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Chapter VII: Ocean Circulation

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

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

  3. 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 Measuring surface currents Figure 7B

  4. 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 Ocean currents

  5. 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 Deep water masses and currents

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

  7. 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 Identification of deep water masses 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 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. Ekman spiral: Wind Driven (τ) Figure 7-6

  12. 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! Ekman 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.

  16. Convergence (Divergence) across a mid ocean gyre

  17. 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

  18. 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)

  19. An example of conservation of vorticity when H stays constant 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. North Pole (High planetary Vorticity f) Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ )=0, ξ must be < 0. The water begins to spin. A parcel of water moves off the equator its vorticity on the equator (f+ ξ)=0. Equator (Zero planetary Vorticity f)

  20. 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 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. A parcel of water moves east (constant latitude) in N.Hemis. Ocean Surface What happens when the parcel leaves the bump? H H Ocean bottom Bump in bottom

  21. 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 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. A parcel of water moves east (constant latitude) in N.Hemis. Ocean Surface H H H Ocean bottom Bump in bottom

  22. 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 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. A parcel of water moves east (constant latitude) in N.Hemis. North Parcel Moves Equatorward From ABOVE H Bump in bottom H South

  23. 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

  24. 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! Geostrophic Balance Coriolis pushes water to right(left). Piles up water. current Sealevel Pressure force current pressure coriolis

  25. 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 Geostrophic Balance Figure 7-7

  26. 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

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

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

  29. 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). A parcel of water moves into an area of downwelling. It becomes shorter (and fatter). f/H must be conserved! Ekman Convergence Ocean Surface Mixed Layer 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. H H Ocean bottom

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

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

  32. 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 Geostrophic Balance Figure 7-7

  33. Sea Surface Height and Mean Geostrophic Ocean Circulation

  34. 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 25o) 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 Current gyres

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

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

  37. 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. Western intensification of subtropical gyres

  38. Back to our example of conservation of vorticity when H stays constant 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. Remember this example? As the western boundary current returns north, this should happen, but it does not. Why? North Pole (High planetary Vorticity f) Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ)=0, ξ must be < 0. The water begins to spin. A parcel of water moves off the equator its vorticity on the equator (f+ ξ)=0. Equator (Zero planetary Vorticity f)

  39. 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. North Pole (High planetary Vorticity f) Parcel wants to spin Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ)=0, ξ must be < 0. The water begins to spin. But can’t due to friction A parcel of water moves off the equator its vorticity on the equator (f+ ξ)=0. Equator (Zero planetary Vorticity f)

  40. Wind-driven surface currents Figure 7-4

  41. 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 Upwelling and downwelling

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

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

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

  45. Antarctic surface circulation Figure 7-13

  46. Ocean surface currentsWhat Currents do you need to know?

  47. 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. The Gulf Stream and sea surface temperatures Figure 7-16

  48. Flows are typically unstable; they meander