1. Ocean circulation

2. What are ocean currents?
• Mass movement or flow of ocean water
• Two types of major currents
– Surface currents
– Deep ocean currents / thermohaline circulation.

3. Surface currents
• Surface currents move water horizontally – parallel to the
Earth’s surface extend to approximately 100-150m depth
(depending on strength of winds)
• Two external forces influence the World Ocean generating
ocean currents - gravitation and the energy flux from the
sun.
• Gravitation includes tidal forces resulting from the
interaction of water mass with the moon and the sun, and
rotation of the Earth..
• The radiation flux from the sun results in wind stress,
heating and cooling of the ocean surface, and evaporation
and precipitation of water.
• A complex process of interaction between these forces
results in a complex and variable pattern of ocean
circulation

4. 1. What drives ocean currents?
The amount of heat
radiation is of
maximum at the
equator. The cold air
at the poles is denser
than the warm air at
the equator; hence, air
pressure at sea level
is higher at the poles
than at the equator.
The high air pressure
in the poles moves
towards the equator.
IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography

5. Current Generation
• Wind acting on the surface of the water, causes a partial
transfer of kinetic energy from the wind to the water.
• Wind-driven currents decline with depth, and are
generally limited by the permanent pycnocline – 100 to
200 m – but in some cases they may go as deep as
1000 m.
• The southeast trades and the northeast trades cause a
general westward current.
• The westerlies cause a general eastward current.
• Because of the continents, these currents are
interrupted. A westward current becomes a southward
current in the southern hemisphere, and a northward
current in the northern hemisphere.
• Eastward currents are forced to turn north in the
southern hemisphere, and south in the northern
hemisphere.

6. Winds on a NON-rotating earth
flow north-south
In fluid and gases pressure
gradients produce flow from
regions of high pressure to
regions of low pressure. If
the earth were not rotating,
the response to these
pressure gradients would
be direct and simple.
IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography

7. 1. What drives ocean currents?
The rotation of the
Earth modifies the
pattern of atmospheric
circulation. As air
moves toward the
equator, the rotation
of the earth shifts
ocean and land
eastward under it. The
result is "easterly"
winds (Polar
Easterlies and Trade
winds).
Traveling from the equator to the pole air in the upper atmospheric layer cools. About
30ºN and 30ºS the air becomes dense enough to fall back to earth surface, forming
two Hadley cells of atmospheric circulation. Similar cells are created between the
poles and 60º latitude. The zones between 30º and 60º are called Ferrel cells, where
"westerly" winds dominate.
IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography

8. Global wind pattern
The global pattern of winds cause the major
ocean currents in the surface layer.

9. The movement of water as
influenced by the Coriolis
Surface Currents effect and gravity.
• Controlled by three factors
– Global winds
– Coriolis Effect
– Continental Deflections
Global Winds cause surface
currents to flow in the
direction the wind is
blowing.
The Coriolis effect is the shifting of winds and surface
currents caused by Earth’s rotation. The Coriolis effect
creates clock-wise gyre in northern hemisphere and anti-
clockwise in southern hemisphere.
Continental Deflections
Shape of continents change the direction of current flow

10. Wind driven circulation
• About 10% of the water is moved by surface currents
• Surface Currents occur at upper 400 m.
• The currents move above the pycnocline.
• Surface currents are primarily driven by the wind and
wind friction
• Move fast relative to thermohaline circulation (1 to 2 m/s)
• Reflect global wind patterns and Coriolis effect!
• The pycnocline separates the surface layer from the
deep thermohaline circulation.

11. Gyre Formation
• Two great circular currents
(gyres) are formed, one the
northern hemisphere and
one on the southern
hemisphere, in both the
Atlantic and the Pacific. winds
• When the wind acts on
water in the open ocean for
a long time, the resultant
currents are about 1-3% of
the wind speed.
• Ocean currents are
massive. They can persist currents
for long times, even if the
winds are light or even
against them.

12. Current gyres
• 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

13. – 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

14. • Western boundary currents – These are narrow,
deep,
• fast currents found at the western boundaries of ocean
basins.
– The Gulf Stream
– The Japan Current
– The Brazil Current
– The Agulhas Current
– The Eastern Australian Current

15. • Eastern boundary currents – These currents are
cold,shallow and broad, and their boundaries are not
well defined
– The Canary Current
– The Benguela Current
– The California Current
– The West Australian Current
– The Peru Current

16. Major Currents of the World Ocean

17. Ekman spiral
• Wind flows over surface and creates drag on water
• Wind driven flow is deflected to right in N hemisphere by
Coriolis effect
• Water flows at only about 3% of the speed of the driving
wind.
• Current flows at 45o to the right of the wind direction in
the northern hemisphere
• But, only the surface feels the wind
• Each layer down only feels the layer above so is
deflected based on the layer above
• Each layer down moves more slowly than the layer
above

18. •Wind creates a drag on surface waters and successive layers
exert drag on each successive layer below.
•Each layer is subject to Coriolis deflection

20. Ekman flow
• Wind exerts frictional drag on water causing a thin
layer of water to move
– Transfer of momentum is not efficient; induced current is
about 2% of wind speed
– Coriolis force causes water to veer right or left of wind
• As the surface layer of water begins to move, it
exerts frictional drag on the layer below
• And so on, each layer moving slower and deflected
relative to the layer above
• Produces a pattern of decreased speed with depth
and increased angle between flow and wind
direction with depth (Ekman spiral)

21. Flow in Ekman layer
• Surface current typically 20-40o to
wind direction
• Average or net flow of water in
Ekman layer is 90o to wind
• Average or net flow in Ekman
layer is the drift current
• Thickness of Ekman layer is
approximately 100m

22. Fig. 5-1

23. Coastal upwelling
Northern hemisphere

24. Coastal downwelling
Northern hemisphere

25. Southern hemisphere

26. Pressure gradients develop in the ocean
because the sea surface is warped into broad
mounds and depressions with a relief of
about one meter.
• Mounds on the ocean’s surface are caused by
converging currents, places where water sinks.
• Depressions on the ocean;s surface are caused by
diverging currents, places from where water rises.
• Water flowing down pressure gradients on the
ocean’s irregular surface are deflected by the
Coriolis effect. The amount of deflection is a
function of latitude and current speed.

27. In the center of gyres
water piles up (converges)
upper ~100 m
Fig. 5-3 (a) Ekman spiral
Fig. 5-3 (b) Ekman transport

28. Downwelling of water
Creation of geostrophic currents as
a result of the pressure gradient
Upwelling of deep water to replace
surface water in areas of divergence
- e.g., along the equator

29. Consequences of Ekman transport
• At the center of the gyres:
– Convergence
– Water tends to pile up
• Convergence is associated with
downwelling
• At the edge of continents, divergence
occurs
• Divergence is associated with upwelling

30. Consequences of Ekman transport,
Coriolis, and gravity
• Ekman transport causes water to pile up in the
middle of gyres
• Gravity then acts to force it down
• Coriolis act in the opposite direction as gravity
• The forces balance, and flow tends to occur
parallel to the topographic contours
• This is called geostrophic flow

31. Geostrophy – a frictionless balance
between the pressure gradient
And the Coriolis acceleration –
generates currents that move
Around the ‘hill’

32. Measured average topography of the North Atlantic (red-high)

33. Permanent convergences and divergences
Convergences - downwelling
– 5 major permanent zones of convergence
∼ tropical convergence at equator
∼ N. subtropical convergence –30° to 40° N and S
∼ S. subtropical convergence –mark the center of the gyres
∼ Antarctic convergence at 50º S
∼ Arctic convergence at 50º N
Divergences - upwelling
– 3 major permanent zones of divergence
∼ N. tropical divergence
∼ S. tropical divergence
∼ Antarctic divergence

34. Upwelling and downwelling
• Vertical movement of water
– Upwelling = movement of deep water to surface
• Brings 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

35. Review of horizontal wind driven circulation
upwelling
downwelling downwelling
upwelling
downwelling
downwelling
upwelling

36. Antarctic
Circumpolar
Circulation

37. Areas of upwelling – coastal upwelling has seasonality (as do winds)

39. May global wind induced upwelling

40. Deep-Ocean Circulation
• Throughout most of the oceans, the layers form three principle
zones: the surface zone ( the mixed layer), the pycnocline zone,
and the deep zone.
• Deep ocean currents are known as thermohaline circulation
• Thermohaline circulation is a density driven flow of water
generated by differences in salinity or temperature.
• Water at the surface is exposed to changes in salinity
through evaporation or precipitation and in temperature
through cooling or heating.
• Based upon depth, surface water masses can be broadly
classified as Central waters (from 0 to 1 km), Intermediate
waters (from 1 to 2 km), and Deep and bottom waters
(greater than 2 km).

41. • Once water sinks and becomes isolated from the
atmosphere, its salinity and temperature are largely set
for an extended period of time up to 1000 years.
• In the polar/subpolar regions the climate is cold and the
sea-water is frozen. When the water freeze only pure
water turns into ice and increases the salinity and
density.
• The major thermohaline currents form the high density
water sink in the polar or subpolar regions and flow
mainly equatorward, their outward flow is confined
between the continents.
• Thermohaline circulation occurs below the pycnocline
• It involves 90% of all ocean water
• It moves slowly

42. Density of surface waters
Pacific Deep North Atlantic
Waters Deep Waters
Antarctic
Intermediate Antarctic
Waters Bottom Waters

43. Buoyancy driven circulation
Precipitation Evaporation Precipitation Evaporation (cold winds)
Heating Cooling
Dense
water
sinks

44. Vertical section of the Atlantic
North Pole South Pole

45. • The Atlantic Ocean has the most complex ocean
stratification containing the following layers: Antarctic
Bottom Water, Antarctic Deep Water, North Atlantic
Deep Water, Arctic Intermediate Water, and
Mediterranean Intermediate Water.

46. Atlantic Ocean subsurface water masses

47. Pacific Ocean
• The Pacific Ocean has a
less complex
stratification, is weakly
layered, displays
sluggish circulation and
is remarkably uniform
below 2000m
Indian Ocean
• The Indian Ocean has
the simplest stratification
consisting of Common
Water, Antarctic
Intermediate Water, and
Red Sea Intermediate
Water.

48. Labeling the world oceans Water Masses
TS Diagrams

49. SUMMARY of Water Masses in the Atlantic
• AABW densest and deepest Sea ice forms ==> salt
• NADW second, fills most of deep Atlantic Salty water moving north
• MOW saltiest but not as dense - sits with
other water masses at medium depths Evaporation ==> VERY
salty

50. Review of horizontal wind driven circulation
Vertical section

51. A model of the vertical overturning circulation

52. REGIONAL TYPES CIRCULATION
Anti-estuarine and estuarine circulation in basins with excess
evaporation and with excess precipitation, respectively, The arid basin
(A) is characterized by downwelling, hence low fertility and high oxygen
content. The estuarine basin (B) is characterized by upwelling and
salinity stratification, hence high fertility and low oxygen content. The
geographic names above the graph give three examples each for anti-
estuarine and estuarine circulation
Fig.7.12 Anti-estuarine and estuarine circulation in basins with excess evaporation and
with excess precipitation, respectively, The arid basin (A) is characterized by downwelling,
hence low fertility and high oxygen content. The estuarine basin (B) is characterized by
upwelling and salinity stratification, hence high fertility and low oxygen content. The
geographic names above the graph give three examples each for anti-estuarine and
estuarine circulation

53. Estuary Types Circulation
• Where river flow is
strong, and tidal
currents weak, salt-
wedge type
estuaries are
favored
• A partially mixed
estuary where river
flow and tidal
mixing are about
equal. Most
estuaries are of this
type.
Low density water mass flow toward the sea and the high
density water flow towards the land
Estuary circulations are found in the humid areas: fjiord and
estuary.

54. Anti-estuary circulation
• Mediterranean type circulation
• High salinity water mass as a result of evaporation in
isolated sea.
• High salinity water mass flows outwards to open ocean
and the low density water from open ocean flow into the
isolated sea.
• For example: Mediterranea- Gibraltar- Atlantic
Persian Gulf- Hormuz Strait- Indian Ocean
Red Sea – Bab el Mandeb- Indian Ocean

55. The outflow of Mediterranean Water