Surface circulation

1. Surface circulation Wind driven, Coriolis modified Ekman and geostrophic flow

2. Surface currents - patterns • Similar in all basins • At low latitudes, have large, “closed” gyres • Gyres elongated in the E-W direction • Gyres centered on the subtropics (~30oN or S) • West-directed flow at N and S equatorial currents • East-directed flow ~ 45oN and S • N-S directed flow at eastern and western boundary currents

3. Surface currents - patterns • West-directed flow driven by tradewinds (coming from N or S-east • East-moving equatorial countercurrents • Western boundary currents are distinct, narrow (< 100km), swift (>100 km/day) and deep (2 km) • Eastern boundary currents are broad (>1000 km), weak (~10s km/day) and shallow (~500 m) • Have smaller, less developed polar gyres in N • Have circumpolar “gyre” in the S

4. Transverse currents • E-W currents driven by the trade winds (easterlies) and mid-latitude westerlies • Link the boundary currents • Equatorial currents • Moderately shallow and broad • Pile up water on west side of basin (W Atl is 12 cm [8”] higher than Pac; W Pac is 1 m higher than E Pac) • Eastward flowing currents at mid-latitudes are weaker (wider and slower) than equatorial currents • Differences in land mass distribution in N and S hemispheres affects flow

5. Change in productivity rates affects atm CO2? Assume these fluxes are balanced Change in burial rates affects atm CO2? Why is this important? Processes in surface, wind-driven layers are different but connected to processes in deep waters. Fig. 8-9

6. Short-term Long term (organic) Long term (inorganic/tectonic)

7. Link with short term C cycle In surface oceans “adds” back CO2 Onset of modern plate tectonics “turns this on”

8. Thermohaline Circulation

9. Thermohaline circulation • Vertical water movement • Driven by density differences (can be very small) • Remember temperature and salinity diagrams and the properties of water • Temperature and salinity profiles (with depth) • Salty water is denser than fresh water • Cold water is denser than warm water • Density gradients with latitude (due to temperature differences of surface waters) • Polar water has the most uniform density (weakest pycnocline) so is least stable

10. Fig. 5-6

11. Polar water column is • least stable; most • uniform density • weakest pycnocline • Surface is cold • Deep water is cold • everywhere • Easiest to rearrange • Formation of ice • excludes salt • Seawater freezes at • –2oC

12. Cooling Cycle: 1. August 2. September 3. October 4. November 5. January Warming Cycle: 1. March 2. May 3. June 4. August

13. Temp. sea water

16. Wind

17. Most of the ocean is cold Temperature degree C

20. Thermohaline circulation • As for the atmosphere, there are convergence and divergence zones where water masses collide or diverge • Important for global heat balance • Deep circulation and basin exchange of water, material, and heat

21. Thermohaline circulation • Deep circulation is driven by density differences • Horizontal movement along density surfaces • Movement is very slow (0.1 m/s) • Three layer ocean • surface mixed layer • Pycnocline • Deep water • Deep water formed at 2 places – N Atlantic and Weddell Sea (Antarctica) • Connection between surface and deep water • Diffusion (slow and along density gradients) • Mixing (e.g., storms) • Upwelling (polar, equatorial and coastal)

22. Deep circulation is like a conveyer belt that moves heat and water

23. Water masses • Possess identifiable properties • Don’t mix easily – flow above or beneath each other • Surface water – to about 200 m • Central water – to bottom of main thermocline • Intermediate water – to about 1500 m • Deep water – to about 4000 m (not the bottom) • Bottom water – in contact with seafloor • Retain characteristics when the mass was formed at the surface (heating/cooling, evaporation/dilution)

24. Formation of deep water • Antarctic bottom water – densest water in the ocean – S (34.65%o), T (-0.5oC), dens (1.0279) • Weddell Sea in winter – when ice freezes and get brines • Sinks and creeps North • Pacific and Atlantic – 100’s to 1000’s of years to get to northern basin • North Atlantic deep water • Formed in the Arctic but escapes only through channels • Warm, salty water chills (heat is transferred to air) • Atlantic and Pacific deep water is less dense than Antarctic bottom water

26. Mediterranean deep water • Salty water (38 %o) • Underlies central water mass in Atlantic • Warmer than other deep water so not as dense

28. Different combinations of temp and salinity can make water of the same density • Isopycnal is a line of constant density • Mixing of water masses can increase the density Increasing density

31. Age of a water mass • Oxygen and C isotopes • Water picks up oxygen and CO2 only at surface by exchange with atmosphere • Loses oxygen as it ages (respiration, reaction with rocks); 14C decays as it ages • Water masses mix slowly • Antarctic bottom water in the Pacific retains its character for up to 1600 years

32. Fig. 8-11

33. -70‰ ∆14C -230‰ -110‰ -210‰ -150‰ -170‰ -150‰ -170‰ This diagram represents the flow at a depth of 4000 m; the strange-looking continent/ocean configuration is what we would obtain if the oceans were drained to this depth. (After W.S. Broecker and T.-S. Peng, Tracers in the Sea, New York: Eldigio Press, 1982, Figure 1-12.)

34. Water residence time • Bottom currents are slow • Antarctic bottom water ~1600 years • Other bottom water – 200-300 years (time it takes to rise to the surface) • Fast bottom currents on bottom around objects • Surface currents are faster • Surface water – on the order of years • N Atlantic gyre may take about 1 year to complete a circuit

35. Deep water circulation • Flow may be slow but still modified by Coriolis

36. Convergence zones • Two water masses meet • Usually one will go under the other (density) • If the same density, may mix to create a new and denser water mass (remember T-S diagram) – this is caballing • Formation of N Atlantic intermediate water, Antarctic intermediate water and Antarctic bottom water produced by mixing

37. S-curve tracks density • with depth • Points a and b on an • Isopycnal so are the • same density, despite • different temperatures • and salinities • If the two water masses • mix, will result in • denser water!

38. Convergence zones • Areas of downwelling • Antarctic convergence zone at 50-60oS • South of this is the Antarctic Circumpolar Current • Subtropical convergence zone (40-50oS) • N boundary of Subantarctic Surface Water • Equatorward of subtropical convergence zone is Central Water which is the warmest and saltiest water • Subtropical convergence (45-60oN) • Arctic convergence – ill-defined because of land mass in the N

40. Divergence zones • Areas of upwelling • Tropical divergence • Antarctic divergence

41. Upwelling • Sinking water must be offset by upwelling of water • Water sinks in a localized area, relatively rapidly • Water rises gradually over larger areas • When water moves to surface, must be transported to a pole to sink again • Diffuse upwelling maintains a permanent thermocline (1 cm/d)

42. Maintained by diffuse upwelling

44. Main thermocline

45. Thermohaline flow • Sinking of surface water most pronounced in the North Atlantic • Water moves at great depths toward Southern hemisphere and wells up into the surface in the Indian and Pacific Oceans (~1000 years) • Slow circulation that crosses hemispheres superimposed on rapid flow of surface water in gyres • Some heat travels from S Pacific, across Indian Ocean and into the Atlantic • Flow distributes gases, solids, nutrients and organisms among ocean basins

46. Fig. 5-12

49. Fig. 8-9

50. The Redfield Equation 106 CO2 + 16 NO3- + HPO42- + 122 H2O  (CH2O)106(NH3)16(H3PO4) + 138 O2 Can also develop similar ratios for trace elements required for growth (e.g., Fe, Mn, Zn, etc.)