Ocean Circulation

Chapter 8: Ocean Circulation

Fig. 8-26

Ocean currents move large amounts of water and heat

Wind-driven surface ocean circulation

Density-driven deep-ocean circulation Both

redistribute heat from warmer regions to cooler Fig. 8-

16

Measuring ocean currents

Direct measurement Floating

objects/instruments Fixed instruments

Indirect measurement Distribution of density Satellite data Doppler flow meter Chemical tracers Distinctive water masses

Surface currents Frictional drag due to winds Surface currents similar to global

winds Continents affect surface current

patterns Other factors:

Gravity Friction Coriolis Effect

Subtropical gyres

Large circular flow Clockwise in northern oceans, e.g., North Atlantic

Counterclockwise in southern oceans, e.g., South Atlantic

4 main currents in each gyre

Ekman spiral and Ekman transport

Ekman transport moves surface seawater about 90o to the right of the wind in the Northern Hemisphere

90o left in Southern Hemisphere

Fig. 8-6a

Western intensification “Hill” of seawater is steeper on western side Western currents are fast, narrow, deep

Subtropical gyres Ekman transport

piles up “hill” of seawater at about 30o N and S

Water flows downhill under gravity and veers right (Northern hemisphere) due to Coriolis Effect

Circular flowFig. 8-7

Upwelling

Ekman transport moves seawater offshore

Ekman transport moves seawater away from another water mass

Fig. 8-11a

Cooler, nutrient-rich water rises vertically toward sea surface

High biologic productivity

Downwelling opposite Fig. 8-11b

Surface currents

Pattern similar in major ocean basins

Northern ocean gyre moves clockwise

Southern ocean gyre moves counter-clockwise

Antarctic circulation

Antarctic Circumpolar Current (or West Wind Drift) Greatest volume Connects main oceans

East Wind Drift Antarctic Divergence

(upwelling)

North Atlantic Circulation

North Equatorial Current Gulf Stream North Atlantic Current

Gulf Stream and North Atlantic Currents warm Europe

Canary Current

North Equatorial Current

Gulf Stream

North Atlantic Current

Canary Current

South Atlantic Ocean

South Equatorial Current Brazil Current Antarctic Circumpolar Current

Greatest volume Connects three main oceans

Benguela Current

South Equatorial current

Brazil Current

Benguela Current

North Pacific Ocean

North Equatorial Current Kuroshio Current North Pacific Current California Current Strong Equatorial Counter

Current

South Pacific Ocean

South Equatorial Current East Australian Current Antarctic Circumpolar Current Peru Current Strong Equatorial Counter

Current

ENSO El Niño-Southern Oscillation

Irregular shift in ocean and atmosphere characteristics every 2-10 years

Affects global climate Harmful and beneficial

consequences

El Niño ENSO Warm Phase Most obvious in Equatorial Pacific Pacific Warm Pool moves eastward across

Equatorial Pacific Changes in pattern of upwelling/downwelling

in eastern Pacific Ocean

Fig. 8-2

ENSO warm phase effects

Warmer seawater higher sea level Warmer seawater less upwelling in

eastern Pacific Lower biologic productivity

Warmer seawater kills some marine life

Shift in atmosphere pressure Shift in areas of precipitation

La Niña ENSO Cool Phase

Stronger tradewinds Warm pool stays in western

Pacific Increased upwelling in eastern

Pacific Shift in precipitation patterns Shift in atmospheric pressure

ENSO data collection TOGA Tropical Ocean Global

Atmosphere TAO Tropical Atmosphere Ocean

Monitor ocean and atmosphere Data indicate what phase ENSO

Pacific Decadal Oscillation Decades-long cycles of warming and cooling in Pacific Ocean

Indian Ocean Circulation North and South Equatorial

Currents Equatorial Counter Current Agulhas Current Antarctic Circumpolar Current West Australian Seasonal shifts (monsoons) of

winds and currents

Thermohaline circulation 90% of world

ocean Below pycnocline Large volumes of

seawater Extremely slow

speed Each ocean basin

is similar in deep-ocean circulation

Sources of deep water

Densest seawater is cold Most deep water masses

from polar oceans Sink to density level and

move horizontally

Antarctic Bottom Water Densest deep

water (coldest) Sinks around

Antarctica Most widespread

deep-water mass Found as far as

40o N Carries O2 into

deep ocean Fig. 8-25

North Atlantic Deep Water Complex mixture

of cold seawater from Norwegian sea mixing with Gulf Stream Mediterranean

Intermediate Water

Other NA water masses

Extends from North Atlantic to about 40o south

Intermediate Water Masses

Antarctic Intermediate Water Sinks at Antarctic Convergence Cold Salinity less than average

Mediterranean Intermediate Water Warm Very salty

Conveyor Belt Circulation Mixture of

surface ocean circulation and deep-ocean circulation

Mixes surface and deep

Mixes waters in different oceans

Fig. 8-26

Langmuir Circulation Alternately converging and diverging

convection cells Wind blowing over calm ocean Alternate rows of upwelling and downwelling

End of Chapter 8: Ocean Circulation