Ocean currents

ocean circulation, surface and deep currents, thermohaline circulation

1. Ocean Currents Geronimo R. Rosario

2. Circulation - water movement in the ocean Currents - c ohesive streams of sea water that circulate through the oceans. Currents are also water masses in motion. Water Mass - a body of water identifiable from its temperature, salinity or chemical content. Upper water mass- includes the well-mixed surface layers of the ocean and the thermocline. Deep water mass- includes the water below the thermocline to the bottom of the ocean. Ocean Currents

3. The ocean is forced from the surface by fluxes of momentum and buoyancy (heat and freshwater). Surface currents are influenced by major wind belts Currents redistribute global heat. Most of the stratification is in the top km or so Thermohaline circulation affects deep currents. The sluggish thermohaline circulation forces ocean overturning reaching in some regions to the sea floor, resulting in the formation of the major water masses of the global ocean: North Atlantic Deep Water (NADW) Antarctic Bottom Water (ABW). Currents affect marine life. Ocean currents

4. Surface currents Surface circulation Wind-driven Warm and cold current Faster movement Primarily horizontal motion 10% of the total ocean water Deep currents Thermohaline circulation Driven by differences in density caused by differences in temperature and salinity Sluggish movement Cold current Vertical and horizontal motions 90% of the total ocean water Types of Ocean currents

5. Surface ocean currents Transfer heat from warmer to cooler areas Similar to pattern of major wind belts Affect coastal climates Deep ocean currents Provide oxygen to deep sea Types of Ocean Currents

6. Ocean currents are driven by the following: 1. Solar Heating 2. Winds 3. Gravity 4. Coriolis Effects of Ocean currents 1. transfer heat from tropical to polar regions 2. influence weather and climate 3. distribute nutrients and scatter organisms Primary forces and effects of Ocean currents

7. Distribution of solar heating is not uniform. The equator receives more heat than the polar regions. Solar heating causes water to expand and move Solar heating

8. Wind is caused by pressure gradient force. Pressure gradient force results in a net force that is directed from high to low pressure The variation of pressure is caused by differential solar heating. Coriolis force modify the movements of the wind creating the global wind belts. Surface currents are wind-driven circulation. Winds

9. pull water downhill or pile against the pressure gradient (high/low) Causes geostrophic current together with Coriolis force influences tides Gravity

10. Caused by Earth’s rotation, faster at equator than at the poles Changes the intended path of all moving bodies (winds and currents). Motions are deflected to the right in northern hemisphere and left in southern hemisphere Causes the gyres and wind belts Coriolis force

11. © 2011 Pearson Education, Inc. Measuring Surface Currents Direct methods Floating device tracked through time Fixed current meter Indirect methods Pressure gradients Radar altimeters Doppler flow meter

12. © 2011 Pearson Education, Inc. Measuring Deep Currents Floating devices tracked through time Chemical tracers Tritium Chlorofluorocarbons Characteristic temperature and salinity Argo

13. Global array of free-drifting profiling floats that will measure the temperature and salinity of the upper 2000 m of the ocean in or near real-time. Argo

14. Surface Currents Occur above pycnocline Frictional drag between wind and ocean Generally follow wind belt pattern Other factors: Distribution of continents Gravity Friction Coriolis effect

15. © 2011 Pearson Education, Inc. Gyres Large, circular loops of moving water Bounded by: Equatorial current Western Boundary currents Northern or Southern Boundary currents Eastern Boundary currents Centered around 30 degrees latitude N and S

16. Five Subtropical Gyres North Atlantic – Columbus Gyre South Atlantic – Navigator Gyre North Pacific – Turtle Gyre South Pacific – Heyerdahl Gyre Indian Ocean – Majid Gyre

17. Subtropical Gyres and Currents

18. Subtropical Gyre Currents Four main currents flowing into one another: Equatorial Currents North or south Travel westward along equator Western Boundary Currents – warm waters Northern or Southern Boundary Currents – easterly water flow across ocean basin Eastern Boundary Currents – cool waters

19. Gyres and Boundary Currents

20. © 2011 Pearson Education, Inc. Other Surface Currents Equatorial Countercurrents – eastward flow between North and South Equatorial Currents Subpolar Gyres Rotate opposite subtropical gyres Smaller and fewer than subtropical gyres Subpolar currents North Atlantic North Pacific North Atlantic current Alaska current Norwegian current Aleutian current Labrador current Oyashio current East Greenland current Southern ocean Antarctic Circumpolar current

21. The westward flow of equatorial surface wind produces anticyclonic current gyres. 1 2 3 4 5 4 3 They start with North Equatorial and South Equa-torial currents, in the Atlantic (1, 2), Pacific (3, 4) and Indian (only 5) oceans. Warm Cold 1 North Equatorial Current in the Atlantic Ocean 2 South Equatorial Current in the Atlantic Ocean 3 North Equatorial Current in the Pacific Ocean 4 South Equatorial Current in the Pacific Ocean 5 South Equatorial Current in the Indian Ocean

22. The westward flow of equatorial surface wind produces anticyclonic current gyres. 1 2 3 4 5 4 3 Warm Cold 6 Kuroshio or Japan Current 7 East Australian Current 8 Gulf Stream 9 Brazil Current 10 Agulhas Current Blocked by land, these currents turn polewards. 6 8 9 7 10

23. The westward flow of equatorial surface wind produces anticyclonic current gyres . Warm Cold 11 California Current 12 Peru Current 13 Canary Current 14 Benguela Current 15 West Auatralian Current They cool down as they reach ~45°, and return as cold water currents 1 2 3 4 5 4 3 6 8 9 7 10 14 12 11 13 15

24. Earth’s rotation produces the Circum-Antarctic or Circum-Polar Current. Warm Cold Also called the West Wind Drift, this cold water current is the only ocean surface current that joins the waters of all the oceans. 1 2 3 4 5 4 3 6 8 9 7 10 14 12 11 13 15 Circum-Antarctic Circulation

25. Warm Cold Equatorial Counter Current is the gravity driven roll-back of warm waters stacked on western margins of the tropical ocean by westward flowing equatorial surface wind. 1 2 3 4 5 4 3 6 8 9 7 10 14 12 11 13 15 Circum-Antarctic Circulation ECC ECC ECC ECC can trigger El Niño

26. Things to consider: 1. Ekman spiral and transport 2. Convergence and Divergence 3. Vorticity 4. Geostrophic balance Understanding the formation of Surface currents

27. The spiraling pattern described by changes in water direction and speed with depth . Surface currents move at an angle to the wind. The result is a surface flow at 45 o to the right in NH. Direction varies and velocity decreases with depth until 100m. Depth of frictional influence - depth at which motion ceases. Ekman spiral Factors: 1.Wind Pushes Water through Wind Stress 2. Coriolis effect pushes water to right(left)

28. the net transport of water by wind-induced motion. the overall water movement due to Ekman spiral Ideal transport is 90º from the wind Transport direction depends on the hemisphere Ekman transport is proportional to the speed of the wind . Higher wind, higher transport! Ekman transport

29. Where water converge, water piles up and causes downwelling . Where water diverge, water level lowers and causes upwelling Convergence and Divergence

30. Converging surface seawater Surface seawater moves towards an area Surface seawater piles up Seawater moves downward Downwelling Low biological productivity

31. Diverging surface seawater Surface seawater moves away Deeper seawater (cooler, nutrient-rich) replaces surface water Upwelling High biological productivity

32. Upwelling – Vertical movement of cold, nutrient-rich water to surface High biological productivity Downwelling – Vertical movement of surface water downward in water column Low biological productivity Upwelling and Downwelling

33. Important areas of downwelling

34. Important areas of upwelling Peru

35. © 2011 Pearson Education, Inc. Coastal Upwelling Ekman transport moves surface seawater offshore. Cool, nutrient-rich deep water comes up to replace displaced surface waters.

36. © 2011 Pearson Education, Inc. Other Types of Upwelling Offshore winds Seafloor obstruction Coastal geometry change

37. © 2011 Pearson Education, Inc. Coastal Downwelling E kman transport moves surface seawater toward shore. Water piles up, moves downward in water column Lack of marine life

38. Langmuir circulation is a complex horizontal helical (spiral) motion that extends parallel to the wind. Langmuir circulation Adjacent helices rotate in opposite directions creating alternating zones of convergence and divergence. Material floating on the surface becomes concentrated in the zones of convergence and form sea stripes which parallel the wind direction.

39. Langmuir circulation

40. Circulation and vorticity are the two primary measures of rotation in a fluid. Circulatio n, which is a scalar integral quantity , is a macroscopic measure of rotation for a finite area of the fluid the fluid. Vorticity , however, is a vector field that gives a microscopic measure of the rotation at any point in the fluid. Vorticity is the tendency for elements of the fluid to "spin. Vorticity can be related to the amount of “circulation” or "rotation" (or more strictly, the local angular rate of rotation) in a fluid. Vorticity

41. Absolute vorticity - vorticity as viewed in an inertial reference frame. Relative vorticit y - vorticity as viewed in the rotating reference frame of the Earth. Planetary vorticity - vorticity associated with the rotation of the Earth. When we talked about Coriolis we introduced the idea of Planetary Vorticity . Every object on earth has a vorticity given to it by the rotation of the earth (except an object on the equator). This vorticity is dependent on latitude. Types of vorticity

42. Geostrophic Balance Most large currents are in Geostrophic balance. All currents are pushed to the right(left). This piles water up on the right(left). This creates a pressure force back towards the current. Eventually a balance is reached. Pressure BALANCES Coriolis ! current Coriolis pushes water to right(left). Piles up water. Sealevel Pressure force current coriolis pressure

43. © 2011 Pearson Education, Inc. Geostrophic Flow Geostrophic flow (current) – a current that develops out of the Earth’s rotation and is the result of a near balance between gravitational force and the Coriolis effect. Ideal geostrophic flow Friction generates actual geostrophic flow

44. © 2011 Pearson Education, Inc. Antarctic Circulation Antarctic Circumpolar Current Also called West Wind Drift and Penguin Gyre Only current to completely encircle Earth Moves more water than any other current

45. © 2011 Pearson Education, Inc. Antarctic Circulation Antarctic Convergence Cold, dense Antarctic waters converge with warmer, less dense sub-Antarctic waters Northernmost boundary of Antarctic Ocean East Wind Drift Polar Easterlies Creates surface divergence with opposite flowing Antarctic Circumpolar Current Antarctic Divergence Abundant marine life

46. © 2011 Pearson Education, Inc. Atlantic Ocean Circulation North Atlantic Subtropical Gyre North Equatorial Current Gulf Stream North Atlantic Current Canary Current South Equatorial Current Atlantic Equatorial Counter Current

47. © 2011 Pearson Education, Inc. Atlantic Ocean Circulation South Atlantic Subtropical Gyre Brazil Current Antarctic Circumpolar Current Benguela Current South Equatorial Current

48. © 2011 Pearson Education, Inc. Gulf Stream Best studied of all ocean currents Meanders and loops Merges with Sargasso Sea Circulates around center of North Atlantic Gyre Unique biology – Sargassum

49. © 2011 Pearson Education, Inc. Gulf Stream Meanders or loops may cause loss of water volume and generate: Warm-core rings – warmer Sargasso Sea water trapped in loop surrounded by cool water Cold-core rings – cold water trapped in loop surrounded by warmer water Unique biological populations

50. © 2011 Pearson Education, Inc. Other North Atlantic Currents Labrador Current Irminger Current Norwegian Current North Atlantic Current

51. © 2011 Pearson Education, Inc. Climate Effects of North Atlantic Currents North-moving currents – warm Gulf Stream warms East coast of United States and northern Europe North Atlantic and Norwegian Currents warm northwestern Europe South-moving currents – cool Labrador Current cools eastern Canada Canary Current cools north African coast

52. © 2011 Pearson Education, Inc. Indian Ocean Circulation Monsoons – seasonal reversal of winds over northern Indian Ocean Heat Capacity Differential Northeast monsoon – winter Southwest monsoon – summer

53. © 2011 Pearson Education, Inc. Indian Ocean Circulation

54. Indian Ocean Monsoon Affects seasonal land weather Affects seasonal Indian Ocean current circulation Affects phytoplankton productivity

55. © 2011 Pearson Education, Inc. Indian Ocean Circulation Indian Ocean Subtropical Gyre Agulhas Current Australian Current Leeuwin Current

56. Pacific Ocean Circulation North Pacific Subtropical Gyre Kuroshio North Pacific Current California Current North Equatorial Current Alaskan Current

57. © 2011 Pearson Education, Inc. Pacific Ocean Circulation South Pacific Subtropical Gyre East Australian Current Antarctic Circumpolar Current Peru Current South Equatorial Current Equatorial Counter Current

58. © 2011 Pearson Education, Inc. Pacific Ocean Circulation

59. Both are shallow(thin layers of fluid) Both are rotating rapidly Both are stratified fluids (usually stably, with lighter fluid on top) The ocean has sidewall boundaries . The ocean has a definitive top while the atmosphere does not. The ocean is almost incompressible. Ocean vs Atmosphere

60. The atmosphere is driven primarily by thermal forcing at its lower boundary; the oceans are driven primarily mechanically driven from the top. The atmosphere has significant internal diabatic heating (latent heat release; radiation); the oceans do not . The oceans are salty, the atmosphere is moist and cloudy The ocean is dense (~1000 times air), with a large heat capacity and large inertia . 2.5m of water holds as much heat as the whole depth of the atmosphere Ocean vs Atmosphere

61. © 2011 Pearson Education, Inc. Atmospheric-Ocean Connections in the Pacific Ocean Walker Circulation Cell – normal conditions Air pressure across equatorial Pacific is higher in eastern Pacific Strong southeast trade winds Pacific warm pool on western side of ocean Thermocline deeper on western side Upwelling off the coast of Peru

62. © 2011 Pearson Education, Inc. Normal Conditions, Walker Circulation

63. El Niño (Spanish for “the Child” in reference to baby Jesus) = 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 La Niña  is a climate pattern that describes the cooling of surface ocean waters along the tropical west coast of South America.  La Nina is considered to be the counterpart to El Nino, which is characterized by unusually warm ocean temperatures in the equatorial region of the Pacific Ocean. El Niño, ENSO and La Niña

64. © 2011 Pearson Education, Inc. El Niño – Southern Oscillation (ENSO) Walker Cell Circulation disrupted High pressure in eastern Pacific weakens Weaker trade winds Warm pool migrates eastward Thermocline deeper in eastern Pacific Downwelling Lower biological productivity Peruvian fishing suffers

65. ENSO Conditions in the Pacific Ocean

66. © 2011 Pearson Education, Inc. La Niña – ENSO Cool Phase Increased pressure difference across equatorial Pacific Stronger trade winds Stronger upwelling in eastern Pacific Shallower thermocline Cooler than normal seawater Higher biological productivity

67. © 2011 Pearson Education, Inc. La Niña Conditions

68. © 2011 Pearson Education, Inc. Occurrence of ENSO Events El Niño warm phase about every 2–10 years Highly irregular Phases usually last 12–18 months 10,000-year sediment record of events ENSO may be part of Pacific Decadal Oscillation (PDO) Long-term natural climate cycle Lasts 20–30 years

69. ENSO Occurrences

70. © 2011 Pearson Education, Inc. ENSO has Global Impacts

71. © 2011 Pearson Education, Inc. Notable ENSO Events 1982 – 1983 1997 – 1998 Flooding, drought, erosion, fires, tropical storms, harmful effects on marine life Unpredictable

72. Predicting El Niño Events Tropical Ocean−Global Atmosphere (TOGA) program 1985 Monitors equatorial South Pacific System of buoys Tropical Atmosphere and Ocean (TOA) project Continues monitoring ENSO still not fully understood

73.   The ocean is divided into three zones: Surface zone - the upper layer of the ocean, containing the least dense water. The surface zone is only about 2% of total ocean volume. Pycnocline - a zone in which density increases with depth, containing about 18% of all ocean water. Deep zone – contains about 80% of all ocean water. There is little change in density throughout this layer. Density Structure of the Ocean

74. Ocean Currents

75. Conditions of the deep ocean: Cold Still Dark Essentially no productivity Sparse life Extremely high pressure Deep Ocean

76. © 2011 Pearson Education, Inc. Deep-Ocean Currents Thermohaline Circulation – deep ocean circulation driven by temperature and density differences in water Below the pycnocline 90% of all ocean water Slow velocity

77. © 2011 Pearson Education, Inc. Thermohaline Circulation Originates in high latitude surface ocean Cooled, now dense surface water sinks and changes little. Deep-water masses identified on temperature–salinity (T–S) diagram Identifies deep water masses based on temperature, salinity, and resulting density

78. © 2011 Pearson Education, Inc. Thermohaline Circulation

79. © 2011 Pearson Education, Inc. T – S Diagram

80. © 2011 Pearson Education, Inc. Thermohaline Circulation Some deep-water masses Antarctic Bottom Water North Atlantic Deep Water Antarctic Intermediate Water Oceanic Common Water Cold surface seawater sinks at polar regions and moves equatorward

81. A current that connects the Ocean’s Surface Waters to Deep waters via Upwelling and Downwelling Also called Thermohaline Circulation Mixes waters within and throughout all oceans Oxygen flows down Nutrients flow up Conveyor Belt Circulation

82. © 2011 Pearson Education, Inc. Conveyor Belt Circulation

83. Conveyor Belt Circulation

84. © 2011 Pearson Education, Inc. Power From Currents Currents carry more energy than winds Florida–Gulf Stream Current System Underwater turbines Expensive Difficult to maintain Hazard to boating