Surface ocean : Wind driven circulation Flashcards
4 Forces controlling ocean current circulation
Fw- Wind stress
Fp- Pressure
Ff- Friction
Fc- Coriolis force
Coriolis force
where an object moving with respect to the Earth tends to be deflected to the right in the Northern hemisphere and to the left in the southern
Centrifugal Force
Tendence for an object to fly away from the axis of rotation
Ekman transport
Wind stress balances the Coriolis force
Geostrophic currents
Pressure gradients balance coriolis force
Rule of geostrophic flow:
Water moves along isobars with the higher pressure on its left in Southern Hemisphere and to its right in Northern Hemisphere
Why do winds circulate around anti-cyclones?
Balance of atmospheric pressure by Coriolis force
Planetary and Relative vorticity
The movement of ocean water creates rotations of various sizes, from small eddies to large gyres. This spinning motion is called local relative vorticity.
Total Vorticity
Every water parcel has a total spin which is the sum of planetary vorticity and local relative vorticity
Planetary vorticity
The inherent spin a water body gets due to Earth’s rotation- increases as you move north (anticlockwise in the N.Hemisphere)
Local relative vorticity
Additional spin the water has due to its own currents
Conservation Principle
As water moves around, its total vorticity needs to stay constant - if planetary vorticity increases (moving North) , local relative vorticity must decrease (spin slower or change direction) to keep total spin balanced.
Wind Torques
=A cyclonic or anticyclonic wind pattern when it exerts a twisting force
Sverdrup transport
- Ekman Pumping provides squashing or stretching
- Water columns must respond. They do this by changing latitude
Local relative vorticity can be lost by …
friction between ocean currents and the continents
Planetary vorticity can be lost by:
Moving towards the equator
Subtropical Gyres
Anticyclonic
Subpolar gyres
Cyclonic
Ekman Suction:
Ekman transport divergence= upwelling
Ekman Pumping
Convergence= Downwelling
Potential Vorticity
Q= (f+ζ)/H
Potential vorticity= (planetary vorticity + relative vorticity )/ Column Height)
Stretching 1/H
height of the water column= denominator since making a column taller makes it spin faster
Conservation of potential vorticity
Planetary vorticity grows, relative vorticity must decrease
Squashing
Equatorward movement
Stretching
Poleward movement
If there is a decrease in H
Decrease in latitude- water moves towards the equator
To compensate for the Sverdrup transport…
Western boundary currents are developed in all ocean basins as a strong and deep poleward flow of water
Eddies
- Friction can create or destroy vorticity by producing a velocity gradient
- Frictional effects are confined to a boundary layer
Conservation of vorticity
Produces eddies beyond the edge of a spit when drag of coastal friction is removed
Gulf stream- North of Cape
Broad Shelf of shallow sea relatively untouched by the warm waters of the Gulf Stream but more influenced by v. cold waters of the Labrador with temperature between 3 and 9 C
Examples of upwelling- Atlantic Ocean
Tropical Atlantic: off the Coast of Africa e.g. Gulf of Guinea
North Atlantic: Canary Islands
What direction do trade winds transport wind from 30S-30N ?
East to West
What latitudes do westerlies occur? W-E
30-60N and 30-60S
