CIRCULATION OF CIRCULATION OF OCEANSOCEANS

OCEAN & ATMOSPHEREOCEAN & ATMOSPHERESIMILARITIESSIMILARITIES

Circulation of Atmosphere & Oceans both Circulation of Atmosphere & Oceans both driven by convection currentsdriven by convection currents

Coriolis force rotates ocean currents in the Coriolis force rotates ocean currents in the same way as wind currentssame way as wind currents

IMPORTANT DIFFERENCESIMPORTANT DIFFERENCES

Density controlled by SALINITY as well asDensity controlled by SALINITY as well as

temperaturetemperature Ocean heated from top rather than bottomOcean heated from top rather than bottom Wind shear importantWind shear important Landmasses interrupt flowLandmasses interrupt flow Velocity of flow much slowerVelocity of flow much slower

TWO TYPES OF CURRENTTWO TYPES OF CURRENT

DENSITY DRIVENDENSITY DRIVEN

Cold, saline water sinks, Cold, saline water sinks, warmer water of lower warmer water of lower

salinity rises salinity rises producing producing

VERTICAL CURRENTSVERTICAL CURRENTS

(Downwelling & (Downwelling & Upwelling)Upwelling)

WIND DRIVENWIND DRIVEN

Wind shear over the Wind shear over the surface produces surface produces

HORIZONTAL HORIZONTAL CURRENTSCURRENTS

andand

VERTICALVERTICAL

UPWELLING UPWELLING

Voyage of the Fram 1893-6Voyage of the Fram 1893-6

WIND DRIVEN CURRENTSWIND DRIVEN CURRENTS

THE EKMAN SPIRAL discovered as a THE EKMAN SPIRAL discovered as a result of Nansen’s observations on the result of Nansen’s observations on the Fram 1893-6Fram 1893-6

Coriolis force not only rotates winds but Coriolis force not only rotates winds but also ocean currents driven by windalso ocean currents driven by wind

Result is huge rotating current gyres Result is huge rotating current gyres

EKMAN SPIRALEKMAN SPIRAL

90o

45o

Ekman Transport(Average of all vectors)

Surface Current< 3% Wind Speed

WESTERLY WINDS

Effect dies outBelow 100 m

Water layers of increasing depth

CURRENT GYRESCURRENT GYRES

AN EXAMPLE AN EXAMPLE

THE NORTH ATLANTIC GYRETHE NORTH ATLANTIC GYRE

First studied by Franklin & FolgerFirst studied by Franklin & Folger

Wind BeltsWind Belts

SUBTROPICAL HIGH

A M

E R

I C

A S

E U

R O

P E

A F

R I C

A

W E S T E R L I E S

N E T R A D E S

North Atlantic Current GyreNorth Atlantic Current GyreA

M E

R I

C A

SE

U R

O P

E A

F R

I C A

North Atlantic Current

North Equatorial Current

GulfStream

CanaryCurrentCanaryCurrent

North Atlantic Gyre

South Atlantic Gyre

Subpolar Gyre

Not Developed(Antarctica)

Equatorial Counter Current

N. Equatorial Current

S. Equatorial Current

West Wind Drift

North Atlantic Current

Polar Current

EKMAN TRANSPORTEKMAN TRANSPORTA

M E

R I

C A

SE

U R

O P

E A

F R

I C A

2m 1m

Effects of Ekman TransportEffects of Ekman Transport

1. Surface water piled into center of gyres1. Surface water piled into center of gyres

Effects of Ekman TransportEffects of Ekman Transport

1. Surface water piled into center of gyres1. Surface water piled into center of gyres 2. Water also pushed to west end of gyres 2. Water also pushed to west end of gyres

because Coriolis force increases with latitudebecause Coriolis force increases with latitude

Effects of Ekman TransportEffects of Ekman Transport

1. Surface water piled into center of gyres1. Surface water piled into center of gyres 2. Water also pushed to west end of gyres 2. Water also pushed to west end of gyres

because Coriolis force increases with latitudebecause Coriolis force increases with latitude 3. Piling of water at west end leads to a 3. Piling of water at west end leads to a

GEOSTROPHIC effect which accelerates GEOSTROPHIC effect which accelerates current at west end of gyre (w intensification)current at west end of gyre (w intensification)

Geostrophic EffectGeostrophic EffectA

M E

R I

C A

SE

U R

O P

E A

F R

I C A

2m

1m

North Atlantic Current

North Equatorial Current

GulfStream(9km/hr)

CanaryCurrent

(0.9 km/hr)

Effect of Ekman Transport

Effect ofGravity

Effect of CoriolisForce

Final Current Vector acceleratesWind-driven current

Geostrophic EffectGeostrophic Effect

Effects of Ekman TransportEffects of Ekman Transport

1. Surface water piled into center of gyres1. Surface water piled into center of gyres 2. Water also pushed to west end of gyres 2. Water also pushed to west end of gyres

because Coriolis force increases with latitudebecause Coriolis force increases with latitude 3. Piling of water at west end leads to a 3. Piling of water at west end leads to a

GEOSTROPHIC effect which accelerates GEOSTROPHIC effect which accelerates current at west end of gyre (w intensification)current at west end of gyre (w intensification)

4. Loss of surface water to west end of gyre is 4. Loss of surface water to west end of gyre is made good by coastal upwelling at east end.made good by coastal upwelling at east end.

Coastal UpwellingCoastal Upwelling

1m

North Atlantic Current

North Equatorial Current

GulfStream9km/hr

CanaryCurrent0.9 km/hr

CanaryCurrent0.9 km/hr

UPWELLING

Warm Surface Water

COASTAL UPWELLINGCOASTAL UPWELLING

Result of migration of surface water to Result of migration of surface water to west end of current gyreswest end of current gyres

Allows nutrient-rich deep water to upwell Allows nutrient-rich deep water to upwell at east endat east end

Upwelling nutrients encourage plankton Upwelling nutrients encourage plankton Rich fishing grounds Rich fishing grounds

EL NINO & LA NINAEL NINO & LA NINA

Coastal Upwelling at east end of S Pacific Coastal Upwelling at east end of S Pacific gyre provides rich fishing off Peru gyre provides rich fishing off Peru

Anchovy of great economic importanceAnchovy of great economic importance Periodically upwelling fails and Anchovy Periodically upwelling fails and Anchovy

industry collapses (El Nino)industry collapses (El Nino) Part of larger phenomenon –Southern Ocean Part of larger phenomenon –Southern Ocean

Oscillation with world-wide effects ( floods & Oscillation with world-wide effects ( floods & droughts)droughts)

La NinaLa Nina

UP

WE

LLING

Warm Surface Water

Strong Trades

Strong Westerlies

El NinoEl Nino

Warm Surface Water

Strong Westerlies

Weak Trades

LA NINA

EL NINO

EL NINO & LA NINAEL NINO & LA NINA

Coastal Upwelling at east end of S Pacific Coastal Upwelling at east end of S Pacific gyre provides rich fishing off Peru gyre provides rich fishing off Peru

Anchovy of great economic importanceAnchovy of great economic importance Periodically upwelling fails and Anchovy Periodically upwelling fails and Anchovy

industry collapses (El Nino)industry collapses (El Nino) Part of larger phenomenon –Southern Ocean Part of larger phenomenon –Southern Ocean

Oscillation with world-wide effects ( floods & Oscillation with world-wide effects ( floods & droughts)droughts)

TWO TYPES OF CURRENTTWO TYPES OF CURRENT

DENSITY DRIVENDENSITY DRIVEN

Cold, saline water sinks, Cold, saline water sinks, warmer water of lower warmer water of lower

salinity rises salinity rises producing producing

VERTICAL CURRENTSVERTICAL CURRENTS

(Downwelling & (Downwelling & Upwelling)Upwelling)

WIND DRIVENWIND DRIVEN

Wind shear over the Wind shear over the surface produces surface produces

HORIZONTAL HORIZONTAL CURRENTSCURRENTS

andand

VERTICALVERTICAL

UPWELLING UPWELLING

THERMOHALINE THERMOHALINE CIRCULATIONCIRCULATION

VERTICAL circulation driven by DENSITYVERTICAL circulation driven by DENSITY Recall that density of sea water controlled Recall that density of sea water controlled

by temperature, salinity and pressureby temperature, salinity and pressure Of these factors temperature is most Of these factors temperature is most

importantimportant Since warm water has low density and Since warm water has low density and

oceans are heated from the top vertical oceans are heated from the top vertical circulation is problematiccirculation is problematic

IMPORTANCE OF VERTICAL IMPORTANCE OF VERTICAL CIRCULATIONCIRCULATION

DOWNWELLINGDOWNWELLING

Carries oxygen to the Carries oxygen to the deep oceandeep ocean

UPWELLINGUPWELLING

Carries carbon dioxide Carries carbon dioxide and nutrients to the and nutrients to the

surfacesurface

THERMOCLINETHERMOCLINE

Warm water heated Warm water heated by sun rests on colder by sun rests on colder unheated water unheated water

Steep temperature Steep temperature gradient is called the gradient is called the thermoclinethermocline

Thermocline at right is Thermocline at right is stablestable

Depth

Temperature

StableThermocline

HALOCLINEHALOCLINE

Depending on climate Depending on climate and precipitation and precipitation surface water is surface water is usually less saline usually less saline than deep water.than deep water.

Salinity of surface Salinity of surface water is increased by water is increased by evaporation and ice evaporation and ice formationformation

Steep gradient is Steep gradient is called the haloclinecalled the halocline

Depth

Saliinity

StableHalocline

PYCNOCLINEPYCNOCLINE

Even when surface Even when surface water is saline it is water is saline it is nearly always less nearly always less dense than deep dense than deep water because of water because of temperaturetemperature

There is a steep, There is a steep, stable density stable density gradient (pycnocline) gradient (pycnocline) between surface and between surface and deep waterdeep water

Depth

Density

StablePycnocline

Effect of LatitudeEffect of LatitudeThermoclinesThermoclines HaloclinesHaloclines

Depth

TemperatureSalinity

P T E P E T

DOWNWELLINGDOWNWELLING Since stratification of oceans is so stable Since stratification of oceans is so stable

how and where is downwelling possible?how and where is downwelling possible?

Depth

Pycnocline

E T P

Only possibleclose to poles especially whenice freezes and expels salt

Density

030 3060 6090 90

Salinity

Temperature

Density

Latitude

Low

High

Norwegian SeaLabrador Sea

E Greenland

Weddell Sea

Areas of DownwellingAreas of Downwelling

North Atlantic Deep Water due to freezing in North Atlantic Deep Water due to freezing in Norwegian, Labrador & E Greenland SeasNorwegian, Labrador & E Greenland Seas

Antarctic Bottom Water due to freezing & Antarctic Bottom Water due to freezing & salt expulsion in Weddell Seasalt expulsion in Weddell Sea

Intermediate Waters due to Arctic and Intermediate Waters due to Arctic and Antarctic ConvergencesAntarctic Convergences

SHALLOW & DEEP SHALLOW & DEEP CIRCULATIONCIRCULATION

SUMMARYSUMMARY

SURFACE CIRCULATIONSURFACE CIRCULATIONCurrent gyres driven by windCurrent gyres driven by wind

UPWELLINGUPWELLINGDriven by wind, important for nutrientsDriven by wind, important for nutrients

DOWNWELLING & DEEP CIRCULATION DOWNWELLING & DEEP CIRCULATION Driven by density, important for oxygenation of Driven by density, important for oxygenation of

deepdeep

Westward IntensificationWestward IntensificationA

M E

R I

C A

SE

U R

O P

E A

F R

I C A

2m

1m

North Atlantic Current

North Equatorial Current

GulfStream

CanaryCurrent