© 2011 Pearson Education, Inc. CHAPTER 7 Ocean Circulation.

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CHAPTER 7Ocean Circulation

What is the temperature of the water in the deepest oceans?

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Chapter Overview

• Ocean currents are moving loops of water.

• Surface currents are influenced by major wind belts.

• Currents redistribute global heat.

• Thermohaline circulation affects deep currents.

• Currents affect marine life.

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Types of Ocean Currents

• Surface currents

– Wind-driven

– Primarily horizontal motion

• Deep currents

– Driven by differences in density caused by differences in temperature and salinity

– Vertical and horizontal motions

Three-Cell Model of Atmospheric Circulation

Remember this wind info from the last chapter???

Density

Waters of different densities form a layered ocean

Global Ocean Currents

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Measuring Surface Currents• Direct methods

– Floating device tracked through time

– Fixed current meter

• Indirect methods– Pressure gradients– Radar altimeters– Doppler flow meter

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Global Surface Current Flow

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Measuring Deep Currents

• Floating devices tracked through time

• Chemical tracers– Tritium

– Chlorofluorocarbons

• Characteristic temperature and salinity

• Argo

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Argo

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Surface Currents

• Occur above pycnocline

• Frictional drag between wind and ocean

• Generally follow wind belt pattern

• Other factors:– Distribution of continents– Gravity– Friction– Coriolis effect

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Subtropical Gyres• Large, circular loops

of moving water• Bounded by:

– Equatorial current– Western Boundary

currents– Northern or Southern

Boundary currents– Eastern Boundary

currents

• Centered around 30 degrees latitude

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Five Subtropical Gyres

• North Atlantic – Columbus Gyre

• South Atlantic – Navigator Gyre

• North Pacific – Turtle Gyre

• South Pacific – Heyerdahl Gyre

• Indian Ocean – Majid Gyre

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Subtropical Gyres and Currents

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Ekman Spiral• Surface currents move

at an angle to the wind.• The Ekman spiral

describes speed and direction of seawater flow at different depths.

• Each successive layer moves increasingly to the right in the Northern Hemisphere– Coriolis effect

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Ekman Transport

• Average movement of seawater under influence of wind

• 90 degrees to right of wind in Northern hemisphere

• 90 degrees to left of wind in Southern hemisphere

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Geostrophic Flow• Ekman transport piles up

water within subtropical gyres.

• Surface water flows downhill and to the right.

• Geostrophic flow – balance of Coriolis Effect and gravitational forces

• Ideal geostrophic flow• Friction generates actual

geostrophic flow

GYRES

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Ocean Currents and Climate

• Warm ocean currents warm the air at the coast.– Warm, humid air– Humid climate on adjoining landmass

• Cool ocean currents cool the air at the coast.– Cool, dry air– Dry climate on adjoining landmass

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Ocean Currents and Climate

August

February

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Upwelling and Downwelling

• Upwelling – Vertical movement of cold, nutrient-rich water to surface– High biological productivity

• Downwelling – Vertical movement of surface water downward in water column

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Coastal Upwelling

• Ekman transport moves surface seawater offshore.

• Cool, nutrient-rich deep water comes up to replace displaced surface waters.

• Example: U.S. West Coast

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Coastal Downwelling

• Ekman transport moves surface seawater toward shore.

• Water piles up, moves downward in water column

• Lack of marine life

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Atlantic Ocean Circulation

• North Atlantic Subtropical Gyre– North Equatorial

Current– Gulf Stream– North Atlantic Current– Canary Current– South Equatorial

Current– Atlantic Equatorial

Counter Current

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Gulf Stream

• Best studied of all ocean currents• Meanders and loops• Merges with Sargasso Sea

– Circulates around center of North Atlantic Gyre

– Unique biology – Sargassum

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Gulf Stream• Meanders or loops may

cause loss of water volume and generate:

– Warm-core rings – warmer Sargasso Sea water trapped in loop surrounded by cool water

– Cold-core rings – cold water trapped in loop surrounded by warmer water

• Unique biological populations

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Climate Effects of North Atlantic Currents

North-moving currents – warm

• Gulf Stream warms East coast of United States and northern Europe

• North Atlantic and Norwegian Currents warm northwestern Europe

South-moving currents – cool

• Labrador Current cools eastern Canada

• Canary Current cools north African coast

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La Nina Conditions

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La Nina – ENSO Cool Phase

• Increased pressure difference across equatorial Pacific

• Stronger trade winds

• Stronger upwelling in eastern Pacific

• Shallower thermocline

• Cooler than normal seawater

• Higher biological productivity

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El Nino – Southern Oscillation (ENSO)

• High pressure in eastern Pacific weakens

• Weaker trade winds

• Warm pool migrates eastward

• Thermocline deeper in eastern Pacific

• Downwelling

• Lower biological productivity– Peruvian fishing suffers

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ENSO Conditions in the Pacific Ocean

La Nina

El Nino

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Occurrence of ENSO Events

• El Nino warm phase about every 2–10 years

• Highly irregular

• Phases usually last 12–18 months

• 10,000-year sediment record of events

• ENSO may be part of Pacific Decadal Oscillation (PDO) – Long-term natural climate cycle– Lasts 20–30 years

La Nina El Nino

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ENSO Occurrences

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ENSO has Global Impacts

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Deep-Ocean Currents

• Thermohaline Circulation – deep ocean circulation driven by temperature and density differences in water

• Below the pycnocline

• 90% of all ocean water

• Slow velocity

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Conveyor Belt Circulation

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Power From Currents• Currents carry

more energy than winds

• Florida–Gulf Stream Current System

• Underwater turbines– Expensive

– Difficult to maintain

– Hazard to boating

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End of CHAPTER 7Ocean Circulation