1
Currents and the Coriolis Force
Ocean Circulation
• Surface Currents: wind driven; ultimateenergy source is the sun.
• Deep Currents: “thermohaline” (density)currents; density driven
• In reality, ocean currents are a combinationof these two convenient classifications
Complicating Factors
• The Earth is Rotating• Moving water (and air) masses are affected
by this rotation• This effect is know as the Coriolis Force• Let’s take a look:
First, let’s think about reference frames:Car 1
Car 2
Car 3
Car 4
35 mph
0 mph
65 mph
65 mph
Reference Frames Summed Up
• Reference Frame: a point of referencerelative to which the properties of otherthings are measured
• Usually Earth is our reference frame• Problems:
– Earth rotates– Laws of physics apply to non-rotating frames
Circular Motion
Release Point of String
Projectile Path/Velocity
Centripetal Acceleration
2
Observations of the phenomenon…
So what’s going on?!
Earth From North Pole
North Pole
MA~42º
Equator
l1
l2
Object at equator ismoving faster thanin MA
“Tangential velocity”increases as you movetoward Equator
Rotation Effect is maximum at the pole
If Coriolis Affected Football…
Tom Brady
Randy Moss
No Rotation
Tom throws ballstraight at Randy
Path appears straightto Tom and Randy
Path appears straightfrom Goodyear Blimp
If Coriolis Affected Football…
Tom Brady
Randy Moss
CCW RotationTom at Pole
Tom throws ballstraight at Randy
By the time ball getsto goal post, Randyhas been rotated frompoint A to point B
AB Ball is deflected to
the right!
Apparent Pathof ball
If Coriolis Affected Football…Tom Brady
Randy Moss
CCW RotationTom at Equator
Tom throws ballstraight at Randy
By the time the ballshould reach Randy,it has moved withappropriate tangentialvelocity to the right!
Apparentpath of ball
Tom and ball have tangential velocitywhen ball is released
Coriolis Effect-Bottom Line• Bodies in NH deflected to right, SH to left• Magnitude increases form 0 at equator to
maximum at poles• Magnitude of the effect is proportional to the
body’s velocity• Coriolis only AFFECTS currents’ paths
– Does not CAUSE currents to flow• Coriolis Effect is equivalent to a hill with a slope
of 0.0007º (6 meter wide road would have a0.1mm change in height)– Too small to be felt in everyday life!
3
Surface Currents
• Created by wind drag on the surface of theocean
• Rarely more than 50-100 meters deep• They are as broad as the winds that create
them
Ekman Transport
Features of Ekman Spiral
• Surface currents at moved 45º to the right(left) of wind
• Net water movement is 90º to the right (left)of wind (Ekman Transport)
• Depth at which flow is 180º to surfacecurrent is called the Ekman Depth– ~100 meters
Coastal Upwelling
Thermohaline Currents
• Driven by density differences (Temp., Sal.)• Water sinks where it is cold and salty• Eventually water upwells back to surface• Estimated “residence time” of water in this
cycle is ~1000 years
Atlantic Deep Water Flow
4
The “Global Conveyor Belt”Additional Points
• Speculation of Deep Water terminationrelated to glacial cycles
• Deep Water is very nutrient rich!