Part 3. Distribution and
Movement of Air
Chapter 8
Atmospheric Circulation and Pressure
Distributions
Planetary Winds
Well-defined pressure patterns exist across the
Earth that induce the global wind patterns on
the planet
Idealized Single-Cell Convection Model for
a Planet
The ITCZ is a band of clouds across the tropics
Ferrel cell -southwesterly winds
at surface
Subtropical high -- Air
subsides (dry climate)
Hadley cell -- tropical
convection cell
Intertropical
convergence zone
(ITCZ) -- surface low
pressure with clouds
and rain
The Three-Cell Model
Polar cell -northeasterly winds
at surface
The three-celled model vs. reality:
• Hadley cells are close approximations of real
world equatorial winds
• Ferrel and polar cells do not approximate the
real world winds very well at all
• Model is unrepresentative of westerly flow
aloft
• Continents and topographic irregularities
cause significant variations in real world
wind patterns compared to the model
Semi-Permanent Pressure Cells are large areas of
higher or lower atmospheric pressure than the
surface average
They may be thermally induced (rising warm air or
subsiding cold air) or they may be caused
dynamically by converging or diverging wind
patterns)
They fluctuate seasonally
Northern hemisphere semi-permanent cells
The Aleutian, Icelandic, and Tibetan lows
Siberian, Hawaiian, and Bermuda-Azores highs
ITCZ (low)
Average atmospheric air pressure and wind patterns in January
Average atmospheric air pressure and wind patterns in July
The Sahel
reflects seasonal
migration
of the ITCZ
The Sahel is rainy during northern hemisphere summer and dry during northern
hemisphere winter as the ITCZ shifts north and south through the year. It is the
reason for annual flooding of the Nile River.
Shifts in the ITCZ affect the Sahel
Mid-latitude winds in the middle and upper troposphere are
controlled b the pressure gradient force and the Coriolis
force, giving the westerly winds
Stronger pressure gradients in winter give stronger
westerlies
Westerly winds in the upper atmosphere
• Air motions directed towards poles
• Redirected by Coriolis deflection
– Westerly winds aloft result
The polar front and jet streams
• Fast stream of air in upper troposphere
– Above polar front
– Stronger in winter
Polar jet profile
Strongest
pgf here
300 mb
500 mb
800 mb
The fastest middle and upper troposphere mid-latitude winds (the jet stream) are
at polar front. There is a sharp temperature contrast and horizontal pgf to the
south and north of the polar front.
The subtropical jet transports low latitude moisture and energy
Subtropical
front and jet
Ridges and trough profile
Ridge
Ridge
Trough
A trough over the mid-continent in (b)
Rossby waves are the wave-like pattern
of ridges and troughs in the upper
troposphere winds. Ridges and troughs
(Rossby waves) will migrate either east or
west with time
Sequence of Rossby wave migration -- the dashed line shows the
migration and evolution of a trough with time across the country
Ocean currents are driven by wind stresses and
are deflected by the Coriolis force. Thus, the
water moves at about a 45° angle to the winds.
The diection of movement of the ocean changes
with depth following a pattern called the Ekman
spiral.
• Western basins usually experience warm
ocean currents
• Eastern basins usually experience cold ocean
currents
Upwelling in along-shore winds
• Cold waters rise due to Ekman transport
Global Ocean Circulation
Infrared Satellite Image of the Gulf Stream
Major Wind Systems of the Earth
Monsoons
• Thermal induced seasonal wind patterns
associated with shifts of the ITCZ
• Monsoons are characterized by dry offshore
winter flow and wet onshore summer flow
• The monsoon in East Asia experiences
orographic enhancement
Winter
monsoon
Summer
monsoon
Topography enhances monsoonal effects
Foehn winds are strong, downslope winds that
adiabiatically compress, raising the air temperature. Foehn
winds are associated with hot, dry, clear weather
Chinook winds are foehn winds along the east slope of the
Rockies (“snow eaters”)
Santa Ana winds are foehn winds that blow from the
deserts and over the mountains into the valleys of southern
and central California
Katabatic winds are cold, dense winds that flow down
mountain slope. They warm as they descend, but they are
still colder than the surrounding air.
• Boras and mistral winds are forms of katabatic winds in
Europe
Southern California Santa Ana induced fires
Sea and land breezes form due to temperature
differences over land and sea. Sea breezes form
during the day, and land breezes form at night.
Valley and mountain breezes form due to heating
and cooling on mountain sides. Valley breezes
form during the day, and mountain breezes form
at night (similar to katabatic winds)
Sea breeze
development
Sea breeze-initiated clouds over Hawaii
Valley and mountain breeze development
Air-Sea Interactions in the Equatorial Pacific
El Niño, La Niña, and the Walker circulation
• El Niño events
– Unusually warm water in the eastern equatorial
Pacific Ocean
– Linked to global weather anomalies
– 2 to 5 year recurrence
• La Niña events -- wind and temperature patterns
reversed of El Niño patterns
• Walker circulation
– Vertical and horizontal tropospheric flow in the
equatorial Pacific that controls areas of heavy
rainfall
The “Normal” Walker Circulation (no El Niño conditions)
ENSO (El Niño/Southern Oscillation) events -oscillations between El Niño and La Niña
conditions
• ENSO results in global teleconnection
patterns (weather effects far from the
equatorial Pacific)
El Niño SST Anomalies
La Niña SST Anomalies
The Southern Oscillation Index
The PDO Index
Warmer waters in western tropical Pacific
Cooler waters in western tropical Pacific