1. WIND-DRIVEN OCEAN CIRCULATION
EOSC 112: THE FLUID EARTH
WIND-DRIVEN OCEAN CIRCULATION
Oc2 Read: Kump et al. Chap.5, p
Objectives:
To describe the dominant forces in the 0-100m layer;
To describe the dominant forces in the m layer.
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2

2. 1. Forces in the top layer (0-100m)
11/20/2018
1. Forces in the top layer (0-100m)
Nansen (1890s) observed icebergs moving 20-40o to right of wind.
Ekman (1905) solution has surface current at 45o to right of wind in Northern Hem. (to the left in S.Hem.) (again… Coriolis effect).
Coriolis effect explained in detail in Atm. Lectures. Hence more natural to do the Atm. lectures before the Oc. lectures.
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2

3. Upwelling & Downwelling
Wind blowing alongshore can generate offshore Ekman transport
=> Upwelling of lower, cool nutrient-rich water
=> Enhanced biological productivity
Onshore Ekman transport => downwelling => poor biological productivity
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2

4. Upwelling occurs under cyclones.
Downwelling occurs under anti-cyclones.
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2

5. 2. Forces in 100-1000m (Geostrophic Currents)
Tilt in sea level (SL) => pressure gradient => PGF force. When PGF is balanced by the Coriolis force => geostrophic current. SL
Coriolis
current
PGF
High P
Low P
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2

6. current Coriolis force Low P High P PGF Coriolis force Low P High P
Gradual buildup of a geostrophic current:
current
Coriolis force
Low P
High P
PGF
Coriolis force
Low P
High P
PGF
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2

7. H H N.Hem.: low-lat. Easterly winds, mid-lat. Westerlies
=> converging Ekman transport & high sea level (SL) at ~30°N
=> geostrophic currents. H
Ekman
transport
High SL
geostrosphic
current
Coriolis
force
45°N
30°N
15°N H
PGF
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2

8. Wind stress, pressure gradient, Coriolis
With eastern and western boundaries, ocean circulation forms large clockwise or anticlockwise cells = gyres.
Pressure gradient from SL tilt disappears by ~1000m depth => geostrophic current only in top 1000m.
SUMMARY
Wind stress, pressure gradient, Coriolis
In Ekman layer (top 100m): Coriolis force + Friction force balancing wind stress.
m: Coriolis force balancing Pressure Gradient Force => geostrophic current.
11/20/2018 8:02:19 AM
EOSC 112 lecture Oc-2