1. Essentials of Oceanography, Thurman and Trujillo Chapter VII: Ocean Circulation

2. Wind-driven surface currents Figure 7-4 Ocean Circulation Animation

3. Measuring surface currents Direct methods Float meters (lagrangian: float with current) Intentional Inadvertent Propeller meters (eularian: stay in one place) Indirect methods Pressure gradients Satellites Doppler flow meters Figure 7B

4. Ocean currents Surface currents Affect surface water within and above the pycnocline (10% of ocean water…I think it is more like 25% of ocean water) Driven by major wind belts of the world Deep currents Affect deep water below pycnocline (90% of ocean water…I think it is more like 75%) Driven by density differences Larger and slower than surface currents NO CLEAR CUT DELINEATION

5. Deep water masses and currents Deep water masses: Form in subpolar regions at the surface Are created when high density surface water sinks Factors affecting density of surface water: Temperature (most important factor) Salinity Deep currents which transport deep waters are also known as thermohaline circulation Characteristics of deep waters are determined AT THE SURFACE

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

7. Identification of deep water masses Deep water masses are identified by measuring temperature (T) and salinity (S), from which density can be determined T-S diagram Characteristics set at surface Figure 7-24

8. Atlantic Ocean subsurface water masses Figure 7-25

9. Conveyer-belt circulation: Deep Currents Figure 7-27

10. Understanding the formation of SURFACE currents 4 Primary things that need to be understood - Ekman transport (and spiral) - The idea of Convergence - Conservation of Vorticity - Geostrophic Balance What drove Deep Currents?

11. Ekman spiral: Wind Driven ( τ ) Ekman spiral describes the speed and direction of flow of surface waters at various depths Factors: Wind Pushes Water through Wind Stress (τ) Coriolis effect pushes water to right(left) Due to shear, water velocity spins to the right(left) with depth. Figure 7-6

12. Ekman transport Ekman transport is 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! Figure 7-6

13. More Realistic Climatological (average) Winds

14. Understanding the formation of currents 4 Primary things that need to be understood - Ekman transport (and spiral) - The idea of Convergence - Conservation of Vorticity - Geostrophic Balance

15. Convergence/Divergence This idea is nothing more then the piling up or moving of water away from a region. Conservation of VOLUME: (du/dx+dv/dy+dw/dz=0) Rearranging... du/dx + dv/dy = -dw/dz If water comes into the box (du/dx + dv/dy)>0 there is a velocity out of the box: dw/dz < 0 DOWNWARD So lets go back to Ekman…and see where water is piled up and where it is emptied.

17. Convergence (Divergence) across a mid ocean gyre

18. Understanding the formation of currents 4 Primary things that need to be understood - Ekman transport (and spiral) - The idea of Convergence - Conservation of Vorticity - Geostrophic Balance

19. Vorticity (I think the 3rd time we’ve talked about it) Vorticity is analagous to angular momentum. Vorticity is a conserved quantity (Conservation of Vorticity) When we talked about Coriolis we introduced the idea of Planetary Vorticity (f). 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. Each object on earth can have Relative Vorticity as well. An ice skater who is spinning has Relative Vorticity. A skater who becomes more skinny spins faster (greater relative vorticity). But remember that water is incompressible. So if a water column becomes ‘skinny’ it MUST become taller at the same time! TOTAL VORTICITY is CONSERVED BY FLUIDS. Planetary (f) + Relative (ξ) = Constant H H is the (tallness, or depth of water column)

20. North Pole (High planetary Vorticity f) Equator (Zero planetary Vorticity f) A parcel of water moves off the equator its vorticity on the equator (f+ ξ )=0. Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ )=0, ξ must be < 0. The water begins to spin. Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative. An example of conservation of vorticity when H stays constant

21. Ocean Surface Ocean bottom A parcel of water moves east (constant latitude) in N.Hemis. As the parcel hits the bump, H decreases. We know that (f + ξ )/H=Constant. So if H decreases, (f + ξ ) must decrease. If f decreases, the parcel moves equatorward. If ξ decreases the parcel spins clockwise. Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative. An example of conservation of vorticity when H doesn’t stay constant H Bump in bottom H What happens when the parcel leaves the bump?

22. Ocean Surface Ocean bottom A parcel of water moves east (constant latitude) in N.Hemis. As the parcel hits the bump, H decreases. We know that (f + ξ )/H=Constant. So if H decreases, (f + ξ ) must decrease. If f decreases, the parcel moves equatorward. If ξ decreases the parcel spins clockwise. Or a combination. Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative. An example of conservation of vorticity when H doesn’t stay constant H Bump in bottom H H

23. North South A parcel of water moves east (constant latitude) in N.Hemis. As the parcel hits the bump, H decreases. We know that (f + ξ )/H=Constant. So if H decreases, (f + ξ ) must decrease. If f decreases, the parcel moves equatorward. If ξ decreases the parcel spins clockwise. Or a combination. Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative. An example of conservation of vorticity when H doesn’t stay constant Bump in bottom H H From ABOVE Parcel Moves Equatorward

24. Understanding the formation of currents 4 Primary things that need to be understood - Ekman transport (and spiral) - The idea of Convergence - Conservation of Vorticity - Geostrophic Balance

25. Geostrophic Balance Most large currents are in Geostrophic balance. Which terms from our momentum equation? 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

26. Geostrophic Balance Geostrophic flow causes a hill to form in subtropical gyres Example in the book of the balance of coriolis and pressure force (gravity). Current is Perpendicular to slope. Current is along constant height Figure 7-7

27. Understanding the formation of currents We’ve been introduced to the 4 Primary things that need to be understood. Let’s put them all together to understand what drives our ocean currents! - Ekman transport (and spiral) - The idea of Convergence - Conservation of Vorticity - Geostrophic Balance

28. More Realistic Climatological (average) Winds Ekman transport creates convergence and divergence of upper waters. Convergence Divergence

29. Upwelling and Downwelling across a mid ocean gyre due to Ekman Transport Convergence causes downwelling! Divergence causes upwelling!

30. Ocean Surface Mixed Layer Ocean bottom A parcel of water moves into an area of downwelling. It becomes shorter (and fatter). f/H must be conserved! We know that (f + ξ )/H= Constant. So if H decreases, (f + ξ ) must decrease. I gave examples before that either f or ξ could change. But in this process; it is f that decreases. f can only decrease by the parcel moving equatorward. With DOWNWELLING, the vertical velocity is downward. This pushes on the column of water, making it shorter (and fatter). What happens when a column of water gets short and fat (Vorticity must be conserved). HH Ekman Convergence

31. More Realistic Climatological (average) Winds Ekman transport creates convergence and divergence of upper waters. Convergence Divergence

32. More Realistic Climatological (average) Winds Ekman transport creates convergence and divergence of upper waters. Equatorward flow Poleward flow Complicated flow 45o N 15o N 15o S 45o S

33. Geostrophic Balance Ekman transport has caused a ‘hill’ to form in the sea surface when convergence occurs (subtropical gyre) Vorticity balance explains equatorward flow (from gyre center to the east) Geostropic current is along constant height (WARM water to right in N Hemis) Current must return back to the north (conservation of mass) Western Boundary Current is that return. Very strong very intense Figure 7-7

34. Sea Surface Height and Mean Geostrophic Ocean Circulation

35. Current gyres Gyres are large circular-moving loops of water Subtropical gyres Five main gyres (one in each ocean basin): North Pacific, South Pacific, North Atlantic, South Atlantic, Indian Generally 4 currents in each gyre Centered at about 30º north or south latitude (I think more like 25 o ) Subpolar gyres Smaller and fewer than subtropical gyres Generally 2 currents in each gyre Centered at about 60º north or south latitude Rotate in the opposite direction of adjoining subtropical gyres

36. Sea Surface Height and Mean Geostrophic Ocean Circulation H-Subtropical Gyre L-Subpolar Gyre H-Subtropical Gyre L-Subpolar Gyre

37. P37 mean dyht and temperature field HK Guam HA SF Sea Surface Height Temperature Field Salinity Field

38. Western intensification of subtropical gyres The western boundary currents of all subtropical gyres are: Fast Narrow Deep Western boundary currents are also warm Western Boundary Currents and Vorticity Conservation…Must conserve.

39. North Pole (High planetary Vorticity f) Equator (Zero planetary Vorticity f) A parcel of water moves off the equator its vorticity on the equator (f+ ξ )=0. Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ )=0, ξ must be < 0. The water begins to spin. Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative. Back to our example of conservation of vorticity when H stays constant Remember this example? As the western boundary current returns north, this should happen, but it does not. Why?

40. North Pole (High planetary Vorticity f) Equator (Zero planetary Vorticity f) A parcel of water moves off the equator its vorticity on the equator (f+ ξ )=0. Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ )=0, ξ must be < 0. The water begins to spin. Back to our example of conservation of vorticity when H stays constant As the water moves up the coast in the VERY Narrow WBC, it rubs against the coast. It removes vorticity through friction. The WBC MUST be narrow, it must get close to the coast. Conservation of Vorticity is valid as an idea. But once an outside force like friction is applied, conservation is not going to happen. Parcel wants to spin But can’t due to friction

41. Wind-driven surface currents Figure 7-4

42. Upwelling and downwelling Vertical 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. Coastal upwelling and downwelling Ekman transport moves surface water away from shore, producing upwelling Ekman transport moves surface water towards shore, producing downwelling Figure 7-11

44. Other types of upwelling Equatorial upwelling Offshore wind Sea floor obstruction Sharp bend in coastal geometry Figure 7-9 Equatorial upwelling

45. Other examples of upwelling (Which one looks like San Diego?)

46. Antarctic surface circulation Figure 7-13

47. Ocean surface currents What Currents do you need to know?

48. The Gulf Stream and sea surface temperatures The Gulf Stream is a warm, western intensified current Meanders as it moves into the North Atlantic Creates warm and cold core rings Rings move west. Argue as given in book for westward intensification. Figure 7-16

49. Flows are typically unstable; they meander

51. Currents and climate Warm current  warms air  high water vapor  humid coastal climate Cool current  cools air  low water vapor  dry coastal climate Figure 7-8a

52. 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

53. Average conditions in the Pacific Ocean Figure 7-18a El Nino/La Nina Animation

54. El Niño conditions (ENSO warm phase) Figure 7-18b

55. La Niña conditions (ENSO cool phase; opposite of El Niño) Figure 7-18c

56. The 1997-98 El Niño Sea surface temperature anomaly map shows warming during severe 1997-98 El Niño Internet site for El Niño visualizations Current state of the tropical Pacific Figure 7-19a

57. El Niño recurrence interval Typical recurrence interval for El Niños = 3-12 years Pacific has alternated between El Niño and La Niña events since 1950 Figure 7-20

58. Effects of severe El Niños Figure 7-21

59. El Nino La Nina

60. Essentials of Oceanography, Thurman and Trujillo End of Chapter VII

61. Measuring currents through satellite Red: High sea level…High sea level is warmer water (water expands when warm)…In N Hemisphere warm water is on the right. ONLY measures anomaly, Must add GEOID.

62. Equatorial Currents are complicated…but they are still driven EXACTLY THE SAME WAY as the gyres. The currents are complicated because the winds are complicated and the equator is present (Why would the equator be important?) f is nearly zero near the equator so swashing and stretching of water columns isn’t the driving force. The process is just ekman convergence/divergence and pressure forces.

63. Topex/Poseidon dynamic topography after GEOID has been added

64. Ocean surface currents Figure 7-14

65. North Atlantic Ocean circulation Figure 7-15 Sverdrup: measure of flow rate (length 3 /time) 1 Sv = 10 6 m 3 /s

66. Pacific Ocean surface currents Figure 7-17

67. Indian Ocean surface currents Figure 7-23 Northeast monsoonSouthwest monsoon