Ocean Currents

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Ocean Currents

• Essentials of Oceanography

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Wind-driven Ocean Circulation

• Surface ocean circulation
• mixed layer above top 100 m
• controlled by winds coriolis
• Overhead wind patterns
• Wind blows against surface- friction sets water
  into motion
• Continents interfere with the winds and redirect
  airflow
• Result- circulation cells within each ocean basin

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Gyres

• Gyre -
• closed, circular flow of water around an ocean
  basin
• 5 gyres
• North Atlantic
• South Atlantic
• North Pacific
• South Pacific
• Indian Ocean
• Plus circulation around Antarctica- closed
  circuit
• wind and water can freely flow around Antarctica

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Surface Ocean Currents
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Western Boundary Currents

• Flow from equator to pole along western margin of
  basins
• Strong, fast, narrow, focused flow
• Transports heat to higher latitudes

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Gulf Stream

• Good example of a Western Boundary
• Current that flows like a river- amount of water
  carried 100x discharge from all rivers!
• First mapped by Ben Franklin
• Major mechanism for transport of heat to North.
• Climate in England vs. Newfoundland

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Eastern Boundary Currents

• Eastern flow more diffuse, wider, slower
• Cold water currents

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Divergence and Convergence

• Where currents or current and land come together
  or split apart
• Convergence leads to downwelling
• Divergence leads to upwelling- brings cold,
  nutrient-rich water up from about 500 m
• 2 important areas of upwelling
• Pacific equatorial region
• Near shore
• Along shore winds force water off the coast -
  creates low water pressure
• Eastern margins of ocean basins - Calif. Coast,
  Peru

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El Niño- Southern Oscillation (ENSO)

• Represents interactions between
• Atmospheric circulation
• Ocean circulation
• Climate
• Begins in equatorial Pacific, but has global
  effects
• Cause is not well understood
• El Nino refers to changes in ocean circulation
• Named for anomalous warm current off Peru that
  occurs at Christmas time
• Normally - cold current off of Peru due to
  upwelling

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El Niño- Southern Oscillation (ENSO)

• Southern Oscillation
• Refers 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.

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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)

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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 lt 0 Normal
• SOI gt 0 ENSO

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Normal
El Nino
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ENSO Comparison

• Normal years
• Lower pressure over Indonesia
• Higher pressure over eastern equatorial Pacific
• Driven by strong trade winds
• Weak 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 years
• Higher pressure over Indonesia
• Lower pressure over eastern Pacific
• Decreased pressure gradient across the equatorial
  Pacific weakens trade winds
• Stronger countercurrent transports warm water to
  the east
• Reduced upwelling
• Shift in rainfall to the east

La Nina is when conditions are more intensely
normal
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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

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Effects of ENSO
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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

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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

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Global Circulation

• NADW 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

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Summary

• Surface circulation is driven by global wind
  patterns
• El Nino is a warming of the west coast of South
  America and causes a disruption of global
  precipitation
• Deep water circulation is driven by gravity
  through density changes caused by temperature and
  salinity