7.3 Wind-Driven Surface and Near-Surface Currents
219
wind is stronger than rv 3.5 mis, a counterrotating, convective water flow pattern of helical vortices is set up parallel to the wind. The lines are typically
spaced about 100 to 200 m apart and sometimes are as much as 500 m long.
This wind-driven flow pattern is known as Langmuir circulation. Each convection cell is 10-50 m broad and 5-6 m deep (Fig. 7.8).
The formation of the helical vortices is due to thermal instability in the surface water. As the counterrotation of adjacent cells exists, floating material,
such as oil-surface films, seaweeds or bubbles, aggregates at zones where currents converge. The Langmuir circulation is a short-term response of the water
surface layer to wind stress, and wind action for only a short duration (several
minutes or hour) is needed to generate it.
Circulation in Tropical Oceans. In Sect. 7.3.3 we mentioned that the Trade
Winds blowing towards the Equator induce the western-flowing North Equatorial Current and South Equatorial Current. Between both Trade Wind systems
exists a zone of weak variable wind, known as the doldrums. In this region,
a vigorous Equatorial Countercurrent flows in opposition to the westward
North and South Equatorial Currents. The current extends large distances;
for example, in the Pacific from the Philippine Islands to Columbia in South
America, a distance of almost 15,000 km. Its width varies seasonally and
reaches 400-500 km during the northern summer. In the Atlantic Ocean, the
countercurrent is even stronger (Pinet, 1992).
Sverdrup's theory, mentioned above, explains the existence of such equatorial
current system consisting of two westward currents and an eastward current.
In the Pacific, this system is not symmetrical about the Equator but is moved
northward. This fact corresponds to a similar displacement of the Trade Winds.
In the Atlantic Ocean, the current distribution is similar. However, in the
Indian Ocean the wind pattern changes seasonally causing the seasonal changes
of the current. Due to monsoonal activity over the ocean, there is only a
two-current system: the wind-driven Southwest Monsoon Current to the east,
crossing the Equator, and the South Equatorial Current to the west, well south
of the Equator.
In the early 1950s, oceanographic surveys in the tropical Pacific established
the existence of an undercurrent beneath the surface, flowing from West to East,
known as the Equatorial Undercurrent. Subsequent experiments have verified the worldwide existence of this current. Maximum speed of the Equatorial
Undercurrent is about 1.5 mls or more, at depths rising from 200 m in the
West to 50 m or less in the East. The dimensions are quite remarkable, with a
length of 10,000 km, width of 400 km, and 200-300 m thickness. The annual
average transport is estimated at about 40 Sv, with maximum transport up to
70 Sv.
A similar current exists in the Atlantic and there is also evidence for an equatorial undercurrent in the Indian Ocean during the northwest monsoon (Pickard
and Emery, 1982). Field experiments and theoretical analysis have shown that
the Equatorial Undercurrent is a geostrophic current with the pressure-gradient
219
wind is stronger than rv 3.5 mis, a counterrotating, convective water flow pattern of helical vortices is set up parallel to the wind. The lines are typically
spaced about 100 to 200 m apart and sometimes are as much as 500 m long.
This wind-driven flow pattern is known as Langmuir circulation. Each convection cell is 10-50 m broad and 5-6 m deep (Fig. 7.8).
The formation of the helical vortices is due to thermal instability in the surface water. As the counterrotation of adjacent cells exists, floating material,
such as oil-surface films, seaweeds or bubbles, aggregates at zones where currents converge. The Langmuir circulation is a short-term response of the water
surface layer to wind stress, and wind action for only a short duration (several
minutes or hour) is needed to generate it.
Circulation in Tropical Oceans. In Sect. 7.3.3 we mentioned that the Trade
Winds blowing towards the Equator induce the western-flowing North Equatorial Current and South Equatorial Current. Between both Trade Wind systems
exists a zone of weak variable wind, known as the doldrums. In this region,
a vigorous Equatorial Countercurrent flows in opposition to the westward
North and South Equatorial Currents. The current extends large distances;
for example, in the Pacific from the Philippine Islands to Columbia in South
America, a distance of almost 15,000 km. Its width varies seasonally and
reaches 400-500 km during the northern summer. In the Atlantic Ocean, the
countercurrent is even stronger (Pinet, 1992).
Sverdrup's theory, mentioned above, explains the existence of such equatorial
current system consisting of two westward currents and an eastward current.
In the Pacific, this system is not symmetrical about the Equator but is moved
northward. This fact corresponds to a similar displacement of the Trade Winds.
In the Atlantic Ocean, the current distribution is similar. However, in the
Indian Ocean the wind pattern changes seasonally causing the seasonal changes
of the current. Due to monsoonal activity over the ocean, there is only a
two-current system: the wind-driven Southwest Monsoon Current to the east,
crossing the Equator, and the South Equatorial Current to the west, well south
of the Equator.
In the early 1950s, oceanographic surveys in the tropical Pacific established
the existence of an undercurrent beneath the surface, flowing from West to East,
known as the Equatorial Undercurrent. Subsequent experiments have verified the worldwide existence of this current. Maximum speed of the Equatorial
Undercurrent is about 1.5 mls or more, at depths rising from 200 m in the
West to 50 m or less in the East. The dimensions are quite remarkable, with a
length of 10,000 km, width of 400 km, and 200-300 m thickness. The annual
average transport is estimated at about 40 Sv, with maximum transport up to
70 Sv.
A similar current exists in the Atlantic and there is also evidence for an equatorial undercurrent in the Indian Ocean during the northwest monsoon (Pickard
and Emery, 1982). Field experiments and theoretical analysis have shown that
the Equatorial Undercurrent is a geostrophic current with the pressure-gradient