The General Circulation of the Atmosphere and Oceans

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The General Circulation of the Atmosphere and
Oceans
ATS 351 Lecture 9 November 2, 2009
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The Global Heat Budget
Collects in tropics Escapes near poles
and aloft

• The Atmosphere has a radiative/convective
  equilibrium in the vertical
• Incoming and outgoing radiation
• Covection and subsidence
• It also has a similar equilibrium in the
  horizontal

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Single-Cell Model

• Assumptions
• Earths surface uniformly covered with water
• Sun is always directly over equator
• The earth does not rotate
• A huge thermally direct cell develops in each
  hemisphere
• Warm air rises and cold air sinks

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Single-Cell Model

• Assumptions
• Earths surface uniformly covered with water
• Sun is always directly over equator
• The earth does not rotate
• A huge thermally direct cell develops in each
  hemisphere
• Warm air rises and cold air sinks

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Three-Cell Model

• Add in rotation
• Three cells in each hemisphere
• Hadley cell
• Ferrel cell
• Polar cell
• Weak winds at the Equator (doldrums) and 30
  degrees (horse latitudes)?
• Boundary between cold polar air and mid-latitude
  warmer air is the polar front.

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Three-Cell Model

• Hadley Cell
• Thermally direct
• Driven by meridional gradient in heating
• Air rises near the equator and sinks near 30
  degrees
• Explains deserts, trade winds, ITCZ
• Ferrel Cell
• Thermally indirect
• Driven by heat transports of eddies (storms)?
• Air rises near 60 degrees and sinks near 30
  degrees
• Polar cell
• Thermally direct

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Inside the Ferrel Cell

• Westerly momentum is transferred from the earth
  to the atmosphere in the trade wind belt
• Westerly momentum is transferred from the
  atmosphere to the earth in the midlatitudes
• Why dont the midlatitude westerlies slow down
  over time?
• Eddies (storms) transfer momentum poleward in the
  upper troposphere
• This momentum transfer weakens the Hadley
  circulation, but drives the Ferrel Cell

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Three-Cell Model
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3-Cell Model Pressure
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The Real World

• Its much more messy!
• Continents, mountains, ice fields and forests all
  get in the way.
• The real wind patterns form around
  quasi-permanent high and low pressure systems
  rather than the banded ones discussed previously.
• During winter, highs form over the land, lows
  over the oceans. This flips in the summer.
  WHY?
• The Bermuda high and Pacific High form near 30
  degrees north in response to convergence aloft
  (mainly in the NH summer)
• ITCZ moves south in January and north in July

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Global Pressure Patterns
Winter
Summer
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Global Precipitation Patterns
January

• Very Wet in the tropics (ITCZ)
• Monsoon Regions
• Seasonal Shift of the ITCZ
• Mid latitudes get more rain in the summer
• Storm tracks
• More variability in the NH

July
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Take Home Concepts

1. Driven by differential solar heating between the
   equator and the poles. Acts to move heat
   poleward.
2. In Hadley cell, warmer air rises and moves
   poleward.
3. Ferrel cell is driven by heat and momentum fluxes
   by eddies.
4. In the NH, air is deflected to the right as it
   moves. Opposite in SH.
5. Pole-to-pole Hadley Cell is unstable in the
   presence of rotation, hence the single cell model
   breaks down.
6. Rotation makes the trade winds, surface
   westerlies, and jet streams

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But What about the Oceans?
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But What about the Oceans?
Heat still needs to be transported poleward
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Temperature Structure of the Ocean

• Mixed Layer
• Temperature in the top 50 m doesnt change much
  with depth
• Thermocline
• Rapid temperature drop
• Deep ocean

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What causes ocean circulation?

• The direction of most ocean currents is
  determined largely by the wind
• Coriolis force acts to turn the moving water to
  the right (in the NH)
• Friction directly opposes motion
• Resulting current at the surface moves 45 to the
  right of the surface wind (in the NH)

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Ocean Gyres

• Idealized gyre is a rotating dome of water
• Real world gyres are asymmetric
• Eastern boundary current is wide and moves slowly
• Western boundary current is much faster
• (e.g. the Gulf Stream)

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Coastal Upwelling

• Along west coast of U.S. prevailing wind is from
  the north in the summer
• Ekman pumping along western coasts leads to ocean
  moving away from the land
• Need upwelling to compensate colder deepwater
  moves up near the coast
• Ocean temperatures off the coast of California
  are typically much colder than at similar
  latitudes on the east coast

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El Nino La Nina
El Nino
La Nina
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El Nino La Nina

• Non-El Nino
• Upwelling and cooler water in eastern Pacific
• Warmer water in western Pacific

• El Nino
• Change in pressure causes trades to reverse
• Reverses figure above - warmer in eastern
  Pacific

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North America and ENSO

• El Nino
• Wet weather and storms into California and the
  southern part of US
• Weak polar jet stream over Canada creates warmer
  than normal weather over a large part of NA
• La Nina
• Moist air from ocean is directed into the Pacific
  NW wet winter for that region
• Winter months in southern part of US tend to be
  warmer and drier than normal

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Thermohaline Circulation

• Circulation in the ocean is also caused by
  density differences
• Density differences come from differences in
  temperature (thermo) and salinity (haline)
• In the north Atlantic, freezing leads to high
  salinity content
• Cold, salty water sinks and forms North Atlantic
  Deep Water
• Warmer water moves in at the surface to replace
  the mass lost

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Thermohaline Circulation
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