1. The Circulation of the Oceans
Geos 110 Lectures: Earth System Science
Chapter 5: Kump et al 3rd ed.
Dr. Tark Hamilton, Camosun College

2. Ocean Water is a Special Fluid
Dense & Viscous but variable due to salinity & temperature
Density also varies with suspended sediment load
High thermal mass & High heat capacity
While the Ocean is the “burner” under the Troposphere, it is mainly heated from above
90% of Sunlight/heat is absorbed in the top 100 m
Large insolation changes achieve minor T°C change
The sea is thermally and density stratified, convecting as a 2 layer system
This is very slow at depth ~ several thousand years
Biology matters, especially for ocean chemistry

3. Winds & Surface Currents: Sea in a Box
 Continent
Easterly Equatorial Trades cause westwards flow in Tropics
Continents deflect flow to North and South in Gyres aided by Westerlies at mid latitudes
Continent 
Winds have mass, momentum and vector directions
Wind Drift Currents: Friction at the sea surface & excites wave motion & lateral flow causing convection & advection in the sea

4. Winds and Surface Currents: Sea in a Box
Warm Iceland/Scotland
Cold Labrador 
Kuroshio Current
Gulf Stream
 Continent
Continent 
Wind Drift Currents confined to upper m
Except big turbulent gyres: Gulf stream, Kuroshio current 1-2 km deep and 100’s of km wide
Coriolis force plays a role: Clockwise N, Anti-Clock S

5. Major Ocean Surface Currents
Westwards Equatorial flow and gyres as predicted
Reality is similar but more complex: warm & cold
Esp. Polar Regions, Northern Indian Ocean, Straits

6. Current Convergence & Divergence
Water does not generally pile up along the East coast of land in tropics
Mid ocean pile up: wind driven currents, rotation & friction all contribute
Nansen 1890’s noted drift at 20-40° to the right of the wind!
Ekman Spiral: viscous shear, thermal dissipation & coriolis, greater angular deflection with depth but less speed, currents at 100 m depth can reverse!
Eckman Transport: net advection at 90° down wind

7. The Ekman Transport & Spiral
Wind Friction Currents & Earth’s rotation
The shallow flow drags & shears the water just below it
Heat losses make each deeper layer flow slower
Each layer still feels Coriolis Force

8. The Ekman Transport & Spiral
In Southern Hemisphere, this reverses as Coriolis force and gyres are Widdershins!
W/strong wind, surface current is <45° to wind
Flow slows & reverses by ~100 m depth
Net transport is 90° to wind, into gyre!

9. Divergence  & Convergence
In Equatorial North Atlantic, NE Trades & Ekman Transport to right of wind deflects water North!
This Produces the North Equatorial Current
Conversely, SE Trades in Equatorial South Atlantic & Ekman Transport to left deflects water South!
This Produces the South Equatorial Current
W Mexico & W Africa have significant divergence
Also diverge off W Ecuador & W South Africa

10. Major Ocean Surface Currents: Divergence
Westwards Equatorial flow and gyres as predicted
Reality is similar but more complex: warm & cold

11. Upwelling  & Downwelling
Where Divergence occurs, the sea thins and sea level drops ~2 m below the GEOID (geopotential surface)
This causes upwelling of cold, micro-nutrient rich deep water: marine biology usually thrives here
Where Convergence occurs, the sea thickens and piles up ~2 m above the GEOID
This causes downwelling of warm, acidic, dust and carbon rich plankton bearing surface waters creating deep sea drifts (linear sediment deposits)

12. Major Ocean Surface Currents: Convergence
Westwards Equatorial flow and gyres as predicted
Reality is similar but more complex: warm & cold

13. Divergence  & Convergence Upwelling  & Downwelling

14. Sea Surface Relief & Geostrophic Flow
The pressure force of a gyre 50 cm high in the middle acts to counteract the inwards directed Ekman transport. The net result is right handed circular flow (Gulf Stream) in N. Atlantic and Kuroshio in NW Pacific! There are similar features set up in the southern oceans between the South Equatorial current and the West Wind Drift.
In practice since the Ekman force is a little less than 90°, there is a build up and downwelling in the center of the gyre.
Gradients are a few m over 100 to 10,000 km!
Slopes of 1 in create outwards Pressure▼
This results in circular geostrophic current ↑‘gyre,↓

15. Sea Surface Relief & Geostrophic Flow
Northern Hemisphere wind stress currents set up the sub-tropical gyres & geostrophic currents

16. Sea Surface Relief & Geostrophic Flow
Pressure gradient force opposes Coriolis force for net outwards deflection

17. Sea Surface Relief & Geostrophic Flow
Pressure gradient force opposes Coriolis force for net outwards deflection tangential & clockwise around the gyre in the Northern Hemisphere

18. Major Ocean Boundary Current Gyres
Westwards Equatorial flow & Subtropical gyres
Clockwise Northern & Anti-clockwise Southern

19. >20°C, 50-70 km wide & 3-10 km/hr
Up to 1 km deep G G
<10°C, 1000 km wide & <4 km/hr
< 500m deep
Canary Current
Boundary Currents are Asymmetric - The Gulf Stream is a Fast Western Boundary Current G G

20. Vorticity: The Tendency for Fluid to Rotate under the influence of Body Forces
Planetary vorticity increases towards the poles due to rotation = Coriolis Force
Relative vorticity
Cyclonic Low P wind shear +
AntiCyclonic Hi P -negative
Ocean current shear in gradients parallel to coasts
Positive vorticity  Counterclockwise (from above)
Negative vorticity  Clockwise

21. Current Shear Producing + or - Vortex
+ Positive Vorticity when current increased to Right
- Negative Vorticity when current increases to Left
Whirlpools & Rip Tides tend to take you offshore!

22. So why are Eastern Boundary Currents Weaker than Western Ones?
Speeding
Vorticity makes the gyres themselves asymmetric.
Wind over water is driven by heat picked up.
Slowing
The divergence to the east & Slowing Westerlies
Equator bound Canary Current

23. Ocean Circulation & Sea Surface T°CW C C T
The North Atlantic Drift also makes Europe’s Ice Cap Grow first as Earth enters an Ice Age!
The Ocean Heat has a long memory and influences climate fluctuations on annual to decadal time scales!
The Labrador Current - Cold outflow from Arctic
North Atlantic Drift – Warms Iceland & Norway

24. Ocean Circulation & Sea Surface T°CW
Benguela current
Humbolt current C C
The Cold Humbolt Current – makes the Atacama
The Cold Benguela Current – makes the Namib Erg

25. Ocean Circulation & Sea Surface T°C
E-W Circulation in Equatorial Troposphere
Where does Rainfall happen the most?
With respect to the atmospheric circulation?
With respect to sea versus land?

26. Ocean Surface Layer – Tropical Pacific La Niña every 2-10 years (enhanced normal)
Strong Easterlies make for upwelling in E Pacific
Colder December-February off Ecuador & Peru

27. ENSO – El Niño Southern Oscillation Pattern of Easterlies Breaks Down
Reverse of Trades: no upwelling in E Pacific
Warmer December-February off Ecuador & Peru
Rains, high tides, coastal flooding

28. Which Comes 1st the Chick or the Egg
Positive feedback loop: Easterlies make warm west
Warm west Pacific makes strong Easterlies!

29. El Niño versus La Niña Correlated Circulation Events (1997-1998 ENSO)
Sea Surface Temperature Maps (& 1989 La Niña)

30. ENSO - Atmospheric Circulation
Shift of Warm water to Central Pacific
Loss of Upwelling & micronutrients in E. Pacific
Leads to massive die back of marine life & seabirds
Droughts/Famines in Africa, Australia & Indonesia

31. Ocean Circulation & Sea Surface T°C
Normal E-W Circulation in Equatorial Troposphere
Rains in South American jungle not Andes
Rains in central Africa
Rains in northern Australia

32. SOI Index – Sea Level Pressures
Negative SOI at Tahiti vs Darwin in warm ENSO
Note extreme ENSO in &

33. Consequences of 82-3, 97-8 ENSO Events
Ecuador & Peru: floods, landslides, 600 dead
crop & property losses > $400M
Guayaquil X 20 normal rainfall
Major erosion, soil loss & sediment transport
Indonesia crop failure & famine
E. Australia worst drought of 100 years
Livestock slaughter
11,000 tons of dust on Melbourne & worst bushfires
Tahiti got 100 year Typhoon & 6 others in 5 months
Flooding of Mississippi & California landslides

34. ENSO Anomalies in Rainfall & T°C
Consistent Pattern in Tropics but variable intensity
Highly Variable Mid Latitude Effects Wet 82Dry76

35. Salinity = g solute per Kg solvent i.e. per mil Seas 35, 1.035
Anions: Cl- > SO4 -2 > HCO3- > Br- > Boric>F-
Cations: Na+ > Mg+2 > Ca+2 > K+ > Sr+2
Iodide is less than Fluoride
Soluble ions come from chemical weathering of common rock forming minerals.

36. The Salt in the Seas Total salt content is 5 x 1019 kg
John Joly assumed this accululated since Earth formed and got Ma
Salts are removed by submarine weathering, biosedimentation, subduction, uplift and evaporation
The modern flux is 4 x 1012 kg/day
Our estimate would be: 5 x 1019 kg / 4 x 1012 kg/day
or 13 x 106 yr
This is not “The Age of the Earth” but the residence time for salts in the ocean! ~ 1/400 of Earth’s age!

37. Thermohaline Structure & Circulation Thermal, Salinity & Density Structure
Shallow water: warmer, fresher & lighter
Deep water is pretty uniformly dense & stable
Deep density currents are slow
Vertical convection is limited, 2 layer convection

38. Water is Special & so is Seawater
As per graphs:
density & salinity decrease as T°C increases
Pure H2O max density at 4°C
Above 4° increase in T°C means decrease in density
But density also decreases below 4°C down to 0°C
For saline H2O max density at 2°C for 10 per mil
& at the Freezing point for 24.6 per mil
The freezing point drops as salinity increases, but density decreases somewhat, still defining freezing

39. Thermohaline Structure & Circulation Thermal, Salinity & Density Structure
Surface zone/Mixed Layer = low density in upper m
Maximum interaction with atmosphere: energy, kinetics, friction, evaporation, concentration, dilution
Thermal: absorption of solar radiation, emission of long wave IR

40. Thermohaline Structure & Circulation Thermal, Salinity & Density Structure
Rapid increase in density with depth ~1 km = pycnocline
Transition zone is Pycnocline Zone
Where density increase is driven by salinity increase, this is Halocline
Most regions the density increase is driven by the temperature decrease thus the Thermocline
Either structure stops mixing and is stable despite seasons

41. Thermohaline Structure & Circulation Thermal, Salinity & Density Structure
Deep Ocean has only slight increase in salinity with depth while temperature continues to decrease slightly for a net constant salinity
Bottom water is the densest & slight lateral salinity gradients drive deep circulation across isopycnals like isobars hi to low in atmosphere
Where dense cold or salty water is produced, the deep ocean is fed
Polar ice margins -1.9°C dense water + salts excluded by freezing

42. Thermohaline Structure & Circulation
The thermocline is evident in the tropics
At high latitude, deep water extends to the surface

43. Thermohaline Structure & Circulation
The thermocline is evident in the tropics
At high latitude, deep water extends to the surface

44. Thermohaline Structure & Circulation
Salinity structure is more complex than thermal
Salinity maxima at surface in tropics (Why?) & deep

45. Thermohaline Structure & Circulation
Salinity structure is more complex than thermal
Salinity maxima at surface in tropics (Why?) & deep
Which ocean has greater salinity layering?

46. Circumpolar Flow & Weddell Sea
Wintertime Ice Factory – Antarctic Outflow Winds
Ice forms at Shore and disperses to north
Thick ice shelf at Weddell Sea from W. Ant. Cap.
Antarctic Bottom Water forms here & sinks

47. 4000 M Ocean Depth/Configuration
NADW forms off Greenland, higher salinities W AT
AABW forms off Weddell Sea
These dominate flow at 4 km depth

49. Δ14C Values: Low = young Hi = old
The difference value is referenced to modern day ocean surface waters.
Off of BC, the reservoir age for modern sea water is 400 years
Not just atmospheric CO2
The reference for the difference value is the modern carbon content of upper oceans.
Here off of BC, the ocean carbon reservoir today gives a radiocarbon age of 400 years
-40 permil off Greenland NADW young
-140 permil off Weddell Sea Antarctica AABW
-220 permil off Bering Sea – N. Pacific = OLD

50. Radioactive Decay – 14 C
Biota take up Carbon including small amounts of radiocarbon as they live and grow.
They stay in equilibrium with their surface reservoir.
After they die they become decay clocks
14N  14C in upper atmosphere by cosmic rays Oxidation to CO2 deposits this in lower atmosphere
14C  14N by β emission at T1/2 = 5730 years

51. Radioactive Decay – 14 C
You are what you eat!
We need to correct 14C dates for reservoir effects!
Tree ring analysis & counts.
Coral annual growth bands.
Living biota: clams, forams etc.
BC Reservoirs: Wood ~400 year
Clams ~6000 year correction
Planktonic Forams ~400 year
Ocean returns Carbon to deeps
14N  14C in upper atmosphere by cosmic rays Oxidation to CO2 deposits this in lower atmosphere
14C  14N by β emission at T1/2 = 5730 years

52. Thermohaline Structure & Circulation
NADW
AABW
2 big Deep Water Factories: NADW & AABW
Return flow is slow over whole ocean pycnocline
& fast in areas of upwelling: W. Broecker LDGO

53. Thermohaline Structure & Circulation
AABW isopycnals: observed & modelled
Note the role of Ice shelves in making dense water

54. Nimbus 7 Coastal Zone Scanner detects pigments from Clorophyll compounds.
Light Colours = greatest plankton & plants
North Atlantic North Pacific
Bahamas
Galapagos
South Atlantic
South Indian
Dead biota recycle nutrients to deep ocean

55. False-colour satellite image of the world's oceans, showing the distribution of phytoplankton in the surface water. The colours represent varying phytoplankton densities from red (most dense) through yellow, green and blue to violet (least dense). Grey areas are data gaps. This image is an average of distributions for October- December 1979 and shows the seasonal build-up of phytoplankton along the equator, especially off the western coasts of Africa & South America. The image was produced from data acquired by the Coastal Zone Colour Scanner, one of the instruments on NASA's Nimbus-7 research satellite.
False-colour satellite image of the world's oceans, showing the distribution of phytoplankton in the surface water. The colours represent varying phytoplankton densities from red (most dense) through yellow, green and blue to violet (least dense). Grey areas are data gaps. This image is an average of distributions for October- December 1979 and shows the seasonal build-up of phytoplankton along the equator, especially off the western coasts of Africa & South America. The image was produced from data acquired by the Coastal Zone Colour Scanner, one of the instruments on NASA's Nimbus-7 research satellite. Dr. Gene Feldman, NASA GSFC Photo Library

56. 1000 m Depth T°C & Salinity ◦/◦◦
Mediterranean is shallow, warm & salty through evaporation & dissolution of Miocene salt beds
See how it makes the halocline
& sets the floor for the Canary Current

57. Thermohaline Structure & Circulation
As NADW & AABW from and sink near poles
Warm water flows poleward at the surface
This is negative feedback in ice ages, buffering climate

58. The Moist End of Chapter 5!