Unit 10 Ocean Currents

Intro video

Bill Nye The Science Guy on Ocean CurrentsBill Nye The Science Guy on Ocean Currents

Ocean currentsSurface currents

Affect surface water within and above the pycnocline (top 10% of ocean water)Driven by major wind belts of the world

Deep currentsAffect deep water below pycnocline (bottom 90% of ocean water)Driven by density differencesLarger and slower than surface currents

Measuring surface currentsDirect methods

Float metersIntentionalInadvertent

Propeller metersIndirect methods

Pressure gradientsSatellitesDoppler flow meters The great Nike sneaker spill

Surface currents closely follow global wind belt pattern

Trade winds at 0-30º blow surface currents to the westPrevailing westerlies at 30-60º blow currents to the east

Atlantic Surface Currents

Wind-driven surface currents

Current gyresGyres are large circular-moving loops of water

Subtropical gyresFive main gyres (one in each ocean basin):

North PacificSouth PacificNorth AtlanticSouth Atlantic Indian

Generally 4 currents in each gyreCentered at about 30º north or south latitude

Current gyresGyres (continued)

Subpolar gyresSmaller and fewer than subtropical gyresGenerally 2 currents in each gyreCentered at about 60º north or south latitudeRotate in the opposite direction of adjoining subtropical gyres

Ekman spiralEkman spiral describes the speed and direction of flow of surface waters at various depthsFactors:

WindCoriolis effect Ekman spiral

Ekman transportEkman transport is the overall water movement due to Ekman spiralIdeal transport is 90º from the windTransport direction (right or left) depends on the hemisphere

Geostrophic flow and western intensification

Geostrophic flow causes a hill to form in subtropical gyresThe center of the gyre is shifted to the west because of Earth’s rotationWestern boundary currents are intensified

Western Intensification

Western intensification of subtropical gyres

The western boundary currents of all subtropical gyres are:

FastNarrowDeep

Western boundary currents are also warmEastern boundary currents of subtropical gyres have opposite characteristics

Currents and climateWarm current

warms air high water vapor humid coastal climate

Cool current cools air low water vapor dry coastal climate

Warm and cool currents

Upwelling and downwellingVertical movement of water

Upwelling = movement of deep water to surfaceHoists cold, nutrient-rich water to surfaceProduces high productivities and abundant marine life

Downwelling = movement of surface water downMoves warm, nutrient-depleted surface water downNot associated with high productivities or abundant marine life

Coastal upwelling and downwellingEkman transport moves surface water away from shore, producing upwellingEkman transport moves surface water towards shore, producing downwelling

Equatorial UpwellingTradewinds blow equatorial waters north and southUpwelling from the deep replaces water moving north and south Equatorial upwelling

Antarctic surface circulation

Surface currents circle Antarctica moving to the eastDriven by prevailing westerlies

The Gulf Stream and sea surface temperatures

The Gulf Stream is a warm, western intensified currentMeanders as it moves into the North AtlanticCreates warm and cold core rings (eddys)

Gulf Stream Current

Gulf Stream

The Gulf Stream Benjamin Franklin’s Map

The Sargasso Sea

North Pacific Garbage PatchVideo - The Great Pacific Garbage Patch

Other surface currentsLongshore currents

Run parallel along coastlinesResponsible for formation of barrier islands and transport of sand along beaches

Rip currentsOccur when coastal waves or currents converge Dangerous for swimmers because they flow off the beach towards the open ocean

El Niño-Southern Oscillation (ENSO)

I am El Nino!El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmas timeLa Niña = opposite of El Niño Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El NiñoENSO describes a combined oceanic-atmospheric disturbance

Normal conditions in the Pacific Ocean

El Niño conditions (ENSO warm phase)

La Niña conditions (ENSO cool phase; opposite of El Niño)

The 1997-98 El NiñoSea surface temperature anomaly map shows warming during severe 1997-98 El NiñoInternet site for El Niño visualizationsMore animationsCurrent state of the tropical Pacific

El Niño recurrence intervalTypical recurrence interval for El Niños = 2-12 yearsPacific has alternated between El Niño and La Niña events since 1950

Effects of severe El Niños

Deep currentsDeep currents:

Form in subpolar regions at the surfaceAre created when high density surface water sinksFactors affecting density of surface water:

Temperature (most important factor)Salinity

Deep currents are also known as thermohaline circulation

Deep ocean characteristicsConditions of the deep ocean:

ColdStillDarkEssentially no productivitySparse lifeExtremely high pressure

Identification of deep currentsDeep currents are identified by measuring temperature (T) and salinity (S), from which density can be determined

Global conveyer-belt

It takes 1,000 years for water to make a complete circuit

The Conveyor Belt and CO2

At the ocean surface, CO2 is dissolved by water and photosynthetic plankton convert it to oxygen and carbonate (shells)CO2 in the water is also circulated on the conveyor belt into the deep oceans where it is sequesteredIn effect, the oceans and conveyor belt current acts as a carbon “sink”, removing it from the atmosphere

Global Conveyor Belt LinksNOS current siteAnimationsNASA VideoThe Next Ice Age?