OCEAN CURRENTS
Objectives:
1. To learn what forces influence ocean currents.
2. To understand simple force balances and the
resulting currents.
3. To understand the processes responsible for midlatitude gyres and coastal upwelling and
downwelling.
1. Global Current Patterns
- 5 major ocean gyres.
- Antarctic Circumpolar Current.
- Local Current Patterns: fronts and eddies, coastal upwelling and
downwelling.
2. Newtons Law: F1 + F2 + F3 + .... = ma
- F = forces, m = mass, a = acceleration.
- For a steady state, a=0, so the forces must balance (equal and
opposite).
3. Types of Forces
- Coriolis Force (CF): 90º to right of velocity in northern hemisphere,
90º to left of velocity in southern hemisphere.
- Pressure Force (PF): from high (H) pressure to low (L).
- Wind Stress (WS): in the direction of the wind.
- Force (Cent F): outward for curved flow.
4. Simple Force Balances (Only 2 forces acting)
- Cyclostrophic Force Balance:
PF = Cent F. Current can
flow either clockwise or
counterclockwise.
- Geostrophic Force Balance:
PF = CF. In the northern
hemisphere, flow is
counterclockwise around a
low and clockwise around a
high.
-
Ekman Layer: WS = CF. In the northern hemisphere, depth average
flow is 90º to the right of the wind.
Vertical Variation of Ekman Layer Flow: Surface velocity V0 is 45º to
right of the wind. At each subsequent depth, the velocity decreases
and turns more to the right. The amount of water transported forward
at the surface is exactly equal to the amount of water transported
backward at depth.
The total transport therefore is 90º to the right of the wind.
5. Mid-Latitude Gyres:
- Clockwise winds over basin
cause Ekman transport towards
the center of the gyre, piling up
about 1 meter of water in the
middle.
- High pressure in the center of
the gyre results in a clockwise
Geostrophic flow around the
gyre.
- Variation of Coriolis Force with
latitude results in intensified
flow on the western side.
6. Wind Driven Coastal Currents (West Coast).
- Downwelling: wind to north, coast on right. 1) Wind driven Ekman
transport is towards shore. 2) Water elevation rises at shore as warm
surface water is transported shoreward, resulting in downwelling. 3)
Higher water elevation at the shore generates an offshore pressure
force. This must be balanced by another Coriolis force, resulting in
additional alongshore flow to the north.
-
Upwelling: wind to south, coast on right. 1) Wind driven Ekman
transport is offshore. 2) Water elevation falls at shore as warm
surface water is transported offshore, resulting in the upwelling of cold
bottom water. 3) Lower water elevation at the shore generates an
onshore pressure force. This must be balanced by another Coriolis
force, resulting in additional alongshore flow to the south.
8. Review Questions.
- Hurricanes are intense low-pressure systems with strong winds.
Construct a diagram illustrating the force balance and determine the
direction of flow.
- Assume the wind is blowing from the south along the New Jersey
coast. Describe the ocean response.