Ocean CurrentsOcean Currents

Essentials of Oceanography

Wind-driven Ocean CirculationSurface ocean circulation

mixed layer above top 100 mcontrolled by winds + coriolis

Overhead wind patternsWind blows against surface- friction sets water into motion

Continents interfere with the winds and redirect airflow

Result- circulation cells within each ocean basin

GyresGyre -

closed, circular flow of water around an ocean basin5 gyres:

North AtlanticSouth Atlantic North Pacific South PacificIndian Ocean

Plus circulation around Antarctica- closed circuitwind and water can freely flow around Antarctica

Surface Ocean Currents

Western Boundary Currents

Flow from equator to pole along western margin of basins

Strong, fast, narrow, focused flowTransports heat to higher latitudes

Gulf StreamGood example of a Western Boundary

Current that flows like a river- amount of water carried = 100x discharge from all rivers!

First mapped by Ben FranklinMajor mechanism for transport of heat to North.

Climate in England vs. Newfoundland

Eastern Boundary CurrentsEastern flow more diffuse, wider, slowerCold water currents

Divergence and ConvergenceWhere currents or current and land come together or split apartConvergence leads to downwellingDivergence leads to upwelling- brings cold, nutrient-rich water up from about 500 m

2 important areas of upwellingPacific equatorial regionNear shore

Along shore winds force water off the coast - creates low water pressure

Eastern margins of ocean basins - Calif. Coast, Peru

El Niño- Southern Oscillation (ENSO)

Represents interactions between:Atmospheric circulationOcean circulationClimate

Begins in equatorial Pacific, but has global effectsCause is not well understoodEl Nino refers to changes in ocean circulation

Named for anomalous warm current off Peru that occurs at Christmas timeNormally - cold current off of Peru due to upwelling

El Niño- Southern Oscillation (ENSO)Southern Oscillation

Refers to changes in atmospheric conditionsOscillation in the distribution of high and low pressure systems across the equatorial Pacific“Affect wind patterns, which affects surface ocean circulation.

El Niño- Southern Oscillation (ENSO)

Develops when:* Sea surface temperatures (SST) in tropical eastern Pacific Ocean are warmer than normal* pressure patterns weaken (and may reverse)* trade winds weaken (and may reverse)

Southern Oscillation Index (SOI)

Pressure values in the eastern Pacific (Tahiti) and western Pacific (Darwin, Australia) are monitored

SOI = pressure departure at Tahiti pressure departure at Darwin

SOI < 0 = NormalSOI > 0 = ENSO

Normal

El Nino

ENSO ComparisonNormal years

Lower pressure over Indonesia Higher pressure over eastern equatorial Pacific Driven by strong trade windsWeak equatorial counter current Strong upwelling near Peru (and Calif)W. Pac ~ 8º warmer than E Pac. Rain in western Pacific, dry in eastern Pacific

El Nino yearsHigher pressure over IndonesiaLower pressure over eastern PacificDecreased pressure gradient across the equatorial Pacific weakens trade windsStronger countercurrent transports warm water to the east Reduced upwelling Shift in rainfall to the east

La Nina is when conditions are more intensely “normal”

PeriodicityENSO periodicity 2-7 years

~1 event every 4 years for past century and 1 strong event per decadeBut duration and extent variable (each unique)

Appear to be becoming more frequent over past few decades

1990-1998 -- prolonged ENSO conditions Natural variability vs. Global warming effects

Effects of ENSO

Oceanic Deep-water Circulation

Subsurface currents arise from the density differences between water masses Produced by the variations in water temperature (thermal effect) and salinity (haline effect)Collectively referred to as thermohaline circulation

Thermohaline CirculationEvaporation and lower temperatures cool surface waters from ~ 45º N and ~ 45º S latitude to the polesCold (and therefore dense) polar water sinks and then drifts equatorward, below warmer, less dense surface waterCold water descends to a depth of corresponding density, 'sliding' under less dense water and over more dense waterDeep waters slowly return to the surface (after ~1000 years) through upwelling along the equator and in coastal regions

Global CirculationNADW sinks and flows southward along the western side of the Atlantic OceanNADW and AABW mix in the Antarctic Circumpolar CurrentMixed water mass of NADW and AABW flows northward into the Indian and Pacific OceansUpwells in the N. Pacific and Indian Oceans and returns to the south as warm shallow waters

SummarySurface circulation is driven by global wind patternsEl Nino is a warming of the west coast of South America and causes a disruption of global precipitationDeep water circulation is driven by gravity through density changes caused by temperature and salinity