Oceanic Movements – Currents - Amazon AWS

Oceanic Movements – Currents

Dr. Vijendra Kumar Pandey

What is a current?

The term "current" describes the motion of the ocean.

Oceanic currents describe the movement of water from one location to another. Currents are generally measured in meters per second or in knots (1 knot = 1.85 kilometers per hour or 1.15 miles per hour).

Ocean water is continuously on the move, affecting climate, ecosystem, and the marine resources. Ocean currents are continuous and directed movements of ocean water. These currents are on the ocean’s surface and in its depths, flowing both locally and globally.

• A current is the mass movement or flow of ocean water.

• There are two types: surface and deep • Currents flow in complex patterns affected

by wind, the water's salinity and heat content, bottom topography, and the Earth's rotation.

• They are very important in determining the climates of the continents, especially those regions bordering the ocean.

What creates Ocean Surface Currents and how are they organized?

Ocean currents develop from friction between the ocean and the wind that blows across its surface. Surface currents occur within and above the pycnocline (layer of rapidly changing density) to a depth of about 1 kilometer (0.6 mile) and affect only about 10% of the world’s ocean water. Worldwide, the pattern of ocean surface currents is affected mostly by the major wind belts of the world but also by a variety of factors including the Coriolis effect, seasonal changes, and the geometry of each ocean basin.

You can see a relationship between ocean currents and Planetary winds !!!

The general movement of a mass of oceanic water in a definite direction is called ocean current which is more or less like surface stream/ river.

Ocean Currents are Categorized:

1. On the basis of temperature

a. Warm currents

b. Cold currents

2. On the basis of velocity, dimensions and directions

a. Drift – the forward movement of oceanic water under the influence of prevailing winds.

b. Current- movement of oceanic water in a definite direction with greater velocity.

c. Stream- ocean stream involves movement of larger mass of ocean water with maximum possible velocity comparatively greater than the current in a definite direction known as stream eg. Gulf Stream.

What Influences Surface Currents?

• Ocean currents are driven by two forces: the Sun (which causes wind) and the rotation of the Earth.

• Although wind blowing across the surface of an ocean can produce currents, these currents don’t continue moving in the same direction as the wind.

• Earth’s rotation causes ocean currents to bend to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• This force that does not allow currents to flow in a straight path is called the Coriolis effect.

Different winds cause currents to flow in different directions.

The trade winds are located just north and south of the equator.

• In both hemispheres, they push currents westward across the tropical latitudes.

The westerlies are located in the middle latitudes.

1. Global Winds

It has been agreed widely that friction caused by the wind sets the Ocean water in motion. Karl Zoppritz demonstrated in 1878 that a steady blowing winds through its friction on sea water drag the water in its direction. HU Severdrup has proved relationship between wind speed and current velocity. He stated that velocity of ocean currents is 1.5% of the wind velocity.

Origin of Currents

The continents are another major influence on surface currents.

They act as barriers to these currents.

When a surface current flows against a continent, the current is deflected

and divided.

2. Continental Barriers

As Earth rotates, ocean currents and wind belts curve.

The curving of the paths of ocean currents and winds due to Earth’s rotation is called the Coriolis Effect.

The wind belts and the Coriolis Effect create huge circles of moving water, called gyres.

3. Coriolis Effect

Main Components of Ocean Surface Circulation

Although ocean water continuously flows from one current into another, ocean surface

currents have a predictable, recurring pattern within each ocean basin.

1. Subtropical Gyres

The large, circular-moving loops of water that are driven by the major wind belts of the

world are called gyres (gyros = a circle).

2. Equatorial Currents

The trade winds, which blow from the southeast in the Southern Hemisphere and from the

northeast in the Northern Hemisphere, set in motion the water masses between the tropics.

The resulting currents are called equatorial currents, which travel westward along the

equator and form the equatorial boundary current of subtropical gyres. They are called

north equatorial currents or south equatorial currents, depending on their position relative

to the equator.

3. Western Boundary Currents

When equatorial currents reach the western portion of an ocean basin, they must turn

because they cannot cross land. The Coriolis effect deflects these currents away from the

equator as western boundary currents, which comprise the western boundaries of

subtropical gyres. Western boundary currents are so named because they travel along the

western boundaries of their respective ocean basins. For example, the Gulf Stream and the

Brazil Current, are western boundary currents. They come from equatorial regions, where

water temperatures are warm, so they carry warm water to higher latitudes.

4. Northern or Southern Boundary Currents

Between 30 and 60 degrees latitude, the prevailing westerlies blow from the northwest in the

Southern Hemisphere and from the southwest in the Northern Hemisphere. These winds direct

ocean surface water in an easterly direction across an ocean basin (see the North Atlantic

Current and the West Wind Drift). In the Northern Hemisphere, these currents comprise the

northern parts of subtropical gyres and are called northern boundary currents; in the Southern

Hemisphere, they comprise the southern parts of subtropical gyres and are called southern

boundary currents.

5. Eastern Boundary Currents

The currents which flow back across the ocean basin, the Coriolis effect and continental

barriers turn them toward the equator, creating eastern boundary currents of subtropical gyres

along the eastern boundary of the ocean basins. Examples of eastern boundary currents include

the Canary and the Benguela Currents. They come from high-latitude regions where water

temperatures are cool, so they carry cool water to lower latitudes.

6. Equatorial Countercurrents

A large volume of water is driven westward by the north and south equatorial currents and

piles up water on the western side of an ocean basin near the equator, creating higher sea level

there. As a result, this bulge of water flows downhill toward the east under the influence of

gravity. This current, called the equatorial countercurrent, is a narrow, easterly flow of water

that occurs counter to and between the adjoining equatorial currents.

Other Factors Affecting Ocean Surface Circulation

Several other factors influence circulation patterns in subtropical gyres, including

1. Ekman spiral and Ekman transport,

2. geostrophic currents, and

3. western intensification of subtropical gyres.

Fact Check The principal ocean surface currents on the Earth consists of large subtropical gyres and smaller subpolar gyres, both of which are big, circular-moving loops of water powered by the major wind belts of the world.

Major wind-driven surface currents of the world’s oceans during February–March.

Delineate these currents on the World Map and classify them (Warm/Cold).

Exercise

1. Conveyer-Belt Circulation and Climate Change.

2. EL NIÑO–Southern Oscillation (ENSO) Conditions.

3. Monsoons and Circulation pattern in Indian Ocean.

4. Ocean Currents and Climate.

Explore these points?

References/Further Reading

1. Tom S. Garrison - Essentials of Oceanography.

2. Alan P. Trujillo & Harold V. Thurman - Essentials of Oceanography.

3. R.C Sharma & M Vatal - Oceanography for Geographers.