Ocean CirculationOcean Circulation

Fig. CO7

Ocean currentsOcean currents Moving seawaterMoving seawater Surface ocean currentsSurface ocean currents

Transfer heat from warmer to cooler Transfer heat from warmer to cooler areasareas

Similar to pattern of major wind beltsSimilar to pattern of major wind belts Affect coastal climatesAffect coastal climates

A A gyre is a circular spiral form and gyre is a circular spiral form and it refers to the circular motion of it refers to the circular motion of water in all the major oceans. It is water in all the major oceans. It is centered in the subtropical high centered in the subtropical high pressure regions.pressure regions.

Types of ocean currentsTypes of ocean currents Surface currentsSurface currents

Wind-drivenWind-driven Primarily horizontal motionPrimarily horizontal motion

Deep currentsDeep currents Driven by differences in density Driven by differences in density

caused by differences in caused by differences in temperature and salinitytemperature and salinity

Vertical and horizontal motionsVertical and horizontal motions

Types of Ocean CurrentsTypes of Ocean Currents Generally, one talks about the cold current and the warm current. These Generally, one talks about the cold current and the warm current. These

two terms are relative. Generally, a poleward-moving current is warm as two terms are relative. Generally, a poleward-moving current is warm as compared with general atmospheric temperatures and those of the compared with general atmospheric temperatures and those of the adjacent waters of the ocean. An equatorward-moving current is cool or adjacent waters of the ocean. An equatorward-moving current is cool or cold because its temperatures are lower than those of the atmosphere cold because its temperatures are lower than those of the atmosphere or of adjacent waters. or of adjacent waters.

(i) Drift (i) Drift - The velocity of a drift varies from 16 to 24 km a day. Thus, - The velocity of a drift varies from 16 to 24 km a day. Thus, drifts are caused by the friction between the winds and the surface drifts are caused by the friction between the winds and the surface water. It covers a broad belt in between latitude 35° and 45° in the water. It covers a broad belt in between latitude 35° and 45° in the northern hemisphere and latitude 30° and 60° in the southern northern hemisphere and latitude 30° and 60° in the southern hemisphere.hemisphere.

(ii) Current (ii) Current - Current moves more rapidly and more definitely than the - Current moves more rapidly and more definitely than the drift. A current may have a velocity of 6 or even 8 km an hour. drift. A current may have a velocity of 6 or even 8 km an hour.

(iii) Stream (iii) Stream - Stream is intensified and more definite means its - Stream is intensified and more definite means its boundaries are clearly discernible and are more definite. lies on the boundaries are clearly discernible and are more definite. lies on the western side, where a cold, south-moving current parallels it on the western side, where a cold, south-moving current parallels it on the landward side.landward side.

Generalised factors causing -Generalised factors causing -Surface currentsSurface currents

wind and its Frictionwind and its Friction DensityDensity Atmospheric PressureAtmospheric Pressure PrecipitationPrecipitation Melting of GlaciersMelting of Glaciers Coriolis effectCoriolis effect TemperatureTemperature GyresGyres or large circular loops of or large circular loops of

moving watermoving water

Modifying factors causing -Surface Modifying factors causing -Surface currentscurrents

Shape of the coastShape of the coast Seasonal ChangeSeasonal Change Bottom TopographyBottom Topography

Wind its frictionWind its friction Wind and its friction Wind and its friction – Wind put friction on surface of the oceans is – Wind put friction on surface of the oceans is

induced as a result of the stress which the winds exert on ocean water. induced as a result of the stress which the winds exert on ocean water. The trade winds blowing from north-east and south-east in the northern The trade winds blowing from north-east and south-east in the northern

and southern hemispheres respectively provided stress as a result north and southern hemispheres respectively provided stress as a result north and south equatorial currents are produced. These currents are present in and south equatorial currents are produced. These currents are present in the Atlantic, the Pacific and the Indian Oceans. the Atlantic, the Pacific and the Indian Oceans.

In the equatorial regions of all the oceans, the Equatorial Counter Current In the equatorial regions of all the oceans, the Equatorial Counter Current develops between the north and south equatorial currents to make up the develops between the north and south equatorial currents to make up the water deficiency at the surface. These counter currents flow eastwards.water deficiency at the surface. These counter currents flow eastwards.

As a matter of fact, the necessary energy needed to maintain the As a matter of fact, the necessary energy needed to maintain the circulation system of the oceans comes from the planetary wind system. circulation system of the oceans comes from the planetary wind system.

It was found that the currents flow at an angle of 20 to 40° from the It was found that the currents flow at an angle of 20 to 40° from the direction of the wind. To summarise, the energy needed by ocean direction of the wind. To summarise, the energy needed by ocean currents is supplied exclusively by winds, which is putting tangential currents is supplied exclusively by winds, which is putting tangential stress at the sea surface and this way initiate and maintain ocean stress at the sea surface and this way initiate and maintain ocean currents.currents.

Density & Atmospheric Density & Atmospheric Pressure Pressure

Density Density - Denser water tends to move downwards, and less dense - Denser water tends to move downwards, and less dense water being lighter tends to move upwards. water being lighter tends to move upwards.

High temperature leads to lower density and lower temperature results in higher High temperature leads to lower density and lower temperature results in higher density. density.

Salinity also affects density. Higher salinity means higher density, Salinity also affects density. Higher salinity means higher density, whereas lower salinity results in lower density of sea water.whereas lower salinity results in lower density of sea water.

Density is controlled by the rate of evaporation also. Rate of evaporation Density is controlled by the rate of evaporation also. Rate of evaporation is high, as in subtropical High pressure regions, the density increases.is high, as in subtropical High pressure regions, the density increases.

Larger amount of precipitation and the supply of fresh water by the rivers Larger amount of precipitation and the supply of fresh water by the rivers and by the melt-water causes lower the density of sea water.and by the melt-water causes lower the density of sea water.

Ocean current flows from lower density to higher density ocean waterOcean current flows from lower density to higher density ocean water Atmospheric pressure - Atmospheric pressure - Because of higher pressure, there is Because of higher pressure, there is

reduction in the volume of surface water which results in lowering of reduction in the volume of surface water which results in lowering of the sea-level. On the contrary, lower pressure on the sea surface the sea-level. On the contrary, lower pressure on the sea surface results in raising the water level and that the water movement starts results in raising the water level and that the water movement starts from the higher water level towards the lower level. from the higher water level towards the lower level.

PrecipitationPrecipitation Precipitation and evaporation Precipitation and evaporation - Water level in areas with greater - Water level in areas with greater

amount of precipitation is higher than that in areas with lesser amount. amount of precipitation is higher than that in areas with lesser amount. Thus, in order to eliminate the difference in water level ocean currents are Thus, in order to eliminate the difference in water level ocean currents are produced. produced.

Besides the difference in the water level, the supply of fresh water results Besides the difference in the water level, the supply of fresh water results in the decrease of salinity, which in turn increases level of sea water. The in the decrease of salinity, which in turn increases level of sea water. The precipitation, therefore, contributes a lot, directly or indirectly, in the precipitation, therefore, contributes a lot, directly or indirectly, in the initiation of ocean currents. The equatorial region for example, receives a initiation of ocean currents. The equatorial region for example, receives a larger amount of precipitation than the mid-latitude regions. That is why larger amount of precipitation than the mid-latitude regions. That is why ocean currents invariably flow from the equatorial regions towards the high ocean currents invariably flow from the equatorial regions towards the high latitudes, where the amount of precipitation is much less. latitudes, where the amount of precipitation is much less.

On the contrary, in the subtropical high pressure belt the climatic On the contrary, in the subtropical high pressure belt the climatic conditions permit a much greater rate of evaporation and the amount of conditions permit a much greater rate of evaporation and the amount of precipitation is much less. Since in this high-pressure region evaporation, precipitation is much less. Since in this high-pressure region evaporation, exceeds precipitation, the salinity and density of the sea water are exceeds precipitation, the salinity and density of the sea water are relatively higher. Therefore the surface currents start flowing from the relatively higher. Therefore the surface currents start flowing from the areas with abundant precipitation to areas with very little precipitation. areas with abundant precipitation to areas with very little precipitation.

Melting of Ice & Coriolis Melting of Ice & Coriolis forceforce

Melting of Ice Melting of Ice - supplies fresh water to oceans so that their level is - supplies fresh water to oceans so that their level is raised. Melt water reduces the salinity in ocean water. Thus, water raised. Melt water reduces the salinity in ocean water. Thus, water movement from regions of high sea level to other regions of movement from regions of high sea level to other regions of relatively lower sea level is quite natural. The Eastern Greenland relatively lower sea level is quite natural. The Eastern Greenland Current is initiated due to the supply of melt water.Current is initiated due to the supply of melt water.

Coriolis force Coriolis force – Coriolis force is a function of the latitude. It is – Coriolis force is a function of the latitude. It is maximum at the poles and minimum at the equator.As Earth rotates maximum at the poles and minimum at the equator.As Earth rotates on its axis from west to east. It always acts towards the right in the on its axis from west to east. It always acts towards the right in the northern hemisphere, and left in the southern hemisphere. Thus, northern hemisphere, and left in the southern hemisphere. Thus, under the influence of this force, the ocean currents in the northern under the influence of this force, the ocean currents in the northern hemisphere tend to turn to their right, and in the southern hemisphere tend to turn to their right, and in the southern hemisphere to their left.hemisphere to their left. Ekman Spiral - Ekman Spiral - regarding the deflection of ocean currents relative to the regarding the deflection of ocean currents relative to the

wind direction. Ekman developed the theory called the wind direction. Ekman developed the theory called the Ekman Spiral. Ekman Spiral. The theory assumes a homogeneous water column that is set in motion The theory assumes a homogeneous water column that is set in motion by wind blowing across its suface. In the northern hemisphere the surface by wind blowing across its suface. In the northern hemisphere the surface current moves in a direction 45° to the right of the wind. current moves in a direction 45° to the right of the wind.

Ekman Ekman transporttransport Average movement of seawater under Average movement of seawater under

influence of windinfluence of wind 9090oo to right of wind in Northern hemisphere to right of wind in Northern hemisphere 9090oo to left of wind in Southern hemisphere to left of wind in Southern hemisphere

Fig. 7.7

Geostrophic Geostrophic flowflow Ekman transport Ekman transport

piles up water piles up water within within subtropical gyressubtropical gyres

Surface water Surface water flows downhill flows downhill (gravity) and(gravity) and

Also to the right Also to the right (Coriolis effect)(Coriolis effect)

Balance of Balance of downhill and to downhill and to the right causes the right causes geostrophic flowgeostrophic flow around the “hill”around the “hill”

Fig. 7.8

Difference in Difference in temperaturetemperature

Difference in temperature Difference in temperature - variation in the amount of - variation in the amount of insolation is the main cause of non-uniformity in the rate insolation is the main cause of non-uniformity in the rate of evaporation in different regions.of evaporation in different regions.

In the low latitude regions the amount of insolation In the low latitude regions the amount of insolation received far exceeds that received in the high latitude received far exceeds that received in the high latitude regions. But due to the greater amount of cloudiness and regions. But due to the greater amount of cloudiness and precipitation, the rate of evaporation is much lower. This precipitation, the rate of evaporation is much lower. This causes both the salinity and density of sea water to be causes both the salinity and density of sea water to be relatively lower. Thus, the light and warm water from the relatively lower. Thus, the light and warm water from the low latitude region is driven by the prevailing winds low latitude region is driven by the prevailing winds towards the high latitude regions.towards the high latitude regions.

Other surface currentsOther surface currents Equatorial countercurrentsEquatorial countercurrents Subpolar gyresSubpolar gyres

Fig. 7.5

Western intensificationWestern intensification Top of hill of water displaced toward Top of hill of water displaced toward

west due to Earth’s rotationwest due to Earth’s rotation Western boundary currents Western boundary currents

intensifiedintensified FasterFaster NarrowerNarrower DeeperDeeper WarmWarm

Eastern Boundary CurrentsEastern Boundary Currents Eastern side of ocean basinsEastern side of ocean basins Tend to have the opposite properties Tend to have the opposite properties

of Western Currentsof Western Currents ColdCold SlowSlow ShallowShallow WideWide

Ocean currents and climateOcean currents and climate Warm ocean currents warm air at Warm ocean currents warm air at

coastcoast Warm, humid airWarm, humid air Humid climate on adjoining landmassHumid climate on adjoining landmass

Cool ocean currents cool air at Cool ocean currents cool air at coastcoast Cool, dry airCool, dry air Dry climate on adjoining landmassDry climate on adjoining landmass

Coastal upwelling and Coastal upwelling and downwellingdownwelling

Ekman Ekman transport transport moves surface moves surface seawater seawater onshore onshore (downwelling) (downwelling) oror

Offshore Offshore (upwelling)(upwelling)

Atlantic Ocean circulationAtlantic Ocean circulation North Atlantic Subtropical GyreNorth Atlantic Subtropical Gyre North Equatorial CurrentNorth Equatorial Current South Equatorial CurrentSouth Equatorial Current Florida CurrentFlorida Current Antilles CurrentAntilles Current Gulf StreamGulf Stream North Atlantic Drift North Atlantic Drift

North East DriftNorth East Drift Norwegian CurrentNorwegian Current Irminger CurrentIrminger Current East Greenland Cold CurrentEast Greenland Cold Current Labrador Cold CurrentLabrador Cold Current

South East DriftSouth East Drift Canary Cold CurrentCanary Cold Current English ChannelEnglish Channel

Atlantic Equatorial Counter Atlantic Equatorial Counter CurrentCurrent

South Atlantic Subtropical South Atlantic Subtropical GyreGyre

South Equatorial CurrentSouth Equatorial Current Brazil CurrentBrazil Current Falkland CurrentFalkland Current South Atlantic Drift ( West South Atlantic Drift ( West

Wind DriftWind Drift Benguel Cold CurrentBenguel Cold Current

Fig. 7.16

Atlantic Ocean circulationAtlantic Ocean circulation

Fig. 7.14

Climate effects of North Climate effects of North Atlantic currentsAtlantic currents Gulf Stream warms East coast of U.S. Gulf Stream warms East coast of U.S.

and Northern Europeand Northern Europe North Atlantic and Norwegian North Atlantic and Norwegian

Currents warm northwestern EuropeCurrents warm northwestern Europe Labrador Current cools eastern Labrador Current cools eastern

CanadaCanada Canary Current cools North Africa Canary Current cools North Africa

coastcoast

Pacific Ocean surface currents

Pacific Ocean circulationPacific Ocean circulation North Pacific subtropical North Pacific subtropical

gyregyre North Equatorial CurrentNorth Equatorial Current Kuroshio CurrentKuroshio Current Tsushima CurrentTsushima Current Kuroshio ExtensionKuroshio Extension Kurile Cold Current Kurile Cold Current

Aleutian Cold CurrentAleutian Cold Current Oyashio Cold CurrentOyashio Cold Current

North Pacific CurrentNorth Pacific Current Alaskan CurrentAlaskan Current California Cold CurrentCalifornia Cold Current

South Pacific subtropical South Pacific subtropical gyregyre

South Equatorial CurrentSouth Equatorial Current Equatorial Counter Equatorial Counter

CurrentCurrent East Australian CurrentEast Australian Current West Wind DriftWest Wind Drift Peru or Humboldt CurrentPeru or Humboldt Current El Nino CurrentEl Nino Current

SummerSummer South West Monsoon South West Monsoon

CurrentCurrent Somali Cold CurrentSomali Cold Current

South Equatorial CurrentSouth Equatorial Current Madagascar Current Madagascar Current Mozambique ChannelMozambique Channel AgulhasAgulhas West Wind DriftWest Wind Drift West Australian Cold West Australian Cold

CurrentCurrent South Indian Subtropical South Indian Subtropical

GyreGyre

Indian Ocean circulationIndian Ocean circulation WinterWinter North East Monsoon North East Monsoon

CurrentCurrent Equatorial Equatorial

CountercurrentCountercurrent

South Equatorial South Equatorial CurrentCurrent

Madagascar Current Madagascar Current Mozambique ChannelMozambique Channel AgulhasAgulhas West Wind DriftWest Wind Drift West Australian Cold West Australian Cold

CurrentCurrent South Indian South Indian

Subtropical GyreSubtropical Gyre

Indian Ocean circulationIndian Ocean circulation

Atmospheric and oceanic Atmospheric and oceanic disturbances in Pacific Oceandisturbances in Pacific Ocean Normal conditionsNormal conditions

Air pressure across equatorial Pacific is Air pressure across equatorial Pacific is higher in eastern Pacifichigher in eastern Pacific

Strong southeast trade windsStrong southeast trade winds Pacific warm pool on western sidePacific warm pool on western side Thermocline deeper on western sideThermocline deeper on western side Upwelling off the coast of PeruUpwelling off the coast of Peru

Normal conditionsNormal conditions

Atmospheric and oceanic Atmospheric and oceanic disturbances in Pacific Oceandisturbances in Pacific Ocean El Niño-Southern Oscillation (ENSO)El Niño-Southern Oscillation (ENSO)

Warm (El Niño)Warm (El Niño) and and cold phases (La Niña)cold phases (La Niña) High pressure in eastern Pacific weakensHigh pressure in eastern Pacific weakens Weaker trade windsWeaker trade winds Warm pool migrates eastwardWarm pool migrates eastward Thermocline deeper in eastern PacificThermocline deeper in eastern Pacific DownwellingDownwelling Lower biological productivityLower biological productivity

Corals particularly sensitive to warmer Corals particularly sensitive to warmer seawaterseawater

El Niño-Southern Oscillation (ENSO): El Niño-Southern Oscillation (ENSO): Warm phase (El Niño)Warm phase (El Niño)

Fig. 7.20b

El Niño-Southern Oscillation (ENSO): El Niño-Southern Oscillation (ENSO): coolcoolphase (La Niña)phase (La Niña) Increased pressure difference across Increased pressure difference across

equatorial Pacificequatorial Pacific Stronger trade windsStronger trade winds Stronger upwelling in eastern PacificStronger upwelling in eastern Pacific Shallower thermoclineShallower thermocline Cooler than normal seawaterCooler than normal seawater Higher biological productivityHigher biological productivity

El Niño-Southern Oscillation (ENSO)El Niño-Southern Oscillation (ENSO)Cool phase (La Niña)Cool phase (La Niña)

Fig. 7.20c