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Wind-driven Ocean Circulation
Surface 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 basin
5 gyres: North AtlanticSouth Atlantic North Pacific South PacificIndian Ocean
Plus circulation around Antarctica- closed circuitwind and water can freely flow around Antarctica
Western Boundary Currents
Flow from equator to pole along western margin of basins
Strong, fast, narrow, focused flow
Transports 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
Divergence and Convergence
Where 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 OscillationRefers to changes in atmospheric conditions
Oscillation 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
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”
Periodicity
ENSO periodicity 2-7 years ~1 event every 4 years for past century and 1 strong event per decade
But 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
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 Circulation
Evaporation and lower temperatures cool surface waters from ~ 45º N and ~ 45º S latitude to the poles
Cold (and therefore dense) polar water sinks and then drifts equatorward, below warmer, less dense surface water
Cold water descends to a depth of corresponding density, 'sliding' under less dense water and over more dense water
Deep 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 Ocean
NADW and AABW mix in the Antarctic Circumpolar Current
Mixed water mass of NADW and AABW flows northward into the Indian and Pacific Oceans
Upwells in the N. Pacific and Indian Oceans and returns to the south as warm shallow waters