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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

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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!

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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

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The “Global Conveyor Belt”Additional Points

• Speculation of Deep Water terminationrelated to glacial cycles

• Deep Water is very nutrient rich!