1. The Oceans in Motion
Surface Currents

2. Ocean Currents Currents large scale water movements
occur everywhere in ocean
both surface and deep
2 main types: surface currents (10%) and subsurface currents (90%)
surface currents are primarily wind driven
subsurface currents are density driven
other forces affecting currents
Coriolis effect
friction
gravity
thermal expansion
geologic shape of ocean basin

3. Formation of Surface Currents
surface currents =horizontal movement of the oceans surface waters
driven by: a) thermal expansion & gravity, b) winds and c) combination
extend to approximately m depth (depending on strength of winds)
thermal expansion and gravity
equatorial surface waters receive more solar radiation than polar surface waters
warmer equatorial waters expand (thermal expansion) – higher elevation
colder polar waters contract – lower elevation
gravity ‘pulls’ the waters from area of higher elevation (equator) to areas of lower
elevation (poles), water moves downhill

4. Formation of Surface Currents (cont’d)
Winds – (What creates winds?)
the primary force setting surface oceans in motion
Tradewinds ‘push’ surface
waters toward the equator
Westerlies ‘push’ suface waters toward the poles

5. Visualization of the Coriolis effect:
Wind:

7. Formation of Surface Currents (cont’d)
winds (cont’d)
add in the Coriolis Effect
doesn’t only deflect (45°) surface water that wind is in direct contact with, but causes each successively deeper layer of water to be deflected as well;
called the ‘Ekman Spiral’,
extends down to approximately 150m (depending on strength of winds)
over the total 150 m depth, the average flow of water is at a 90° angle to the direction of the wind (right in N. hemisphere, left in S. hemisphere)
this net flow of water is called
‘Ekman Transport’
150
wind
wind
net flow
‘Ekman Transport’
‘Ekman Spiral’
see figs. 7.15a,b (Intro 7e) or 7.25a,b (Fund. 4e)

8. 1 2

10. What would happened in the Northern Hemisphere?

11. Formation of Surface Currents (cont’d)
combine effects of thermal expansion & gravity (pulling water masses) and winds (pushing water masses)
end up with circular flow
clockwise in the northern hemisphere
counter clockwise in the southern hemisphere
circular pattern flowing along the edge of basins are called ‘gyres’
Gyres = large circular, wind-driven, oceanic flow

12. Surface Currents major surface currents: N. Atlantic Gyre
S. Atlantic Gyre
N. Pacific Gyre
S. Pacific Gyre
Indian Ocean Gyre
Westwind Drift (or Antarctic Circumpolar Current) 3 1 5 4 2 6
see fig (Intro 7e)
or 11.2 (Fund. 4e)
NASA

13. Major oceanic circulation systems

14. Other Effects of Wind on Water Movement
convergences and divergences
convergences
results when two wind-driven surface currents collide OR
when a wind-driven surface current collides with a land mass
results in downwelling
typically areas of low nutrients and productivity
two surface currents colliding
surface current colliding with land mass

15. Other Effects of Wind on Water Movement (cont’d)
convergences and divergences (cont’d)
divergences
results when two wind-driven surface currents move away
from each other OR
when a wind-driven surface current moves away from a land mass
results in upwelling
typically areas of high nutrients and productivity
surface current pushed away from land mass
two surface currents pushed in opposite directions

16. Other Effects of Wind on Water Movement (cont’d)
permanent convergences and divergences
ocean zones:
Convergences - downwelling
5 major permanent zones of convergence
tropical convergence at equator
N. subtropical convergence
S. subtropical convergence
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
30° to 40° N and S
mark the center of the gyres
at either side of the tropical convergence
see figs (Intro 7e) or 7.17 (Fund. 4e)

17. Other Effects of Wind on Water Movement (cont’d)
permanent convergences and divergences (cont’d)
coastal zones:
Divergences - upwelling
off west coasts of Africa and South America
Tradewinds ‘push’ water off shore
end up with divergence along the coasts
get continuous upwelling along these coasts
big fish catch!! very productive fisheries

18. Other Effects of Wind on Water Movement (cont’d)
seasonal convergences and divergences
coastal
off west coast of North America
winter convergence
coastal wind blows from the south
net water transport onshore (convergence)
get downwelling
summer divergence
coastal wind blows from the north
net water transport offshore (divergence)
get upwelling
i.e. San Francisco fisheries and fog