General Ocean Circulation

1. General Ocean Circulation

2. 75% of the Earth’s surface Couples atmospheric processes with tectonic processes Important in regulating atmospheric CO2 Important in global heat transport

3. Effect of differential heating and cooling of the earth’s surface • Temperature and salinity (density) gradients (thermohaline circulation) • Atmospheric circulation, wind cells and surface ocean currents (drift currents)

4. Oceans • 75% of Earth’s surface • Important for heat transport • Cycling in the ocean important for elements • Couples shorter atm-ocean cycles with longer tectonic cycles • Ultimately involve burial in marine sediments • Regulation of atm CO2

5. Ocean layers • Thin surface mixed layer (~50 – 100 m) box • Sunlight penetrates • Net primary prod/ps drives biogeochem cycles • OM produced here sinks out and is remineralized below • Important for surface heat transport • Large –dark, cold and deep water box • Most of the ocean volume (~90%) • Isolated from surface for long periods of time (~500 – 1000 yr) = average mixing time for bottom waters • Important for understanding oceanic CO2 uptake • Processes within and between boxes • Ocean zones defined by density differences

7. Surface currents • Move large volumes of water at basin-scale • Transport heat • Work simultaneously with thermohaline circulation • Some independence • Some interaction (complex 3D circulation

8. Wind driven circulation • About 10% of the water is moved by surface currents • Surface currents are primarily driven by the wind and wind friction • Move fast relative to thermohaline circulation (1 to 2 m/s) • Most water moved is above the pycnocline • Reflect global wind patterns and Coriolis effect!

9. Surface circulation • But, surface flow is NOT parallel to wind • Why? • Coriolis force • Ekman flow

10. Coriolis Effect • Earth rotates on its axis 1x/day (CCW with N at top) • Radius of earth = 6371 km • Circumference of earth = 2pr = ~40,000 km • Speed of object at equator is ~40,000 km/d (~1668 km/h) • At 60o, radius is smaller by factor of 2 (cos 60 = 0.5) • So speed is half that at equator (834 km/h) • At 30o speed is 1442 km/h (cos 30 = 0.866)

11. Coriolis force Variations in rotational speed. • Apparent deflection of things moving long distances due to rotation of the earth

12. Coriolis force Consequences of Coriolis deflection. • Apparent deflection of things moving long distances due to rotation of the earth

13. Relative speeds of objects at different radii moving at the same angular speed

15. Inertia keeps objects moving at their original speed • We could see this if viewed from space but we’re moving as well • Has an effect when things move between latitudes

16. Result for wind cells is you go from 2 cell model to 6-cell model

18. Surface Ocean Circulation • Nansen first connected wind with currents • Showed his measurements to Ekman who formulated a mathematical explanation of surface currents

19. Ekman spiral • Wind flows over surface and creates drag on water • Wind driven flow is deflected to right in N hemisphere by Coriolis effect • Water flows at only about 3% of the speed of the driving wind. • Current flows at 45o to the right of the wind direction in the northern hemisphere • But, only the surface feels the wind • Each layer down only feels the layer above so is deflected based on the layer above • Each layer down moves more slowly than the layer above

20. Wind creates a drag on surface waters and successive layers • exert drag on each successive layer below. • Each layer is subject to Coriolis deflection

22. Ekman flow • Wind exerts frictional drag on water causing a thin layer of water to move • Transfer of momentum is not efficient; induced current is about 2% of wind speed • Coriolis force causes water to veer right or left of wind • As the surface layer of water begins to move, it exerts frictional drag on the layer below • And so on, each layer moving slower and deflected relative to the layer above • Produces a pattern of decreased speed with depth and increased angle between flow and wind direction with depth (Ekman spiral)

23. Ekman spiral • Velocity vectors at different depths trace out a spiral around a line perpendicular to the surface • Steady wind induces flow at depth at 90o and 180o or more to wind direction • We say the wind “penetrates” to a depth where flow is 180o to the wind (flow at this depth is about ~4% of flow at surface) • Water above this depth is the Ekman layer • Wind speed, water viscosity and the Coriolis effect all affect the depth of wind penetration • Winds penetrate deeper at low latitudes except right at the equator

24. Flow in Ekman layer • Surface current typically 20-40o to wind direction • By definition, current at base of Ekman layer is 180o to wind direction • Average or net flow of water in Ekman layer is 90o to wind • Average or net flow in Ekman layer is the drift current Wind direction Surface current direction Direction of net Transport within the Ekman layer

25. Ekman flow • Water doesn’t really spiral downward • At some depth water flow will be opposite surface flow and at this depth friction dissipates horizontal flow • Effects of surface wind felt to approximately 100m • The net motion of the water movement, after the sum of the effects of the Ekman spiral is the Ekman transport or flow • In theory, Ekman transport is 90o to the right of the wind in the N hemisphere • In nature, it barely reaches 45o because of the interaction between the Coriolis effect and pressure gradient

27. Surface currents - wind • First order control by predominant wind pattern – friction between atm and surface ocean • More complex in the real world • Position of continents • Ekman transport • Friction plus Coriolis • Pushes water to center of gyres • Regions of convergence and divergence • Geostrophic flow – interaction between pressure gradient associated with Ekman transport/convergence and Coriolis effect • Broad general subtropical gyres

29. Fig. 5-1

30. Surface currents • Moving water “piles up” in the direction the wind is blowing • Continents and land masses also deflect flow in E-W direction • Water pressure increases where its piled up so tries to slide back along a pressure gradient • Coriolis effect intervenes deflecting currents to the right of wind direction (in N hemisphere)

33. Ocean gyres • Circular flow around the periphery of an ocean basin • This flow is often broken down into interconnected currents (e.g., North Atlantic gyre) • Why doesn’t flow spiral toward center because of Coriolis force?

34. Pressure gradients develop in the ocean because the sea surface is warped into broad mounds and depressions with a relief of about one meter. • Mounds on the ocean’s surface are caused by converging currents, places where water sinks. • Depressions on the ocean;s surface are caused by diverging currents, places from where water rises. • Water flowing down pressure gradients on the ocean’s irregular surface are deflected by the Coriolis effect. The amount of deflection is a function of latitude and current speed.

35. Downwelling of water Creation of geostrophic currents as a result of the pressure gradient Upwelling of deep water to replace surface water in areas of divergence - e.g., along the equator Fig. 5-4

36. In the center of gyres water piles up (converges) upper ~100 m Fig. 5-3 (a) Ekman spiral Fig. 5-3 (b) Ekman transport

37. Water piles up in the direction of flow so piles up in middle of gyres due to Ekman transport and creates a pressure gradient in the opposite direction.

38. Pressure gradient Pressure gradient

39. Ocean gyres • Circular flow around the periphery of an ocean basin • Westerly-driven ocean currents in the trade winds, easterly-driven ocean currents in the Westerlies and deflection of the ocean currents by the continents result in a circular current, called a gyre. • This flow is often broken down into interconnected currents (e.g., North Atlantic gyre)

43. Fig. 5-2

44. Gyre circulation • To deflect further than 45o, water would have to move uphill against a pressure gradient • To deflect away from the pressure gradient would defy the Coriolis effect • So water circulates clockwise around the gyre balanced between the pressure gradient in the center of the gyre and the Coriolis deflection • - Coriolis deflection versus gravity • Higher sea surface height at the center of gyres and maintained by wind energy

45. Geostrophic gyres/flow • Gyres in balance between pressure gradient and Coriolis effect • Their currents are geostrophic currents • Because of wind patterns and positions of continents, major gyres are largely independent of each other in each hemisphere. • Six great surface current circuits in the world, one is technically not a geostrophic gyre • The Antarctic circumpolar current (west wind drift) moves eastward around Antarctica driven by westerly winds and is never deflected by a continent

47. Sea surface height Hill is offset to the western side of basins because of western intensification

48. Western intensification • Earth turns CCW • Water piles up against land on west side of basins

49. Less (or no) Coriolis force at equator so water doesn’t turn until it hits and obstacle (land) • More Coriolis at mid and high latitudes so current turns sooner Geostrophic Flow Around the North Atlantic Ocean

50. Sea surface height