Ocean Currents Definition and features Measurement of currents

Ocean Currents • • • Definition and features Measurement of currents Forces acting on seawater surface Drift currents Ekman - Spiral EKMAN SPIRAL Upwelling and down welling Gradient current Thermohaline circulation EI - Nino and current types Types of ocean currents

Introduction • Large scale movement of water in ocean • Depend on atmospheric circulation for propulsion • Surface currents are wind driven • Interaction of atmosphere- Wind driven circulation Density circulation

Measurement of currents Direct method Currents can be measured directly by observing the motion of water relative to the solid earth or by observing the motion of the solid earth relative to the water.

Indirect Method • Geotropic technique in which the current can be measured by measuring the slope of density contours due to current flow. • Method involves the principle of electricity and magnetism.

Forces acting on seawater surface

Ekman-spiral • Wind blown- clrags the water – which sets the layer beneath too in motion –process continues downward and the current speed decreases with increasing depth • The change in current direction and speed – with depth form spiral called Ekman spiral • Under strong winds wind drift cuttents may be as deep 100 meters below surface.

Upwelling and Down welling • • • Upward movement of water Equator major upwelling area Caused by winds and change of coriolic effect Upwelling causes increased productivity Much upwelling occurs near the coast Cells of upwelled water take plumes of coldwater

Thermohaline circulation • Circulation of water-due to water density-induced by temperature and salinity • Generated by density differences between water masser and result in Horizontal and vertical flow • Density of water increases as it becomes cooler and more saline • It sinks to a level below which all water are more dense and above it are lessdense. • In sup polar and polar region- cold dense water descends and flows towards equator-They mix with adjacent water mass and loose indenting • In tropical region: Excess evaporation-produce dense saline water-sink-to appropriate density level. Eg: Mediterranean sea • In estuaries-As river flows in to estuary the less dense freshwater forms-a wedge that displaces denser seawater-and the wedge thins seaward as the freshwater spreads outwards and slowly mixes with seawater.

EI- Nino and current types Ø Wind driven reversal of pacific equtorial current – resulting in movement of warm water towards coasis of america Ø Warm surface water off shore current from perer Ø It is the preiodic elevation of the sea surface temperature in the equitorial pacific ocean Ø The warmer water over rides the cold water of eastern pacific and deepens the termocline

Characteristics of currents. Ocean currents • The currents may be conveniently divided in to 3 groups: – 1. Currents that are related to the distribution of density of sea – 2. Currents that are caused directly by the stress that the wind exerts on the sea surface – 3. tidal currents and currents associated with internal wave. • 1. Currents related to the distribution of density • The well known large scale currents of the oceans, such as Gulf stream, the Kuroshio, the Equatorial currents, the Benguela Current and others. All of these currents transport large amounts of water. • The nature of the currents that are related to the distribution of density in the sea based upon a few simple laws of physics. – The acceleration of a body equals the sum of the forces that act per unit mass of the body-this law which is applicable to any part of a fluid, implies that if a body moves with a uniform velocity, the forces that act on the body balance each other.

• In middle latitudes even the strongest surface currents rarely have velocities above 100 cm/sec. • In latitude 45⁰ the corresponding slope of the sea surface is 1. 05 x 10 that is the surface drops or rises of about 1 cm in 1 km or about 1 m in 100 km. The sea surface always considered as an isobaric surface. • The slopes of any sea surface relative to an isobaric surface at any depth below the surface can be determined from the density distribution. • Form relative slopes relative currents can be computed. • In the Northern hemisphere the current is so directed that the isobaric surfaces slope upward to the right of an observer looking in the direction of flow and that the distances between isobaric surfaces increased with decreasing density.

World Ocean currents • The three major oceans, the Atlantic, Indian and the Pacific can be, considered as deep bays that are in open communication with the Antarctic ocean, to the south but are closed at their northern ends • Of these oceans the Atlantic extends farthest to the North, and several large adjacent seas, the Mediterranean, the Caribbean, the Gulf of Mexico, Baffin Bay, the Norwegian sea and the North Polar sea exercise characteristic effects on the waters of North Atlantic. • The communication between the North Atlantic ocean and the antarctic takes place through the South Atlantic Ocean, which is of constant width and without adjacent seas.

• The three major oceans, the Atlantic, Indian and the Pacific can be, considered as deep bays that are in open communication with the Antarctic ocean, to the south but are closed at their northern ends • Of these oceans the Atlantic extends farthest to the North, and several large adjacent seas, the Mediterranean, the Caribbean, the Gulf of Mexico, Baffin Bay, the Norwegian sea and the North Polar sea exercise characteristic effects on the waters of North Atlantic. • The communication between the North Atlantic ocean and the antarctic takes place through the South Atlantic Ocean, which is of constant width and without adjacent seas.

Currents of the South Atlantic Ocean • The most conspicuous feature of the currents of the South Atlantic is represented by the counter clock wide gyral with the cold Benguela current on the eastern side the warm Brazil, current on the western side, the south equatorial current flowing west on the northern side, and the south Atlantic current flowing east on the southern side. • It is a system of shallow currents • The water within the southern and Eastern parts of the current system is of somewhat lower salinity than that of northern and western parts.

• In the south admixture of sub Antarctic water lowers the salinity, but in the North admixture of saline surface water increases the salinity • The net transport to the south below 1400 m amounts approximately to 15 million cubic meter/sec. • The value of Northward transport of about 6 million cubic meters/sec of upper waters

Currents of the Equatorial region of Atlantic Ocean • The currents near the equator where a counter current towards the eastt, the equatorial counter current, is imbedded between the equatorial currents of the two hemisphere.

Currents of the North Atlantic ocean • The system of current in the North Atlantic is dominated by the North Equatorial current to the South the Gulf Stream system to the north. • The north equatorial currents flows from east to west in the trade wind region. • In the western part of the Atlantic ocean the North Equatorial current joins the branch of South Equatorial current which crosses the Equator. • Thus, the part of North equatorial current continues into the Caribbean sea carries water which is mixed with water of South Atlantic origin, whereas the northern branch of the North equatorial current which flows along the northern side of the Great Antilles as the Antilles current carries water which is identical with that of the Sargasso sea. • The north equatorial current terminates in the Yucatan channel and the Antilles current. The continuation of these currents represents the beginning of the “Gulf Stream system” which dominates the circulation of a great part of the North Atlantic ocean.

Gulf stream system • Following the nomenclature of Iselin (1936), the term “ Gulf Stream System” is used to include the whole northward and eastward flow beginning at the straits of Florida. The Gulf Streams System can be subdivided in to the following parts: – 1. The Florida current – 2. The Gulf Stream – 3. The North Atlantic Current • 1. The Florida current – The north ward moving water from the straits of Florida to a point off cape Hatteras where the current ceases to follow the continental slope. The Florida current can be traced directly back to the Yucatan Channel, because the greater part of the water flowing through this strait continues on the shortest route to the straits of Florida and a small amount only sweeps into the Gulf of Mexico, later to join the Florida current. After having passes the straits of Florida the current is reinforced by the Antilles current, but the name ‘Florida Current’ is retained as far as to Cape Hatteras.

The Gulf Stream • The mid sector of the system, from the region where the current first leaves the continental slope off cape Hatteras to the region to the east of the Grand Banks in about long 45⁰W. Where the stream begins to fork. • The middle portion of the Gulf Stream system continues as a well defined and relatively narrow current which, in contrast to the Florida current, flows at some distance beyond the continental shelf. • To the right of the current if Sargasso seawater, but to the left are now found two water masses, the coastal water which covers the shallow shelf areas and the slope water which at temperatures between 4⁰and 10⁰ is very similar to Gulf Stream water, but at higher temperatures is of lower salinity. • The surface velocities of Gulf stream are very high in latitude 36⁰N, longitude 73⁰ more than 120 cm/sec (lselin, 1939) • Between 38 and 57 million m 3/sec of Sargasso sea water and deep water have been added to Florida-Gulf stream after Antilles current.

• A net transport of 55 million m 3/sec. • 15 -20 million m 3/sec circulate in the gyral to the right of the Gulf stream. • The slope water on the coastal side of the Gulf stream is essentially of the same character as the Gulf stream water but has a slightly lower salinity owing to admixture with coastal water the velocity of ‘slope current’ is about 10 cm/sec. • The two gyrals on both sides of the Gulf stream and in addition an ‘eddy’ is indicated between the slopewater gyral and the Gulf stream where the latter is at a greater distance form the coast.

• One of the best known of these swift, narrow western boundary currents is the Gulf Stream, which is located just off the eastern continental margin of North America. • Having a width of about 50 to 75 km and a depth of 1 ½ to 2 km, it flows north eastward with a velocityof 3 to 10 km per hour. • Daily tidal oscillations move the entire Gulf Stream back and forth some 6 to 8 km per day. • At higher latitudes the Gulf Stream meanders. Off the New England Nova Scotia coasts these snake like windings generally have amplitudes of 15 to 45 km and wavelengths of 150 to 400 km, which have the ability to move up to 25 km sideways in one day. • There may be several “flops” per month in any one of these loops, and is seems at times that they are somehow bouncing off the continental slope. Sometimes a loop gets so pronounced that the Gulf Stream doubles back on itself, and the loop breaks off in an eddy and drifts away

North Atlantic current • • • The name is used as a general term covering all the easterly and northerly currents of the north Atlantic from the region to the east of the Grand Banks where the Gulf stream divides. The branches of the North Atlantic current are often masked by shallow and variable. The North Atlantic current represents the continuation of the Gulf Stream, after it leaves region to the east of the ‘tail’ of the Grand Banks. Beyond this region the Gulf stream losses its characteristics as a well defined current and divides into branches that are often separated by countercurrents or eddies. Some of the branches turn south but others continue towards the east across the mid Atlantic ridge. The branch of the North Atlantic Current that crosses the Mid Atlantic Ridge turns to right and continues as an irregular flow toward south. Some of this water enters the Mediterranean as a surface current and flows out again across the still in the strait of Gibraltar as water of very high salinity. The larger amount of upper water masses continues towards the south and finally joining the North equatorial current. The Antarctic intermediate water enters the North Atlantic ocean along the coast of South America.

• The North Atlantic current runs into northwest Europe (the British Isles), and deflects to the right. • Now the water flows southward as the Canary current to complete the North Atlantic Gyre. • Part of the North Atlantic splits off and flows northward forming the Norway current. • Part of the water flows between Greenland Canada, into the Arctic Ocean. • It comes back south ward as the Labrador Current, bringing icebergs that calved off western Greenland with it. • Another part flows between iceland Scandinavia, into the Arctic Ocean, and comes back out into the Atlantic between Greenland Iceland as the East Greenland Current, again, bringing icebergs from east Greenland with it. • These branches off the North Atlantic Gyre are what sunk, various ships in the North Atlantic.

Currents of Pacific Ocean • The Pacific Ocean also has a gyre in the northern and the southern part: • In the Northern Hemisphere: • The trade winds push the water westward across toward the Phillippines as the North Equatorial Current. • This is deflected to the right and becomes the Kuroshio Current, which is the North Atlantic’s equivalent to the Gulf stream • If flows past Japan. The Westerlies push on this current, and the Coriolis Effect deflects it to the right across as the North Pacific Current.

• It runs into North America and is deflected right to become the California current and complete the North Pacific Gyre. • As in the Atlantic, there are couple of branches northward, forming the Alaska Curent and the Kamchatka Current. • Notice the water along the west coast of the U. S. (the California current) is coming down from the north where it’s been chilling out for weeks. • This is why the pacific coast is cold for swimming. • In the Southern Hemisphere, you can follow the arrows: South Equatorial Current to East Australian Current, across as the South Pacific Current and then to the Peru Current.

Currents of South Pacific ocean-Peru current • At the northern boundary of the Peru coastal Current certain characteristic seasonal changes takes place. During the northern summer the Peru coastal current extends just beyond the Equator where it converges with the Equatorial counter current, the waters of which in summer mainly turns towards the north • In winter this counter current is displaced further to the south and part of the warm but low salinity water of the counter current turns south along the coast of Ecuador, crossing the equator before converging with the Peru coastal current. • The warm south flowing current along the coast is known as El Nino and is a regular phenomenon in February and March. Occasionally major disturbances occur which appear to be related to changes in atmospheric circulation. The surface temperature of the water in March, 1925 was nearly 7⁰ above the average.

Equatorial region of the Pacific ocean • The North and South equtorial currents flowing towards the west and between them the equatorial counter current flowing toward the east. • The South equatorial current is present on both sides of the Equator and extends to about 5⁰N but the North Equatorial current remains in the Northern hemisphere. • Off South America the flow is directed more or less parallel to the coast line, turning gradually west when approaching the Equator. • The equatorial counter current is remarkably well developed in the pacific ocean where, it is present at all seasons of the year, lying always in the Northern Hemisphere. The velocities of the ECC reaching up to 2 knots at the surface.

Currents of Pacific Ocean • The pacific and Atlantic oceans have an Equatorial Counter Current to keep water from pilling up on the west side of the ocean. • Warm equatorial water bathes the north east coast of Australia, but cold polar water bathes the west coast of Australia. • This is why there is so much more reef growth (the Great Barrier Reef) on the east side of Australia than on its west.

Currents of the Indian Ocean • In the southern summer the current bends north before reaching the Australian continent and is joined by a current which flows from the Pacific to the Indian Ocean to the South Australia. In the winter the current appears to reach Australia. • South Equatorial current flows from east to west, reaching its greatest velocity during the southern winter. In this season the current is reinforced by the water from the pacific ocean, which enters the Indian ocean to North of Australia

Currents of the Indian Ocean • The Indian Ocean differs from the Atlantic and Pacific • There is hardly any ‘North Indian Ocean’- that space is occupied by Asia. • The equator crosses the Indian Ocean just south of india. • There is a well-developed gyre in the southern Indian ocean, with Trade Winds blowing a South Equatorial Currents (S. Eq. C. ) westward towards Africa. • In the southern part of the Indian ocean a great anti cyclonic system of currents appears to prevail and it is subject to greater annual variations. Between South Africa and Australia the current is directed in general from west to east.

• The surface currents of the Indian ocean vary greatly from winter to summer owing to the different character of the prevailing winds. • During February & March when the north west monsoon prevails the North Equatorial current is well developed. • In August, September when the South West monsoon blows, the North Equatorial current disappears and is replaced by monsoon currents, which flows from west to east. The equatorial counter current does not appear to be present in this season. • The Agulhas stream transports 20 million m 3/sec.

• The current is deflected to the left, and flows southward as a Gulf-Streamtype current called the Agulhas current. • There is an Equatorial Counter Current (Eq. C. C. ) to balance the westward flow. • There is also a North Equatorial Current (not labeled on the map) which flows south around India and loops in the Arabian Sea (red arrow above the Eq. C. C. ). • There is another loop in the Bay of Bengal, to the east of India. • This pattern changes, however! The pattern during the winter flow of the current is different. • When the high mountains of the Himalayas are warmed in the summer, air rises from them. • This is rising air drawns air from Lower Lying land, namely, the Indian subcontinent (“ I” on the map), causing winds to blow northward across India in the opposite direction of the trade winds. • This reverses the North Equatorial Counter Current so that it flows eastward (as shown on the map at left). The two small gyres in the Arabian Sea (on the left side of India) and the Bay of Bengal (on the right side of India) reverse direction.

The Currents of the Southern Ocean • The Southern Ocean is the water around Antarctica, the continent at the South Pole. • The Southern Ocean is actually the southern ends of the Atlantic, India, and Pacific Oceans. • Because it has a unique current system, most oceanographers recognize it as a separate ocean. • It has little connection with the gyres of any other ocean, although it does downwell and circulate in the subsurface back to the equator. • There’s a lot of other things going but the one to notice is the West Wind Drift.

World ocean currents-Antarctic ocean • The more of less permanent currents of the Antarctic Ocean consist principally of two types, – The relative currents which are associated with the distribution of mass and – The wind drifts of the surface layers • The currents run from west to east around the Antarctic continent