Ocean currents


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Ocean currents

  1. 1. Ocean Currents
  2. 2. Why is Ocean Circulation Important? • Transport ~ 20% of latitudinal heat – Equator to poles • Transport nutrients and organisms • Influences weather and climate • Influences commerce
  3. 3. Non-rotating Earth Convection cell model
  4. 4. Add rotation and add landmassesunequal heating and cooling of the Earth
  5. 5. Physical properties of the atmosphere: Density• Warm, low density air rises• Cool, high density air sinks• Creates circular- moving loop of air (convection cell)
  6. 6. Physical properties of the atmosphere: Water vapor• Cool air cannot hold much water vapor, so is typically dry• Warm air can hold more water vapor, so is typically moist• Water vapor decreases the density of air
  7. 7. Physical properties of the atmosphere: Pressure
  8. 8. 90o High pressure, dry climate60o Low pressure, wet climate High pressure, dry climate30o ITCZ intertropical0o convergence zone= doldrums Low pressure,30o wet climate60o90o
  9. 9. The Coriolis effect• The Coriolis effect – Is a result of Earth’s rotation – Causes moving objects to follow curved paths: • In Northern Hemisphere, curvature is to right • In Southern Hemisphere, curvature is to left – Changes with latitude: • No Coriolis effect at Equator • Maximum Coriolis effect at poles
  10. 10. The Coriolis effect on Earth• As Earth rotates, different latitudes travel at different speeds• The change in speed with latitude causes the Coriolis effect
  11. 11. North Pole Buffalo moves 783 mph Quito moves 1036 mphBuffalo 15oQuito N Buffalo 79oW Quito South Pole
  12. 12. A) Idealized winds generated by pressure gradient and Coriolis Force.B) Actual wind patterns owing to land mass distribution..
  13. 13. Ocean CurrentsSurface Currents The upper 400 meters of the ocean (10%).Deep Water Currents Thermal currents (90%)
  14. 14. Surface Currents Forces 1. Solar Heating (temp, density) 2. Winds 3. Coriolis
  15. 15. Wind-driven surface currents
  16. 16. Wind-Driven and Density-Driven Currents• Wind-driven currents occur in the uppermost 100 m or less• Density differences causes by salinity and temperature produce very slow flows in deeper waters.
  17. 17. Sailors have know about ocean currents for centuriesSailors have know that ―rivers‖ flow in the seas since ancient times. They used them to shorten voyages, or were delayed by trying to stem them.If navigators do not correct to deflection by currents, they may be far away from where they think they are and meet disaster.
  18. 18. Ben Franklin and the Gulf Stream
  19. 19. Matthew Fontaine MauryThe first systematic study of currents was done by Maury based on logbooks in the US Navy’s Depot of Charts and Instruments.His charts and ―Physical Geography of the Sea‖ assisted navigators worldwide. http://www.npg.si.edu/exh/brady/gallery/97gal.html
  20. 20. Winds and surface water• Wind blowing over the ocean can move it due to frictional drag.• Waves create necessary roughness for wind to couple with water.• One ―rule of thumb‖ holds that wind blowing for 12 hrs at 100 cm per sec will produce a 2 cm per sec current (about 2% of the wind speed)
  21. 21. Top-down drag• Wind acts only on the surface water layer.• This layer will also drag the underlying water, but with less force.• Consequently, there is a diminution of speed downward.• Direction of movement is also influenced by the Coriolis Effect and Ekman Spiral
  22. 22. Ekman spiralEkman spiral describes the speed and direction of flow of surface waters at various depths• Factors: – Wind – Coriolis effect
  23. 23. Ekman transportEkman transport is the overall water movement due to Ekman spiral• Ideal transport is 90º from the wind• Transport direction depends on the hemisphere
  24. 24. Ekman TransportWater flow in the Northern hemisphere- 90o to the right of thewind directionDepth is important
  25. 25. Currents in the ―Real‖ Ocean Currents rarely behave exactly as predicted by these theoretical explanations due to factors such as• Depth—shallow water does not permit full development of the Ekman spiral• Density—deeper currents moving in different directions influence the overlying surface movement
  26. 26. Geostrophic Flow Surface currents generally mirror averageplanetary atmospheric circulation patterns
  27. 27. Current GyresGyres are large circular-moving loops of water Five main gyres (one in each ocean basin): • North Pacific • South Pacific • North Atlantic • South Atlantic • Indian • Generally 4 currents in each gyre • Centered about 30o north or south latitude
  28. 28. Geostrophic flow and western intensification• Geostrophic flow causes a hill to form in subtropical gyres• The center of the gyre is shifted to the west because of Earth’s rotation• Western boundary currents are intensified Figure 7-7
  29. 29. Western intensification of subtropical gyres• The western boundary currents of all subtropical gyres are: – Fast – Narrow – Deep• Western boundary currents are also warm• Eastern boundary currents of subtropical gyres have opposite characteristics
  30. 30. Boundary Currents in the Northern HemisphereType of Current General Features Speed Special FeaturesWestern boundary Currents warm swift sharp boundaryGulf Stream, Kuroshio narrow w/coastal circulation, deep little coastal upwellingEastern Boundary Currents cold slow diffuse boundariesCalifornia, Canary broad separating from coastal shallow currents, coastal upwelling common
  31. 31. Pacific Ocean surface currents
  32. 32. ―Hills and Valleys‖ in the Ocean balance between the • A Ekman transport and Coriolis effect produces ―hills‖ in the center of the gyres and ―valleys‖ elsewhere • Gravitational effects from sea floor features also produce variations in sea surface topographyhttp://earth.usc.edu/~stott/Catalina/Oceans.html
  33. 33. What do Nike shoes,rubber ducks, andhockey gloves have todo with currents?
  34. 34. Lost at Sea
  35. 35. Duckie Progress•January 1992 - shipwrecked in the Pacific Ocean, offthe coast of China•November 1992 - half had drifted north to the BeringSea and Alaska; the other half went south toIndonesia and Australia•1995 to 2000 - spent five years in the Arctic ice floes,slowly working their way through the glaciers2001 - the duckies bobbed over the place where theTitanic had sunk•2003 - they were predicted to begin washing uponshore in New England, but only one was spotted inMaine•2007 - a couple duckies and frogs were found on thebeaches of Scotland and southwest England.
  36. 36. 2004-2007Barber’s Point
  37. 37. North Pacific Subtropical Gyre• ―Great Pacific Garbage Patch‖• Estimate: 46,000 pieces of floating garbage/mi2.
  38. 38. North Pacific Subtropical Gyre 135° to 155°W and 35° to 42°N
  39. 39. North Pacific Subtropical GyreGreat Pacific Garbage Patch- Good Morning America 2010http://www.youtube.com/watch?v=uLrVCI4N67M&feature=player_embedded http://marinedebris.noaa.gov/info/patch.html#6
  40. 40. EddyA circular movement of water formed along the edgeof a permanent currentIn an average year, 10-15 rings are formed150-300 km in diameterSpeed 1 m/secWarm core ring1. Rotates clockwise2. Found on the landward side of the currentCold core ring (cyclonic eddy)1. Rotates counterclockwise2. Forms on the ocean side of the current
  41. 41. Sargasso Sea
  42. 42. Upwelling and downwellingVertical movement of water () – Upwelling = movement of deep water to surface • Hoists cold, nutrient-rich water to surface • Produces high productivities and abundant marine life – Downwelling = movement of surface water down • Moves warm, nutrient-depleted surface water down • Not associated with high productivities or abundant marine life
  43. 43. upwelling downwelling
  44. 44. Langmuir Circulation
  45. 45. Satellite Observations• TOPEX/Poseidon, Jason 1, and other satellites have observed patterns of change over the past few years• Animation of seasonal and climatically- influence shifts available athttp://seawifs.gsfc.nasa.gov/OCEAN_PLANET/MOVIES /Topex_Dynamic_Ocean_Topography.mpg
  46. 46. El Niño-Southern Oscillation (ENSO)• El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmastime• Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño• ENSO describes a combined oceanic- atmospheric disturbance
  47. 47. El Niño• Oceanic and atmospheric phenomenon in the Pacific Ocean• Occurs during December• 2 to 7 year cycle Sea Surface Temperature Atmospheric Winds Upwelling
  48. 48. Normal conditions in the Pacific Ocean
  49. 49. El Niño conditions (ENSO warm phase)
  50. 50. La Niña conditions (ENSOcool phase; opposite of El Niño)
  51. 51. Non El Niño El Niño 1997
  52. 52. Non El Niño upwelling El Niño thermocline
  53. 53. El Niño events over the last 55 yearsEl Niño warmings (red) and La Niña coolings (blue) since1950. Source: NOAA Climate Diagnostics Center
  54. 54. World Wide Effects of El Niño • Weather patterns • Marine Life • Economic resources El Nino Animationhttp://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html
  55. 55. Effects of severe El Niños
  56. 56. Surface and Deep-Sea Current InteractionsUnifying concept: ―Global Ocean Conveyor Belt‖ http://seis.natsci.csulb.edu/rbehl/ConvBelt.htm
  57. 57. Heat Transport by Currents• Surface currents play significant roles in transport heat energy from equatorial waters towards the poles• May serve as ―heat sources‖ to cooler overlying air, ―heat sinks‖ from warmer• Evaporation and condensation participate in latent heat exchanges
  58. 58. Matter Transport and Surface Currents• Currents also involved with gas exchanges, especially O2 and CO2• Nutrient exchanges important within surface waters (including outflow from continents) and deeper waters (upwelling and downwelling)• Pollution dispersal• Impact on fisheries and other resources
  59. 59. Global ocean circulation that is driven by differences inthe density of the sea water which is controlled bytemperature and salinity.
  60. 60. White sections represent warm surface currents.Purple sections represent deep cold currents
  61. 61. What effect does global warming play in thermohaline circulation?http://www.youtube.com/v/MZbsMlr9WRI?version=3
  62. 62. 1 2 3 4 CO2 fossil fuel Atmospheric and Subtropical High latitude combustion ocean temp evaporation precipitation & runoff North Atlanticregional cooling Deep water formation Nordic seas & thermohaline salinity & deep Potential feedback circulation convection of increased tropical salinity 6 5 Global climate interconnections
  63. 63. Inquiry1. What is a convection cell?2. Which direction do currents get deflected in the Southern Hemisphere?3. What depth should the water be for an Ekman spiral to occur?4. How are surface currents created?5. What is a gyre?6. How can an El Nino impact upwelling?7. Coriolis Effect is strongest near the _____?