7.3 Wind-Driven Surface and Near-Surface Currents
213
Undercurrent (known also as the Cromwell Current in the Pacific Ocean);
both currents are weak and change locations with the seasons.
The global circulation of surface currents, consisting of water moving in large
gyres, is geostrophic in character, i. e. pressure-gradient forces balance the
Coriolis effects. However, it is not a perfectly closed system and the volume
of water circulating in the gyres is not constant. A classic example is the
water removed from the North Atlantic gyre due to the separation of the Gulf
Stream from the main flow. This removed water must be replaced by a flow
from another source, presumably by some upwelling mechanisms.
Modern observation techniques, such as satellite monitoring of the sea surface,
free-floating and bottom anchored buoys, and sophisticated research vessels
have revealed the very complicated structure of the main circulation gyres. In
particular, the presence of large mesoscale eddies (with diameters of 200-400
km) and rings (of 100-300 km in diameter and ::; 3 km thick), drifting slowly
both with and against main current have been detected. It was shown that
mesoscale eddies may contain a substantial portion of the kinetic energy of the
oceans. The significance of these structures for the ocean life is enormous and
we will discuss this subject in Part III.
The volume of water transported by the large ocean currents usually is measured in Sverdrup units, Sv, named in honour of Harald Sverdrup. One Sv
is equal to 10 6 m 3 /s. Typical values of volume flow are: Pacific Equatorial
Current 10-70 Sv, Gulf Stream 50-150 Sv, and Circumpolar Current up to 290
Sv.
Both wind driving and the effects of density changes are important for the
overall circulation, but the former probably dominates in the upper 1000 m in
most regions of the ocean. We will consider wind-driven flows in next section,
while density effects will be discussed in Sect. 7.4.
7.3.4 Types of Surface Current Flows
Until now we have discussed the spatial patterns of the surface currents, neglecting their vertical structure. As was explained in Chap. 1, once a water
particle is set in motion, it exerts frictional drag on the particles beneath.
This process continues, slowly transferring the wind's momentum down into
the water column. One of the patterns of such a transfer is the Ekman's spiral.
Ekman's Spiral Flow. During the Norwegian North Polar Expedition in
1893-1896, on the famous research vessel Pram, Fridtjof Nansen observed that
the drift of ice with respect to the wind did not follow the wind direction,
but deviated to the right by 20° to 40°, when looking in the direction of the
wind. On Nansen's suggestion, the Swedish physicist V. Walfried Ekman (1905)
investigated the problem mathematically and laid the foundation for one of
the most important theoretical developments in dynamic oceanography. The
physical reasoning behind Ekman's solution is as follows. Consider the water
column as composed of infinitesimally thin horizontal layers and assume that
213
Undercurrent (known also as the Cromwell Current in the Pacific Ocean);
both currents are weak and change locations with the seasons.
The global circulation of surface currents, consisting of water moving in large
gyres, is geostrophic in character, i. e. pressure-gradient forces balance the
Coriolis effects. However, it is not a perfectly closed system and the volume
of water circulating in the gyres is not constant. A classic example is the
water removed from the North Atlantic gyre due to the separation of the Gulf
Stream from the main flow. This removed water must be replaced by a flow
from another source, presumably by some upwelling mechanisms.
Modern observation techniques, such as satellite monitoring of the sea surface,
free-floating and bottom anchored buoys, and sophisticated research vessels
have revealed the very complicated structure of the main circulation gyres. In
particular, the presence of large mesoscale eddies (with diameters of 200-400
km) and rings (of 100-300 km in diameter and ::; 3 km thick), drifting slowly
both with and against main current have been detected. It was shown that
mesoscale eddies may contain a substantial portion of the kinetic energy of the
oceans. The significance of these structures for the ocean life is enormous and
we will discuss this subject in Part III.
The volume of water transported by the large ocean currents usually is measured in Sverdrup units, Sv, named in honour of Harald Sverdrup. One Sv
is equal to 10 6 m 3 /s. Typical values of volume flow are: Pacific Equatorial
Current 10-70 Sv, Gulf Stream 50-150 Sv, and Circumpolar Current up to 290
Sv.
Both wind driving and the effects of density changes are important for the
overall circulation, but the former probably dominates in the upper 1000 m in
most regions of the ocean. We will consider wind-driven flows in next section,
while density effects will be discussed in Sect. 7.4.
7.3.4 Types of Surface Current Flows
Until now we have discussed the spatial patterns of the surface currents, neglecting their vertical structure. As was explained in Chap. 1, once a water
particle is set in motion, it exerts frictional drag on the particles beneath.
This process continues, slowly transferring the wind's momentum down into
the water column. One of the patterns of such a transfer is the Ekman's spiral.
Ekman's Spiral Flow. During the Norwegian North Polar Expedition in
1893-1896, on the famous research vessel Pram, Fridtjof Nansen observed that
the drift of ice with respect to the wind did not follow the wind direction,
but deviated to the right by 20° to 40°, when looking in the direction of the
wind. On Nansen's suggestion, the Swedish physicist V. Walfried Ekman (1905)
investigated the problem mathematically and laid the foundation for one of
the most important theoretical developments in dynamic oceanography. The
physical reasoning behind Ekman's solution is as follows. Consider the water
column as composed of infinitesimally thin horizontal layers and assume that