Unit 9: Circulation Patterns of the
Atmosphere
Simple models of the atmospheric
circulation
Actual circulation of the atmosphere
Global, Regional
Upper atmosphere, jet streams
Action in the atmosphere—cloud
formation over Easter Island in the South
Pacific Ocean. (Authors’ photo)
OBJECTIVES
• Develop a simple model of the global atmospheric
circulation
• Discuss the pressure systems and wind belts of that
model circulation, and the complications that arise
when the model is compared to the actual
atmospheric circulation
• Introduce the basic workings of the upper
atmosphere’s circulation
• Understand the importance of the upper level jet
stream and its seasonal variability
Thermal Cell Circulation based on
uneven heating, but no rotation
Hypothetical atmospheric circulation on a featureless, nonrotating Earth. Polar high pressure and
equatorial low pressure would result in northerly surface winds in the Northern Hemisphere and
southerly surface winds in the Southern Hemisphere. The rotation of the Earth, the annual variation in
the latitude of the vertical noon Sun, the distribution of oceans and continents, and land/water heating
contrasts at the surface prevent this simple general circulation from developing.
Simplified Model of Atmospheric Circulation
including uneven heating, rotation, and
conservation of angular momentum
A conceptual model of the global atmospheric circulation pattern showing the major surface
pressure belts, the prevailing surface wind systems that develop from them, the upper-level jet
streams, and the Coriolis deflection of surface winds.
General Atmospheric Circulation
Figure 4.13b
Features of the Idealized 3-Cell Model
•
Equitorial Low or Inter-Tropical Convergence Zone (ITCZ)
warm air rises near the equator where trades meet; cloudy, rainy
ITCZ shifts with movement of sun, north in our summer, south in our winter.
Shift is greater over continents.
•
Subtropical Highs occur where rising tropical air at the equator moves poleward, cooling and
sinking creating warm, dry conditions.
•
Trade Winds. Air flowing from subtropical highs towards ITCZ deflects by Coriolis forces into
NE trades and SE trades. Trade winds are the most consistent winds in the atmosphere.
•
Westerlies. Air flowing poleward from subtropical highs are deflected by Coriolis force into
westerlies at mid-latitudes.
•
Polar Highs occur at the two polar regions as cold air sinks and spreads equatorward.
Coriolis deflection turns these high latitude winds into polar easterlies.
•
Polar front. Where cold polar easterlies meet warmer westerlies, low pressure and rising air
forms the polar front, a zone of traveling cyclones.
Global mean sea-level
pressure (mb) patterns in
January (A) and July (B).
Features of the Global Seasonal Pressure Patterns
Equitorial Low or Inter-Tropical Convergence Zone (ITCZ)
warm air rises near the equator where trades meet; cloudy, rainy
ITCZ shifts with movement of sun, north in our summer, south in our winter. Shift is
greater over continents.
Subtropical Highs occur where rising tropical air at the equator moves poleward, cooling
and sinking creating warm, dry conditions. These highs also shift with the sun, being larger,
stronger over ocean basins in summer.
Westerlies. Air flowing poleward from subtropical highs are deflected into westerlies at
mid-latitudes.
Polar Highs occur at the two polar regions as cold air sinks and spreads equatorward.
Coriolis deflection turns these high latitude winds into polar easterlies. Polar highs become
more dominant in winter over high latitude continents, such as Siberian High.
Subpolar lows occur over high latitude oceans in winter, such as the Icelandic and Aleutian
Lows. These are mean positions of frequent cyclones that move along the polar front into
N. America and Eurasia. In the S. Hemisphere subpolar lows form a year-round belt of
stormy weather.
Fraction of surface area covered by land as a function of latitude.
The Hadley Cell Circulation
A cross-section of the Hadley Cell circulation. Surface convergence of the trade winds
feeds convective lifting in updraft zones that are embedded along the Equatorial Low
(ITCZ). Westerly acceleration of upper-level winds leads to the formation of the
Subtropical Jet Stream (STJ), which initiates convergence of air and general subsidence
at about 30 latitude. The resulting Subtropical High (STH) is a broad and continuous
belt of high pressure that encircles the Earth.
The Inter-Tropical Convergence Zone
Mean positions of the Inter-Tropical Convergence Zone (ITCZ) in January and July. Note the
larger migration of the ITCZ over continents and oceans surrounded by continents.
Secondary Surface Circulation: Monsoon Winds
Monsoon is a seasonal reversal of
winds caused by differential
heating of land and ocean.
In winter, the land cools with high
pressure and offshore winds.
In summer, the land heats rapidly
creating low pressure and onshore
winds. This is also the rainy season
for much of the subcontinent.
General airflow patterns over the Indian subcontinent during the winter (top) and summer monsoon season (bottom).
The regional circulation system involves strong seasonal changes in the heating patterns over the Indian subcontinent
and Tibetan plateau, seasonal reversal of offshore and onshore winds over India, shifts in the position of the
Subtropical and Equatorial Jet Streams, and distinct winter and summer precipitation patterns.
Monsoon Winds
The Himalayas form a giant barrier to summer monsoon winds pushing them westward into the Ganges River Valley.
Circulation of the Upper Atmosphere
In the upper troposphere, Coriolis force turns poleward flowing air into westerly winds.
The strongest westerlies is called the jet stream. Note the winter jet stream is stronger and
further south than in summer.
Zonal mean wind speeds (m s-1) by latitude and altitude averaged across all longitudes for
multiple decades of time for the Northern Hemisphere in summer and winter. Easterly
zonal winds are shaded, westerly zonal winds are unshaded.
Upper Air Rossby Waves
Upper air westerlies form waves with flow moving equatorward producing
troughs, while poleward flow forms ridges. How many waves can you see?
Mean 500-mb height (m) in the Northern Hemisphere for January.
Wind
Portrait
of the
Pacific
Ocean
Figure 4.6