1. Circulation of the Ocean

2. Ocean Currents
The mass movement of water, either vertically or horizontally, in the ocean
Surface currents are wind-driven movements of water at or near the surface of the ocean
Thermohaline currents are the slow, deep currents that affect the vast bulk of water beneath the pycnocline
Named this because they depend on density differences caused by variations in water’s temperature and salinity

3. Surface Currents
About 10% of the water in the ocean is involved in surface currents
A surface current includes the water that is flowing horizontally in the uppermost 1,300 ft
Most surface currents move water above the pycnocline
These currents are driven mainly by wind friction
Most of the wind energy is concentrated in the trade winds and westerlies
Due to the forces of gravity, the Coriolis effect, solar energy, and solar winds, water often moves in a circular pattern called a gyre.

4. Winds Cause Surface Currents

5. Surface Currents and the Coriolis Effect
Gyres in the N. Hemisphere move clockwise
Gyres in the S. Hemisphere move counterclockwise

6. Gyres
Surface currents flow around an ocean basin to create a gyre, which is typically made up of multiple surface currents
Gyres flow continuously without obvious changes between currents
The end of one current blends into the start of the next current
Each current has distinct characteristics which is why each is considered a separate current
Geostrophic gyres are gyres in balance between the pressure gradient and the Coriolis effect
The geostrophic gyres are independent of one another due to wind patterns and the positions of the continents

7. Gyres
There are 6 great surface circuits, but only 5 of these are considered geostrophic gyres.
N. Atlantic
S. Atlantic
N. Pacific
S. Pacific
Indian Ocean
The 6th circuit does not travel around an ocean basin (it travels around a continent) so it’s not considered a gyre
Antarctic Circumpolar Current (aka West Wind Drift)
Flows endlessly around Antarctica and is driven by the Westerlies

9. Western Boundary Currents
Found on the western edge of each gyre (off the east coast of a continent)
Fastest, deepest, and narrowest currents
Move warm water from the equator to the poles
5 of them:
Gulf Stream (N. Atlantic)
Kuroshio Current (N. Pacific)
Brazil Current (S. Atlantic)
Agulhas Current (Indian)
East Australian Current (S. Pacific)

10. Eastern Boundary Currents
Found at the eastern edge of ocean basins (off west coast of continents)
Shallow, slow moving, and broad
Carry cold water from poles toward equator
There are 5:
Canary Current (N. Atlantic)
Benguela Current (S. Atlantic)
California Current (N. Pacific)
West Australian Current (Indian)
Peru Current (S. Pacific)

11. Transverse Boundary Currents
Most of the power for currents is derived from the trade winds at the fringes of tropics and from mid-latitude Westerlies
The stress of winds on the ocean in these bands gives rise to the transverse currents
These currents flow from east to west and west to east, linking the eastern and western boundary currents on both the north and south ends of each gyre.

12. Transverse Currents

13. Ekman Spiral
So in a gyre, why does water flow around the periphery of the ocean basin instead of spiraling to the center?
Not only does the Coriolis play a role in this movement, so do wind patterns
When driven by the wind, the topmost layer of ocean water in the N. Hemisphere flows at about 45˚ to the right of the wind direction.

15. Ekman Spiral What about the water below the top layer?
The lower layers can’t “feel” the wind at the surface; However, it “feels” only the movement of the water immediately above
The deeper layer of water moves at an angle to the right of the overlying water (due to Coriolis)
The same thing happens in the layer below that and so on and so on
This continues to happen until a depth of about 100m
This process is known as Ekman Spiral

16. Ekman Spiral
Even though its known as the Ekman Spiral, the water itself is not spiraling.
The term spiral refers to what you would see on a diagram

17. Ekman Spiral

18. Ekman Transport
Ekman Transport = the net motion of water down to about 100m after the allowance for the summed affects of Ekman spiral
In theory, the direction of Ekman transport is 90˚ to the right of the wind direction in the N. Hemisphere and 90˚ to the left in the S. Hemisphere
In actuality, the Ekman transport in gyres is 45˚ or less
This difference is caused by the interaction of the Coriolis effect and the pressure gradient

19. Ekman Transport
So, why doesn’t the water spiral to the center of each ocean basin?
There are hills of water at the center of each ocean basin
In order for the water to spiral toward the center, it would have to “climb” this hill and go against the gravity

21. Countercurrents and Undercurrents
Usually found near equatorial currents
Flow on the surface in the opposite direction of the main current
Caused by the lack of persistent winds at the equator
Undercurrents
A countercurrent that occurs beneath the surface currents
Found under most major currents

22. Surface Currents and Climate
Surface currents (along with wind) distribute tropical heat worldwide
Warm water flows to higher latitudes, transfers heat to the air and cools, moves back to low latitudes, and absorbs heat again
The climate along a coastline greatly depends on the nearby surface current
A warm current typically leads to a warm and humid climate (Florida and any of the Gulf states)
A cold current typically leads to cooler temperatures with lower humidity (California and the other west coast states)

23. Surface Currents and Climate

24. Vertical Movement of Water
The wind-driven horizontal movement of water can sometimes cause vertical movement of water
The movement is called wind-induced vertical circulation
2 types
Upwelling
Downwelling

25. Upwelling Upwelling is the upward movement of water
The water from within and below the pycnocline is often rich in the nutrients needed by marine organisms for growth
Coastal upwelling - friction of wind blowing along the ocean surface parallel to the shore causes water to begin moving, the Coriolis effect deflects it to the right and the resulting Ekman transport moves it off shore
Fuels primary productivity (photosynthesis)
Equatorial upwelling - water moving in the currents on either side of the equator is deflected slightly poleward and replaced by deeper water

26. Coastal Upwelling

27. Coastal Upwelling (in red)

28. Equatorial Upwelling

29. Downwelling
Water driven toward a coastline will be forced downward, returning seaward along the continental shelf
Helps supply the deeper ocean with dissolved gases and nutrients
Moves oxygen gas from the photic zone to the aphotic zone, which assists in distribution of living organisms
Has no direct effect on climate or productivity of the coast

30. Downwelling

31. El Niño vs Normal Conditions during a normal year (a non-El Nino yr):
Surface winds across most of the tropical Pacific normally move from east to west
Trade winds blow from the normally high-pressure area over the eastern Pacific (S. America) to the normally stable low-pressure area over the western Pacific (N. Australia)
Strong upwelling currents in the western Pacific
Nutrient rich waters off the coast – leads to great biological diversity
Sometimes these conditions are reversed for unknown reasons
Occurs at irregular intervals of roughly 3-8 yrs

32. El Niño - The Conditions
Conditions during an El Nino year:
Pressure systems reverse
High pressure builds in the western Pacific; low pressure dominates the eastern Pacific
Winds across the tropical Pacific reverse direction and blow from west to east; the trade winds either weaken or reverse
The change in atmospheric pressure is called the Southern Oscillation

33. El Niño – What are the Effects?
The reversed winds move the warmest waters from the western Pacific to the eastern Pacific
The eastern Pacific is greatly affected:
Disrupts upwelling which causes a change in nutrients found in the waters, which affects the animals living in the area – they either die or migrate
Causes high precipitation in normally dry areas
Coastal storms are intensified
Sea levels can rise up to 8 inches
Water temp can increase up to 13˚F
How is the western Pacific affected?
Long periods of drought
Crops don’t grow; people starve

34. Non El Niño Year El Niño Year
Animations of Normal vs El Nino vs La Nina:

35. La Niña Normal circulation returns with surprising strength
Produces strong currents, powerful upwelling; chilly and stormy conditions along S. American coastline
The powerful return that causes colder than normal events is called La Nina.
As conditions in the eastern Pacific cool off, the ocean in the western Pacific warms rapidly
Trade winds become stronger than normal resulting in above average upwelling in the eastern Pacific
Increased upwelling pushes cold water further out into the Pacific, pushing the warm waters closer to the edge of the western Pacific

36. La Niña

37. Thermohaline Circulation
Thermohaline circulation is the movement of water due to differences in density
This type of circulation is mainly found below the pycnocline in the deep zone and moves water vertically and horizontally within this zone
This process is responsible for the large-scale vertical movement of ocean water and the circulation of the global ocean as a whole

38. Thermohaline Circulation

39. Thermohaline Circulation

40. Water Masses
Each water mass has specific temperature and salinity characteristics
These differences prevent water masses from easily mixing with each other
Water masses usually flow above or beneath each other
These differences also lead to density stratification of the ocean

41. Water Masses
Water masses are named according to their relative position
In temperate and tropical waters, there are 5 layers:
Surface water – goes to a depth of about 200 meters
Central water – from approx. 200m down to the bottom of the thermocline
Intermediate water – from bottom of thermocline down to about 1500m
Deep water – water below the intermediate water, but not in contact with the bottom (to a depth of approx. 4000m)
Bottom water – water in contact with the ocean floor
These layers don’t exist near the poles.
Why?

42. AAIW = Antarctic Intermediate Water. AABW = Antarctic Bottom Water
AAIW = Antarctic Intermediate Water AABW = Antarctic Bottom Water NADW = N. Atlantic Deep Water

44. Studying Currents – Float Method
Depends on the movement of a drift bottle or other free- floating object
Drift bottles and drift cards – for surface currents
Especially useful in determining coastal circulation
Do not record the path taken
Drift buoys – for surface currents
Path can be tracked for more precise circulation patterns
Tracked by radar or satellite
Swallow float – for intermediate currents
Detects the drift of intermediate water masses
Descends to a specific density
Emits sonar “pings” as it drifts so that it can be tracked

45. Drift Bottles

46. Drift Buoys

47. Swallow Float

48. Studying Currents – Flow Method
Flow meters measure the speed and direction of a current from a fixed or stationary position
Ekman flow meter
Uses rotating vanes to measure speed and a recording compass to measure direction

49. Studying Currents – Slocum Glider
A glider that “flies” smoothly up and down through the water column
Collects data on temperature, salinity, density, direction and speed of deep water currents
Powered by gravity and buoyancy
Reach the surface periodically to transmit data to satellites before returning to the depths of the sea to collect more data
Can spend years at sea before being collected

50. Slocum Glider