1. Ocean Circulation
At 50°N, you’ll find both polar bears and palm trees….
Polar Bear Provincial Park; Ontario, Canada
Scilly Isles; Britain

2. 50°N latitude

3. Ocean Circulation
Why the great difference in climate between interior Canada and the British Isles?
Cold air flowing over Canada’s cold interior loses heat, but is warmed when it reaches the Atlantic Ocean and the Gulf Stream
The Gulf Stream is a current, a moving mass of water driven by wind and differences in water density

4. Ocean Circulation There are 2 major types of currents:
Surface currents are wind-driven movements of water at or near the ocean’s surface (uppermost 400 meters of ocean); involve ~10% of the world’s ocean water
Thermohaline currents are slow, deep currents that arise from density differences caused by variations in water’s temperature and salinity

5. Surface currents
Surface currents move water horizontally, and are primarily driven by winds
Waves on the sea surface transfer energy from the moving air to the water by friction; the water flowing beneath the wind forms a surface current
Only 2% of wind speed is transferred to ocean current (resulting current is 2% of wind speed)
Water literally “piles up” in the direction of the wind and gravity pulls the water down slope

6. Surface currents
When water moves down slope, the Coriolis effect intervenes!
Because of the Coriolis effect, surface currents in the Northern Hemisphere flow to the right of the wind direction, and in the Southern Hemisphere flow to the left of the wind direction
Additionally, continents and submarine topography frequently block or deflect flow into a circular pattern, called a gyre

7. (Winds driven by uneven solar heating)
The formation of gyres 
Clock-wise
Counter clock-wise
(Winds driven by uneven solar heating)

8. The North Atlantic Gyre

9. Surface currents
The trade winds which blow from the southeast in the Southern Hemisphere, and from the northeast in the Northern Hemisphere set the current in motion between the tropics (equatorial currents)
When equatorial currents reach the western boundary of the ocean basin, they must turn because they cannot cross land; Coriolis deflects these currents away from the equator (western boundary currents)

10. Western boundary currents
Equatorial currents

11. The Gulf Stream is a western boundary current

12. Western boundary currents
Western boundary currents are fast, narrow, and deep surface currents that carry warm water from the equator to the poles
Eastern boundary currents, on the other hand, flow back across the ocean basin carrying cool water from the poles to the equator (also deflected by continents and the Coriolis effect)

13. Wind-driven surface currents

14. Ekman Spiral and Transport
The collision of air molecules (in wind) with water molecules at the sea surface generates the water current
Once this surface film of water molecules in set in motion, they exert a frictional drag on the water molecules immediately beneath them, getting these to move as well
If the wind blows persistently, motion is transferred downward into the water column

15. Ekman Spiral and Transport
As this wind-driven current deepens, its speed diminishes (decreases) because of the growing distance from the driving force (the wind)
The current’s flow direction also changes with depth, the result of Coriolis deflection!
In the Northern Hemisphere, surface current flows to the right of the wind direction
When this topmost layer sets the underlying layer of water in motion, this deeper layer also moves to the right of the direction of flow with each successively deeper layer deflected to the right of the layer immediately above it

16. This spiraling flow pattern is called the Ekman spiral

17. Ekman Spiral and Transport
Under the influence of a strong, persistent wind, the Ekman spiral may extend downward to a depth of meters (~ feet)
The net transport* over this entire wind-driven spiral is 90° to the right of the wind direction in the Northern Hemisphere
*net transport represents the average of all directions and speeds of the Ekman spiral

18. Ekman Transport
*The immediate surface water moves in a direction of 45°; the overall transport of the water is 90°

20. Ekman Transport and Surface Currents
The Ekman transport describes an ideal situation, but ideal conditions rarely exist in nature, so the actual movement of the surface currents deviate slightly from that expected from the Ekman spiral
In shallow waters, for example, the Ekman transport can be very near the same direction as the wind

21. Ekman Transport and Surface Currents
Because Ekman transport deflects surface water 90° to the right (in the Northern Hemisphere), as gyres rotate clockwise, a convergence of water occurs in the middle of the gyre
Causes water to literally pile up in the center of the gyre
Creates a hill of water that is up to 2 meters (6.6 feet) in height

22. Dome of water formed by Ekman Transport

23. Yes, Virginia, there really is a hill of water in the middle of the North Atlantic

24. Geostrophic Flow
Gyres are in a constant balance between the pressure gradient formed by Ekman Transport and the Coriolis effect
Coriolis wants to move water uphill – against the concentration gradient – and gravity wants to move the water downhill – against the Coriolis deflection
Resulting current flows parallel to the slope

25. Geostrophic flow (currents)
Water flows downhill but is deflected to the right by Coriolis effect!

26. Water flows downhill under the influence of gravity, but the Coriolis effect deflects it to the right (in the Northern Hemisphere)
All currents in a gyre move as a result of geostrophic flow!

27. Western boundary currents are very fast and deep because there is a westward intensification of water piling up due to the eastward rotation of the Earth & Coriolis

28. North America
Europe
Equator

29. The Gulf Stream
Remember, the Gulf Stream is a western boundary current
Transports warm, tropical water northward
Together with its eastward extension, the North Atlantic Current or Drift, keeps Ireland and the west coast of Great Britain warm, and parts of Norway ice- and snow-free year round

30. The Gulf Stream sometimes meanders, forming rings or eddies that trap cold or warm water in their centers
Warm-water eddies bring coconuts and tropical fish to Long Island!

31. Wind can cause vertical movement of water
Wind-driven water is usually horizontal in nature, but can sometimes induce vertical movement in the surface water
Upwelling is the vertical movement of cold, deep, nutrient-rich water to the surface
Enhances productivity, which can support incredible numbers of large marine life
Downwelling is the vertical movement of surface water to deeper parts of the ocean
Decreases productivity, but transports necessary dissolved oxygen to organisms on the deep-sea floor

32. Upwelling
When surface waters move away (or diverge) from an area on the ocean’s surface, upwelling occurs
Upwelling commonly occurs along the equator (equatorial upwelling) and along the west Coast of the United States (coastal upwelling)
Creates areas of high productivity that are some of the most prolific fishing grounds in the world

33. Equatorial Upwelling

34. Equatorial Upwelling
As the SE trade winds pass over the equator, they cause water in the northern hemisphere to veer to the right, and water in the southern hemisphere to veer to the left (Coriolis deflection).
Surface water diverges, resulting in equatorial upwelling

35. Coastal Upwelling

36. Coastal Upwelling (Global)

38. Coastal Downwelling

39. El Niño
Under ‘normal’ circumstances, upwelling brings cold, nutrient waters to the coasts of Peru, driving an important anchovy fishery
Pacific Warm Pool

40. Normal conditions Pacific Warm Pool
The Pacific Warm Pool contains some of the warmest water on Earth; very low in nutrients

41. El Niño
Every few years, a current of warm water occurs off the coast of Peru, reducing the commercial catch of anchovies
Sea birds and seals that depend upon the anchovies for food suffered as well
The warm current brought with it increased rainfall (good for Peru; usually arid)
Usually occurred around Christmas and was named “El Niño” (“the child”); today known as the El Niño – Southern Oscillation (ENSO)

42. El Niño
During an El Niño, the high pressure zone along the coast of South America weakens, reducing the pressure gradient difference (high to low)
Causes southeast trade winds to diminish (or in extreme cases, to blow in the opposite direction!)
Without the trade winds, the warm water pool on the western side of the Pacific begins to flow towards South America

43. El Niño
The Pacific Warm Pool creates a band of warm water that stretches across the equatorial Pacific Ocean
Begins moving in Sept and reaches the coast of Peru ~Dec or Jan; temperature of seawater off Peru can increase <10°C (18°F) during this time!
Sea level can increase as much as 8 inches, simply due to thermal expansion of water along the coast!

44. El Niño (underway)

45. El Niño (established)

46. El Niño

47. El Niño
As the warm water increases sea surface temperature, corals become bleached
Near Peru, coastal upwelling brings warmer, nutrient-poor water to the surface, instead of cool, nutrient-rich water limiting productivity
During a strong El Niño, the low pressure zone migrates (and remains) over South America, while high pressure moves towards Indonesia, bringing very dry conditions to Indonesia and Australia

48. Normal conditions

49. Normal conditions

51. La Niña
Occasionally, conditions opposite of El Niño occur; these are known as ENSO cool phase, or La Niña
La Niña is characterized by normal conditions intensified, resulting in stronger trade winds, causing more upwelling, and a band of cooler water stretching across the equatorial Pacific
Usually occurs after an El Niño event; enhanced productivity (good)

52. El Niño and La Niña

53. El Niño and La Niña
El Niño conditions occur on average every 2 to 10 years, but on a highly irregular basis
Increased global warming may be promoting, and/or enhancing El Niño events; increased sea surface temperatures can trigger more frequent and more severe events
Strong El Niño events can alter global weather patterns, resulting in flooding, droughts, fires, tropical storms, and erosion

55. Deep Ocean (Thermohaline) Circulation
Subsurface, or deep ocean currents, arise from density differences between water masses produced by variations in temperature (thermal effect) and salinity (haline effect)
For this reason, they are collectively referred to as thermohaline circulation
As you saw in lab, when 2 water masses with different densities come into contact, the denser water mass slips beneath the less dense water mass

56. Deep Ocean (Thermohaline) Circulation
Deep water currents move large volumes of water and are much slower than surface currents
It takes a deep water current an entire year to travel the same distance as a western intensified surface current can move in one hour!
Although temperature and salinity both affect the density of seawater, temperature has a greater influence on density

57. Deep Ocean (Thermohaline) Circulation
Most water involved in deep-ocean currents originated at the surface in high latitudes
There, the surface water cooled and its salinity increased as sea ice formed (both increasing its density)
When this surface water becomes dense enough, it sinks, initiating deep ocean currents

58. Deep Ocean (Thermohaline) Circulation
Once the water sinks, it is removed from the physical processes that increased its density in the first place, so its temperature and salinity remain largely unchanged
Thus, a temperature-salinity (T-S) diagram can be used to identify deep-water masses based on their characteristic temperature, salinity, and resulting density

60. Sources of deep water
Huge masses of deep water form beneath sea ice off Antarctica
Here, rapid freezing produces very cold, high density water that sinks and becomes Antarctic bottom water, the densest water of the open ocean
Large masses of deep water also forms in the North Atlantic during sea ice formation; this dense mass is known as North Atlantic Deep Water

61. Sources of deep water

62. Conveyer-Belt Circulation
For every liter that sinks from the surface into the deep ocean, a liter of deep water must return to the surface somewhere else
Beginning in the North Atlantic, surface water carries heat to high latitudes via the Gulf Stream; cooling in the North Atlantic during winter increases the density of this surface water until it sinks and flows southward towards the equator

63. Conveyor-Belt Circulation

64. Conveyor-Belt Circulation
In 2005, researchers noticed that the flow of the northern Gulf Stream had decreased by ~30% since 1957
Researchers also noted that the water of the North Atlantic was becoming fresher (less saline) as the Earth continues to warm (increasing precipitation and polar ice melting)

65. A Global Warning?
Freshening of the North Atlantic from global warming could slow or stop the sinking of cold, salty water, which drives the conveyor belt , and brings warm water via the Gulf Stream to the North Atlantic
This warm water gives up heat to the atmosphere and moderates temperature in many parts of the globe

66. The Great Pacific Garbage Patch, and other interesting tidbits