Elements of Physical Oceanography 2.5 Wind-Forced Ocean Processes 37
Part A | 2.5
ary of all the ocean basins, exceeding 200 cm=s in the
Gulf Stream of North Atlantic and in the Kuroshio
of the North Pacific. The total transport of the Gulf
Stream has been estimated to be in the range of 5090
10
6 m
3
=s; with that of the Kuroshio being somewhat
less (Table 2.6).
Despite the greater intensity of winds in the Southern Hemisphere, the intensity of the surface western
boundary currents is less than that for either the Gulf
Stream or Kuroshio. The Brazil current in the western
South Atlantic has typical speeds of 50100 cm=s and
penetrates to depths of only about 400 m. in contrast to
800 m for the Gulf Stream. The reduced transport of the
Brazil Current may be related to the presence of deeper
western boundary undercurrents and their role in transporting heat poleward. The reduced intensity of the Australian current in the western Pacific and the Agulhas
Current in the western Indian Ocean may be related to
the lack of complete continental blockage of zonal flow.
The equatorward flow along the eastern boundaries
of the wind-driven gyre as represented by the Canary
and California Currents is generally less intense and
less organized than the western boundary currents. The
exception is the Benguela Current (Fig. 2.34b), which
is an apparent extension of the Agulhas Current in the
South Atlantic.
At tropical latitudes straddling the equator, significant trade winds can drive surface currents in the zonal
circulation patterns shown in Figs. 2.34 and 2.37, patterns that exist in all major oceans. The wind-driven
westward flow (Coriolis effects are weak near the equator) in a zone bracketing the equator is blocked by the
presence of land masses. A resulting eastward pressure
gradient force drives a return equatorial undercurrent jet
along the equator and a seasonal countercurrent in the
doldrum region. These countercurrents, while primarily
zonal, are also very important connection between the
major ocean basins.
The circulation at poleward latitudes greater than
50
ı differs greatly in the different oceans because
of differences in the geography of the Northern and
Southern Hemisphere ocean basins. The North Atlantic
Ocean effectively ends at the sills at about 60
ı N and
is weakly associated with the shallower Arctic Ocean,
which has a circulation of its own. In the South Atlantic,
there is a zonal ocean which circles the entire Antarctic
continent. The winds are important in driving the broad
Circumpolar Current (Fig. 2.34). Density adjustments
in the interior produce pressure gradient forces which
balance the Coriolis forces to produce a current profile which diminishes from about 15 cm=s at the surface
to near zero at 3 km depth. So although Circumpolar
Current speeds are considerably less than Gulf Stream
speeds, the transports associated with the Circumpolar
Northeast trade winds
Southeast
trade winds
Doldrums
Water depth (m)
Depth (m)
0
200
400
0
200
400
5 ° S
5 ° N
E C C
N E C
S E C
E U C
1 0 ° N
1 5 ° N
0 °
Fig. 2.37 Generalized Scheme of equatorial currents. Schematic of
the westward trade wind-driven south (SEC) and north equatorial
currents (NEC) bracketing the eastward flowing equatorial countercurrent (ECC) and equatorial undercurrent (EUC) (after [2.7])
Current are 130190 10
6 m
3
=s and larger than Gulf
Stream transports. As such, this current has the largest
transport of any in the world’s oceans and plays a crucial role in controlling the deep ocean circulation.
2.5.6 Gulf Stream Ring Flows
Before leaving the discussion of the wind-driven surface currents, it is important to note that western
boundary currents are unstable leading to considerable
changes on monthly time scales. For example, the Gulf
Stream meanders and on occasion sheds pairs of eddies; warm core rings containing warm Sargasso Sea
water or cold core rings (Fig. 2.38) containing colder
waters with origins in the north. These rings are characterized by high currents – Gulf Stream-like currents
of 50100 cm=s. This eddy-shedding process enables
effective poleward heat transport. Once these rings are
shed, with the large Gulf Stream-like ocean currents,
propagate affecting their local environments. Cold-core
rings south of the Gulf Stream also propagate southwestward to be eventually entrained by the Gulf Stream
in the region of Cape Hatteras. Warm-core rings with
clockwise circulation are observed to propagate slowly
southwestward along the continental margin on the
northern side of the Gulf Stream. Satellite infrared imagery is very helpful in tracking these anomalously
warm and cold eddies.
In reality, all of the upper oceans of the world are
filled with eddy-like current structures embedded in the
large-scale, wind-driven gyres described above. Typical currents in these eddies are about 1020 cm=s. in
contrast to the 1 cm=s gyre mean flow, and have lateral extents of about 100 km. Such eddies have been
observed to propagate through a region in a period of
months. The role of these eddies in the overall transport
Part A | 2.5
ary of all the ocean basins, exceeding 200 cm=s in the
Gulf Stream of North Atlantic and in the Kuroshio
of the North Pacific. The total transport of the Gulf
Stream has been estimated to be in the range of 5090
10
6 m
3
=s; with that of the Kuroshio being somewhat
less (Table 2.6).
Despite the greater intensity of winds in the Southern Hemisphere, the intensity of the surface western
boundary currents is less than that for either the Gulf
Stream or Kuroshio. The Brazil current in the western
South Atlantic has typical speeds of 50100 cm=s and
penetrates to depths of only about 400 m. in contrast to
800 m for the Gulf Stream. The reduced transport of the
Brazil Current may be related to the presence of deeper
western boundary undercurrents and their role in transporting heat poleward. The reduced intensity of the Australian current in the western Pacific and the Agulhas
Current in the western Indian Ocean may be related to
the lack of complete continental blockage of zonal flow.
The equatorward flow along the eastern boundaries
of the wind-driven gyre as represented by the Canary
and California Currents is generally less intense and
less organized than the western boundary currents. The
exception is the Benguela Current (Fig. 2.34b), which
is an apparent extension of the Agulhas Current in the
South Atlantic.
At tropical latitudes straddling the equator, significant trade winds can drive surface currents in the zonal
circulation patterns shown in Figs. 2.34 and 2.37, patterns that exist in all major oceans. The wind-driven
westward flow (Coriolis effects are weak near the equator) in a zone bracketing the equator is blocked by the
presence of land masses. A resulting eastward pressure
gradient force drives a return equatorial undercurrent jet
along the equator and a seasonal countercurrent in the
doldrum region. These countercurrents, while primarily
zonal, are also very important connection between the
major ocean basins.
The circulation at poleward latitudes greater than
50
ı differs greatly in the different oceans because
of differences in the geography of the Northern and
Southern Hemisphere ocean basins. The North Atlantic
Ocean effectively ends at the sills at about 60
ı N and
is weakly associated with the shallower Arctic Ocean,
which has a circulation of its own. In the South Atlantic,
there is a zonal ocean which circles the entire Antarctic
continent. The winds are important in driving the broad
Circumpolar Current (Fig. 2.34). Density adjustments
in the interior produce pressure gradient forces which
balance the Coriolis forces to produce a current profile which diminishes from about 15 cm=s at the surface
to near zero at 3 km depth. So although Circumpolar
Current speeds are considerably less than Gulf Stream
speeds, the transports associated with the Circumpolar
Northeast trade winds
Southeast
trade winds
Doldrums
Water depth (m)
Depth (m)
0
200
400
0
200
400
5 ° S
5 ° N
E C C
N E C
S E C
E U C
1 0 ° N
1 5 ° N
0 °
Fig. 2.37 Generalized Scheme of equatorial currents. Schematic of
the westward trade wind-driven south (SEC) and north equatorial
currents (NEC) bracketing the eastward flowing equatorial countercurrent (ECC) and equatorial undercurrent (EUC) (after [2.7])
Current are 130190 10
6 m
3
=s and larger than Gulf
Stream transports. As such, this current has the largest
transport of any in the world’s oceans and plays a crucial role in controlling the deep ocean circulation.
2.5.6 Gulf Stream Ring Flows
Before leaving the discussion of the wind-driven surface currents, it is important to note that western
boundary currents are unstable leading to considerable
changes on monthly time scales. For example, the Gulf
Stream meanders and on occasion sheds pairs of eddies; warm core rings containing warm Sargasso Sea
water or cold core rings (Fig. 2.38) containing colder
waters with origins in the north. These rings are characterized by high currents – Gulf Stream-like currents
of 50100 cm=s. This eddy-shedding process enables
effective poleward heat transport. Once these rings are
shed, with the large Gulf Stream-like ocean currents,
propagate affecting their local environments. Cold-core
rings south of the Gulf Stream also propagate southwestward to be eventually entrained by the Gulf Stream
in the region of Cape Hatteras. Warm-core rings with
clockwise circulation are observed to propagate slowly
southwestward along the continental margin on the
northern side of the Gulf Stream. Satellite infrared imagery is very helpful in tracking these anomalously
warm and cold eddies.
In reality, all of the upper oceans of the world are
filled with eddy-like current structures embedded in the
large-scale, wind-driven gyres described above. Typical currents in these eddies are about 1020 cm=s. in
contrast to the 1 cm=s gyre mean flow, and have lateral extents of about 100 km. Such eddies have been
observed to propagate through a region in a period of
months. The role of these eddies in the overall transport