Coriolis Effect

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Coriolis Effect

• If air moves in the meridional (north-south)
  direction
• It is changing its distance to the axis of
  rotation (the poles)
• Let's consider this in terms of its effect on
  angular momentum
• At a given latitude it is equal to the angular
  velocity times the square of the distance to the
  axis of rotation
• Angular momentum is also conserved in rotating
  systems (such as the Earth)

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Coriolis Effect

• If one moves northward in the Northern
  Hemisphere, one is decreasing the distance to the
  pole
• Thus to conserve angular momentum, the angular
  velocity must increase
• Think of a figure skater who puts out or pulls in
  their arms to change how fast they spin
• To increase the angular velocity, an eastward
  acceleration must occur
• Thus the deflection to the east

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Coriolis Effect

• Thus the Coriolis force arises from the
  conservation of angular momentum
• For air travelling northwards in the northern
  hemisphere, there is an eastward acceleration
• For air travelling southwards in the northern
  hemisphere, there is a westward acceleration
• For air travelling northwards in the southern
  hemisphere, there is an westward acceleration
• For air travelling southwards in the southern
  hemisphere, there is a eastward acceleration

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Coriolis Effect

• If air moves in zonal (east-west) direction
• If it moves eastward, it would take less time to
  complete one entire rotation as it is travelling
  faster than the earths surface
• If it moves westward, it is opposing the earths
  rotation and would take longer to complete a
  rotation

Segar, 2007
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Coriolis Effect

• Another way of saying this is that if you are
  moving in the zonal direction, your angular
  velocity changes
• It will be greater than that of the Earth if the
  travel is to the east
• It will be less than of the Earth if the travel
  is to the west

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Coriolis Effect

• Thus, for motion to the east in the Northern
  Hemisphere
• An increase in angular velocity
• Thinking of a ball orbiting on a string, an
  increase in angular velocity mean an increase in
  radius of the orbit
• The same thing applies here, with an increase in
  radius meaning movement away from the axis of
  rotation, the pole
• towards the Equator

Segar, 2007
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Coriolis Effect

• Eastward motion in the northern hemisphere leads
  to a southward acceleration
• Westward motion in the northern hemisphere leads
  to a northward acceleration
• Results are opposite in the southern hemisphere
• Eastward motion in the southern hemisphere leads
  to a northward acceleration
• Westward motion in the southern hemisphere leads
  to a southward acceleration

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Large-Scale Atmospheric Circulation

• We can thus link our basic ideas of vertical
  motion in the atmosphere with the effects of the
  Coriolis Force to
• Gives us a simplified picture of the average
  north-south overturning circulation into 3 cells
  (Hadley, Ferrel and Polar)
• The surface winds produced by these cells are
  then deflected by the Coriolis Force to drive
  east-west winds above the surface of the ocean

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Climatic Winds

• Hadley Cell
• Warm, moist air rises at Equator
• Flows poleward (and is deflected to right)
• Sinks in subtropics and returns as surface flow
• Ferrel Cell
• Poleward surface flow, rising in mid-latitudes
• Returns equatorward at height and sinks in
  sub-tropics
• Drives northeasterly TRADES in sub-tropics
• Drives WESTERLIES in mid-latitudes

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Climatic Winds

• Since rising air is often moist and since air
  cools as it rises, that moisture cant be held,
  producing clouds and precipitation
• Intertropical Convergence Zone
• Sinking air can hold more moisture (as it is
  warming as descending), which normally means dry,
  clear conditions
• Thus, downwelling in the sub-tropical divergence
  zone means dry conditions and deserts