1. Gyres of the world oceans

2. Gyres of the World Oceans: Wind-driven Ocean Circulation

3. Wind-Driven Circulation: Motion of the Surface Ocean
Ekman Transport - p
Geostrophic Currents (gyres) - p
Wind-Driven Upwelling - p

4. p. 138
Ekman Spiral
The prevailing winds create a drag (wind stress) on the ocean surface.
The momentum gained at the surface is transferred deeper, but energy is lost with increasing depth.
The Coriolis Effect deflects the moving water of the upper m.
A decrease in current speed coupled with continuous deflection with increasing depth creates a theoretical spiral of moving water called the Ekman Spiral.

5. Ekman Transport The surface current is ~45o to the prevailing wind.
Adding all the vectors (magnitude and direction) of the Ekman Spiral yields a net current direction that is ~90o to the prevailing wind.
This composite current is called Ekman Transport and it controls the motion of the surface ocean.

6. Ekman Spiral Ekman Transport Surface current
what hemisphere does this example come from?
Ekman Spiral
change in direction
and current velocity with
increasing depth
Northern Hemisphere - to the right of the prevailing wind
Surface current
(~45° to the prevailing wind)
Ekman Transport
= composite of Ekman Spiral;
~90o to the prevailing wind
p. 138

7. Seeing the Ekman Spiral in 3-D: each
layer of surface water moves in a different direction due to the Coriolis Effect

8. to the right of the prevailing winds in the N. Hemisphere,
Ekman Transport
~90o to the
prevailing winds
90° N
(North Pole)
90° S
(South Pole)
Trade Winds
(NE Trades)
Polar Easterlies
Westerlies
to the right of the prevailing
winds in the N. Hemisphere,
to the left of the prevailing
winds in the S. Hemisphere
p. 133

9. Predict ocean circulation based on your knowledge of the prevailing
winds and Ekman transport
In the Northern
Hemisphere:
clockwise
circulation in
the low to
mid-latitudes;
counter-clockwise
the high latitudes
Circulation has the opposite
sense in the S. Hemisphere

10. Wind-Driven Ocean Circulation
Circulation of the upper water masses is set in motion by the energy of the prevailing winds
Ekman transport is responsible for this wind-driven circulation
Gyres are large horizontal current systems
The subtropical gyres transport warm waters poleward along the western edges of the ocean basins, and cool waters equatorward along the eastern sides p

11. http://www. miracosta. cc. ca

12. Geostrophic Currents
Ekman transport and the Coriolis effect cause surface waters to converge (“pile-up”) in the subtropics, and diverge (move apart) at the equator and in subpolar waters.
Convergence and divergence of surface water masses create subtle relief (“domes” and “depressions”) on the ocean surface (<2 m, or <6 ft.).
Gravity acts on the water to pull it downslope, while the Coriolis effect works in the opposite direction.
Geostrophic flow represents the partial balance between gravity and Coriolis.
p. 134

13. p. 133
Ekman transport causes surface waters to converge (pile-up) in subtropical regions creating a hill or dome
divergence
convergence
divergence
convergence
Ekman transport causes surface waters to diverge (move apart) in subpolar regions & at the equator creating a valley or depression
divergence

14. Divergence creates valleys or depressions,
Convergence produces hills or ridges.

15. divergence of water masses results in upwelling and high
productivity in subpolar waters
convergence of water masses
causes near-surface waters
to pile-up in the subtropics
divergence of water masses
results in upwelling and high
productivity at the equator

16. Geostrophic currents represent the
Profile
Rotation of Earth
Geostrophic currents represent the
partial balance between gravity & Coriolis;
most surface currents are geostrophic.

17. Western Boundary Currents
Earth’s rotation from west to east, compounded by the typically strong Trade Winds, cause tropical & subtropical surface waters to “pile-up” on the western sides of the ocean basins
The “hill” is displaced to the west of center causing water on the western side of the hill to flow faster than the eastern side
In other words, the surface currents are forced through a narrower passage between the continents and the crest of the hill causing them to flow faster; this is called “western intensification”.
Strong western boundary currents mark the western sides of the subtropical gyres.
p. 134

18. Note: the dome is not in the center, but has been offset to the west
p. 135
Map View
Note: the dome is not in the center, but has been offset to the west

19. warm core ring
cold core ring
The Gulf Stream is a western boundary current of the subtropical gyre
“Sargasso Sea”

21. The Gulf Stream and North Atlantic Drift warm Europe

22. Notice the warm currents flowing poleward on the western sides of the ocean basins and the cool currents flowing equatorward on the eastern sides. Also note the Antarctic Circumpolar Current that flows around the isolated continent of Antarctica; this current serves to connect ocean circulation between the three major ocean basins.

23. Lots of garbage collects here!
The north Pacific Geostrophic Subtropical Gyre
Nike shoes and toys found along North American beaches.
Lots of garbage collects here!

24. Coastal Upwelling Map View Profile Surface waters are replaced by
nutrient-rich deeper waters
resulting in high biological
productivity over the outer
shelf and upper slope
Ekman transport causes
surface waters to be
displaced seaward
(i.e., pushed away
from the coast)

25. Dark greens on
land and light
greens & reds in
the oceans are
areas of highest
chlorophyll and
plant/algae
abundances. In
the oceans this
means that either
upwelling or
continental
runoff has
enriched the
ocean in nutrients.

26. Oceanic Divergence (e.g., Equatorial Upwelling) Map View Profile
Change in the direction of the
Coriolis effect at the equator
causes divergence of the
surface waters
Surface waters are replaced by
nutrient-rich deeper waters
resulting in high biological
productivity

27. chlorophyll concentrations on land & sea Northern Hemisphere summer
notice strong equatorial upwelling as well as
coastal upwelling off Africa & Peru

28. 1 2 3 1. equatorial upwelling (divergence) 2. coastal upwelling off
Africa (several
locations)
3. high latitude upwelling
(divergence) around
Antarctica 2 1 3

29. Summary of Wind-Driven Circulationp
Summary of Wind-Driven Circulation
divergence
1. Prevailing winds (Trades, Westerlies, Polar Easterlies) impart energy to set the upper water masses in motion (near-surface circulation)
2. Due to the Coriolis Effect, net transport of water is approximately at a right angle (~90o) to the direction of the prevailing winds (=Ekman Transport)
90o to the right in the N. Hemisphere
90o to the left in the S. Hemisphere
3. Ekman Transport causes near-surface waters to pile-up (converge) in subtropical regions (creating “hills”), and to move apart (diverge) in subpolar regions and at the equator (creating “valleys”)
convergence
divergence
convergence
divergence
divergence of water masses
causes upwelling of deeper waters

30. Lots of garbage collects here!
Nike shoes and toys found along North American beaches.
Lots of garbage collects here!

31. The “Great Pacific Garbage Patch”

32. The remnants of a Laysan Albatross chick which was fed plastic by its parents resulting in death.

33. Geostrophic currents flow around subtle
“hills” and “valleys” on the ocean surface

34. Summary of Wind-Driven Circulation
4. The major near-surface currents are geostrophic, that is, the water masses flow around subtle “hills” and “valleys” on the ocean surface (balance between Coriolis effect & gravity)
e.g., the subtropical gyres flow around “hills” of the Atlantic, Pacific & Indian oceans
5. Near-surface currents are stronger on the western sides of the subtropical gyres (=western boundary currents)
the water is piled-up towards the west because of the Earth’s rotation and the strong Trade winds; currents flow faster where the “hills” are steepest & narrowest (i.e., the west side of the hill)
6. Western boundary currents transport
warm waters poleward; eastern
boundary currents transport cool waters
equatorward. p