, Chapter 4.4). Correctly simulating these
processes will be important for reliable projections
of possible changes in ENSO events as reported in
recent studies (e.g. Timmermann et al., 1999a).
Boundary currents
Western boundary currents are necessary to satisfy
volume and vorticity conservation. Intense western
SECTION 1 THE OCEAN AND CLIMATE
18
and the mid-latitude westerlies (Fig. 1.2.2). It
forces anticyclonic subtropical and cyclonic subpolar gyres in each ocean basin (Fig. 1.2.3, see
Plate 1.2.3, p. 44, and Niiler, Fig. 4.1.4). On the
rotating earth, the wind stress curl acts to force
water into (out of) the interior of the subtropical
(subpolar) gyre. To conserve potential vorticity the
water column moves equatorward (poleward) in the
subtropical (subpolar) gyre. This Sverdrup balance
is one of the main building blocks of ocean circulation theory (Webb and Suginohara, Chapter 4.2).
In the subtropical gyres, it generates an equatorward transport of several tens of millions of cubic
metres per second.
Volume and vorticity conservation require that
water moved meridionally by the gyres circulates
back to its initial latitude within western boundary
currents. These currents are narrow (ϳ100 km),
intense (ϳ1 m s
91 ) and deep (91000 m) baroclinic
jets carrying warm water to higher latitudes (see
below). The resulting warm sea surface temperatures and the sharp horizontal gradients generate
intense atmospheric low pressure systems in
the western boundary current regions (e.g. Cione
et al., 1993).
In the absence of a continent to provide a meridional boundary in the Southern Ocean, the strong
westerlies drive the energetic eastward flow of the
Antarctic Circumpolar Current, resulting in large
interbasin transfers of heat and mass (Fig. 1.2.3,
see Plate 1.2.3, p. 44). This circulation feature is
central in the oceans’ role in climate (Rintoul et al.,
Chapter 4.6).
Near the equator, the Coriolis force reduces to
zero. The increasing contribution of non-geostrophic
dynamics in both the ocean and atmosphere
results in a complex system of zonal equatorial
currents (Godfrey et al., Chapter 4.3) which play
an important role in interhemispheric exchanges
(Fratantoni and Richardson, 1999). The only
direct low-latitude ocean basin interconnection is
between the Pacific and Indian Oceans via the
Indonesian Throughflow. This warm, westwardflowing low-salinity transport plays a central role
in determining the oceanic heat and freshwater
budgets of both the Pacific and Indian Oceans and
in closing the global ocean thermohaline circulation (Gordon, Chapter 4.7). There is evidence that
the strength of the exchange is linked to the phase
of El Niño-Southern Oscillation (ENSO); (Godfrey,
1989; Gordon, Chapter 4.7).
As the wind-driven circulation moves water
around the subtropical gyres, it also moves it vertically from the surface into the main thermocline
and then back into the surface. Surface density
(determined by the combination of ocean circulation and air–sea exchange, Nurser and Large,
Chapter 5.1) increases with increasing latitude. In
those parts of the gyre where the water is moved
equatorward, it moves into regions of lighter surface
water and hence it descends or is subducted into the
ocean interior as it flows along surfaces of constant
density (Price, Chapter 5.3). Once subducted, these
waters are transported around the gyres at depth
within the main thermocline. When they return to
their original latitude 10–50 years later (the longer
times being for the denser and deeper waters), they
will again enter the surface layer through their
participation in winter convection. Heat, salt and
other properties carried into the main thermocline
through this mechanism remain isolated from the
atmosphere for several decades. The volume of this
reservoir is five to ten times greater than the layer
that participates directly in the seasonal cycle.
Using low values of diapycnal mixing as
measured in the main thermocline (Toole and
McDougall, Chapter 5.2), and specified wind stress
and surface density fields, the subduction process, or
thermocline theory (Price, Chapter 5.3; Pedlosky,
1996), has successfully explained many of the major
features of the density field of the upper 1000 m of
the main gyres. Examples include the tongues of
low-salinity Antarctic Intermediate Water penetrating northward near the base of the subtropical gyres
at depths of 800–1000 m in the Atlantic and Pacific
Oceans (Figs 1.2.4 and 1.2.5, see Plate 1.2.4, p. 44,
and Plate 1.2.5, p. 44) and the tongue of low-salinity
North Pacific Intermediate Water penetrating southward in the North Pacific at depths of about 500 m
(Fig. 1.2.5, see Plate 1.2.5, p. 44). The subduction
process also operates in the subtropics and sets the
properties of the equatorial thermocline (Liu and
Philander, Chapter 4.4). Correctly simulating these
processes will be important for reliable projections
of possible changes in ENSO events as reported in
recent studies (e.g. Timmermann et al., 1999a).
Boundary currents
Western boundary currents are necessary to satisfy
volume and vorticity conservation. Intense western
SECTION 1 THE OCEAN AND CLIMATE
18