Wind Driven (Surface) Circulation
• Result from the
combined effects of:
– Wind
– Coriolis effect
– Interaction with
Continents
(impediments)
Interaction of Wind and Surface:
•
Recall near-surface winds
– Caused by differential
heating of surface
– These winds are deflected
by Coriolis effect separating
into circulation cells
•
Winds of these circulation
cells drive ocean-surface
circulation!
– Easterlies (Trades)
– Westerlies
Transfer of Wind Energy to Water
•
Wind Energy is transferred to the ocean surface by Friction
•
•
This Energy transfer drives flow.
Notice surface water is deflected 45º (to right in N
hemisphere) by Coriolis Effect.
The transfer of wind energy changes with Depth
•
As surface current flows (45º
from wind)
– Shears layer below
– Lower layers flow, but also
deflected by Coriolis
– Flow in lower layers diminishes
because of inefficient energy
transfer
– This produces the Ekman Spiral.
Ends at ~100 m depth (no wind
energy transferred below this
depth)
•
Ekman Spiral results from:
•
NET FLOW = Ekman FLOW
– Coriolis
– Inefficient E transfer w/Depth
– 90º to wind direction!
– Note Surface Current ≠ Ekman
Flow
What surface currents will result (in N. Hemisphere)?
A. Westward flow in tropics,
and eastward flow in midlatitudes (30º-60º)
B. Eastward flow in tropics, and
westward flow in midlatitudes (30º-60º)
C. Northward flow in tropics,
and southern flow in midlatitudes (30º-60º)
Let’s draw together
Resulting surface flow pattern
•
•
•
N-easterly Trades and
Westerlies result in Ekman
flow toward ‘center’
This will continue across
ocean until continents deflect
flow.
Results in a Gyre or spiral
flow
Result of surface flow pattern
• Elevated convergence!
– See how water is pilling up
in the center of the Gyre?
• This creates a gravity
imbalance…
Development of Geostropic Gyre
•
Balancing Ekman Flow and gravity flow produces a Geostropic Gyre
– Pressure or gravity flow from elevated convergence (down hill flow)
– Surface & Ekman flow toward the center (against gravity/pressure gradient)
•
•
•
Resulting flow direction - balancing Ekman Flow and gravity flow results in a gyre
or flow around an elevated convergence,
Speed of these Geostropic Currents are 1/100 th of wind speed (~1 to 0.5 m/s)
Size: the scale of an ocean basin: Easterlies drive surface flow to west, Westerlies
drive surface flow to the east. Continents deflect flow N or S.
Example of Geostropic Gyre
North Atlantic Elevated Convergence
Earth’s 7 major surface currents
•
North Atlantic Gyre, South Atlantic Gyre, North Pacific Gyre, South Pacific
Gyre, Indian Ocean Gyre, Arctic Gyre and Antarctic Circumpolar Current
Why is Antarctic Circumpolar Current not a Gyre?
A.
B.
C.
D.
E.
Because there are no
easterly winds at high
latitude
Because there is no
Coriolis effect at high
latitude.
Because there is no
continent to deflect the
current
None of the above
I am not in class
Arctic Gyre (7th current)
Animation of
Surface currents
Note flow in and out is primarily through North Atlantic
Western Boundary Currents:
•
Caused by
– Strong Easterlies at
low latitude
– Deflection of
surface flow by
continents
– Increasing Coriolis
effect with latitude
•
Examples
– Kuroshio & Brazil
– And…
Gulf Stream
• Largest of all
• 55 x 10 6 m3/s
• Equal to 300 x
Amazon flow
Review Questions
•
•
•
•
How does Coriolis Effect produce Ekman Flow?
Explain the Ekman Spiral.
How does Ekman Flow produce an elevated convergence?
How does an Elevated convergence become a geostropic
gyre?
• Why do ocean basins have western boundary currents?
• Why are western boundary currents important to society?