Download Wind: Global Systems - Cal State LA

Chapter 10
General Circulation of the Atmosphere
General refers to the average air flow,
actual winds will vary considerably.
 Average conditions help identify driving
forces.
 The basic cause of the general circulation
is unequal heating of the Earth’s surface

 Warm air is transferred from the Tropics to the
Poles
 Cool air is transferred from the Poles to the
Tropics
General Circulation of the Atmosphere
Single Cell Model


Assume
uniform water surface
Sun always directly overhead the Equator
Earth does not rotate
Result: huge thermally direct convection cell
(Hadley)
1.
2.
3.
Three Cell Model




Allow earth to spin = three cells (Hadley,
Ferrell, Polar)
Alternating belts of pressure starting with L at
Equator
Alternating belts of wind with NE just North of
Equator
General Circulation of the Atmosphere

Average Surface Wind and Pressure:
The Real World
 Semi-permanent high and lows
 Northern vs. Southern Hemisphere
 Major features shift seasonally with the high
sun
○ North in July
○ South in December
General Circulation of the Atmosphere

General Circulation and Precipitation
Patterns
 Rain where air rises (low pressure)
 Less rain where air sinks (high pressure)

Average Wind Flow and Pressure
Patterns Aloft
 North-South temperature and pressure
gradient at high altitudes creates West-East
winds, particularly at mid to high latitudes.
Jet Streams
100-200 kt winds at 10-15km, thousands
of km long, several 100 km wide and a
few km thick (polar and subtropical)
 Observations: Dishpan Experiment

 Illustrates waves, with trough and ridge,
develops in a rotating pan with heat on the
exterior and cold at the center.
Jet Streams

Polar and Subtropical Jet
 Established by steep temperature and pressure
gradients between circulation cells.
 Between tropical-mid-latitude cell (subtropical)
and mid-latitude-polar cell (polar)
 Gradients greatest at polar jet

Topic: Momentum
 Low-latitudes: atmosphere gains momentum
 High-latitudes: atmosphere losses momentum
 Conservation of Momentum
Jet Streams

Other Jet Streams
 Tropical easterly jet stream
 Low-level jet (nocturnal)
 Polar night jet streams
Atmosphere Ocean
Interactions

Global Winds and Surface Ocean
Currents
 Ocean surface dragged by wind, basins
react to high pressure circulation forming
gyres
 Cold current, flowing north to south, on west
side of continent
 Warm current, flowing south to north, on
east side of continent
 Oceanic front
Stepped Art
Fig. 10-14, p. 273
Atmosphere Ocean
Interactions

Upwelling
 Ekman spiral, Ekman transport
 Water moving away from the coast causes
upwelling

El Nino and the Southern Oscillation
 El Nino: irregular warm episode off west coast of




South America
Southern Oscillation: rise in pressure over W
Pacific, fall in the E Pacific, equatorial
countercurrent
ENSO
La Nina
teleconnection
Atmosphere Ocean
Interactions

Pacific Decadal Oscillation
 Reversal in Pacific Ocean temperatures
 Warm = more Pacific storms
 Cool = cool, wet NW North America, wetter
over the Great Lakes, salmon fisheries
decline
Atmosphere Ocean
Interactions

North Atlantic Oscillation
 Reversal of pressure in North Atlantic Ocean
affecting weather in Europe and eastern
coast of North America
 Positive = strong Westerlies, storms in N
Europe, wet and mild in eastern US
 Negative = wet southern Europe and
Mediterranean, cold and dry in eastern US
Atmosphere Ocean
Interaction

Arctic Oscillation
 Closely related to NAO
 Pressure changes between Arctic and
adjacent southern areas causes changes
upper-level winds
 Positive = mild winter in US and W Europe
 Negative = cold US, cold dry Europe, wet
Mediterranean