Back to Notes Page
Ask a question
Home

Continuation of the notes

Discussion:

Wind-Driven Surface Currents - Keynote pdf
Ekman Spiral
Geostrophic Currents
Pattern of Surface Currents in the Pacific - Keynote pdf and Graph

Currents

Wind-Driven Surface Currents

Two types of current:

  1. Wind-driven surface currents
  2. Thermohaline (density-driven) subsurface currents

Ocean currents result from the unequal heating of the earth’s surface.

The general pattern of ocean currents represents average directions of flow.
Currents are similar to the wind, on any given day they can flow in any directions and change repeatedly throughout the day.

General wind-driven circulation is driven by strong Trade Winds and Westerlies.
The Trade Winds form in the tropical climatic region between 0o and 30o.
The Westerlies form in the temperature climatic region between 30o and 60o.

The primary features of surface circulation are the subtropical gyres.
Subtropical gyres are composed of geostrophic currents.

Ekman Spiral

The Ekman spiral is a mathematical model used to explain how the wind can blow in one direction, yet the water, on average, flows perpendicular to the wind direction.

Wind stress sets the water at the ocean's surface into motion.
General wind-driven circulation is driven by strong Trade Winds and Westerlies.
This friction only affects the water molecules at the surface of the ocean.
Transfer of energy is inefficient, so the surface layer of water molecules moves at a fraction of the wind speed - approximately 2-4% of wind speed.

As the surface layer of water moves slower than the wind, it deflects more because of the Coriolis effect.
The surface layer eventually deflects 45o to the right of the wind direction in the N. Hemisphere, and 45o to the left of the wind direction in the S. Hemisphere.

The surface layer of water sets the underlying layer into motion.
This layer moves slower and thus deflects more.

The process continues to a depth of no motion, which averages about 100 m deep.
At the base of the Ekman spiral, water is just barely moving in the opposite direction of the wind.

As water is moving at different speeds and directions from 45o-180o of the wind direction, an average direction of flow, called Ekman transport, is used.
Ekman transport - the average direction of transport from the Ekman spiral.

Ekman transport is 90o to the right of the wind direction in the N. Hemisphere, and 90o to the left of the wind direction in the S. Hemisphere.

The wind must blow for several days for the Ekman spiral to develop fully.
Therefore the Ekman spiral and transport likely are seldom fully developed.

Geostrophic Currents

Geostrophic currents result from the interaction of three mechanisms:

  1. Wind stress
  2. Coriolis effect
  3. Gravity

The Trade Winds and the Westerlies pump water to 30o north and 30o south.
Ekman transport is 90o to the right of the wind direction in the N. Hemisphere, and 90o to the left of the wind direction in the S. Hemisphere.
This convergence of flow forms a hill of water at 30o, the subtropical climatic region.
The hill of water is 1-2 m high.

Gravity pulls water downslope.
The Coriolis effect deflects the water, to the right in the N. Hemisphere and to the left in the S. Hemisphere.
Eventually, the current becomes balanced on a slope, between Coriolis effect deflecting the flow upslope and gravity pulling the flow downslope.

The current is now called a geostrophic current.
Geostrophic currents in the subtropical gyre flow in a circular pattern around the subtropical gyre high.
The subtropical gyre circulation is clockwise (to the right) in the N. Hemisphere and counterclockwise (to the left) in the S. Hemisphere.

The highest part of a subtropical gyre is towards the western side of an ocean.
The western shift is a result of the rotation of Earth and the Coriolis effect.

Pattern of Surface Circulation in the Pacific

Draw and label the pattern of surface currents in the Pacific Ocean. - Graph

The general pattern is diagramed by describing the North Pacific Subtropical Gyre and the South Pacific Subtropical Gyre.

The North Pacific Subtropical Gyre:

Between 0o and 30o N, the North Pacific Equitorial Current flows west.
The North Pacific Equatorial Current is pushed by the Northeast Trade Winds.
The North Pacific Equatorial Current is a warm-water current.

On the western side of the North Pacific Subtropical Gyre, the Kuroshio Current flows north.
The Kuroshio Current is a western boundary current.
The Kuroshio Current is a warm-water current.

Between 30o and 60o N, the North Pacific Current flows east.
The North Pacific Current is pushed by the Westerlies.
The North Pacific Current is a cold-water current.

On the eastern side of the North Pacific Subtropical Gyre, the California Current flows south.
The California Current is an eastern boundary current.
The California Current is a cold-water current.

The South Pacific Subtropical Gyre:

Between 0o and 30o S, the South Pacific Equitorial Current flows west.
The South Pacific Equatorial Current is pushed by the Southeast Trade Winds.
The South Pacific Equatorial Current is a warm-water current.

On the western side of the South Pacific Subtropical Gyre, the East Australian Current flows south.
The East Australian Current is a western boundary current.
The East Australian Current is a warm-water current.

Between 30o and 60o S, the Antarctic Circumpolar Current flows east.
The Antarctic Circumpolar Current is pushed by the Westerlies.
The Antarctic Circumpolar Current is a cold-water current.

On the eastern side of the South Pacific Subtropical Gyre, the Peru Current flows north.
The Peru Current is an eastern boundary current.
The Peru Current is a cold-water current.

The notes continue in Waves