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GEU 0027: Meteorology
Lecture 10
Wind: Global Systems
Global Circulation
• In the absence of rotation, air would tend to flow
from the equator toward the poles.
Hot, less dense air
rising at the equator,
becomes denser as
it cools and descends
at the poles, traveling
back to tropical areas
to heat up again.
Global Circulation
• Because of earth’s rotation we have several
circulation cells not just one per hemisphere
Three-cell Model
• With the 3 cell structure of wind circulation and the
combination of PGF and Coriolis, semi-permanent
circulation patterns are established.
Semi-permanent Pressure and Winds
Semi-permanent structures in the atmosphere provide
consistent wind patterns and breeding grounds for
air masses.
Semi-Permanent Pressure (January)
500-mb streamline and isotherms in January
Stronger Winter PGF
Semi-Permanent Pressure (July)
500-mb streamline and isotherms in July
Weaker Summer PGF
Intertropical Convergence Zone
• ITCZ
• Hot equatorial air rises in convection.
• Air moves away from the equator toward the poles.
• Low Pressure results around the equator.
• A band of convective thunderstorms circles the
tropical areas of the globe.
ITCZ
• Equatorial Cumulus and Thunderstorms
India Monsoon Precipitation
Monsoons (dry)
Monsoons (wet)
Subtropical High and the ITCZ (Sahara)
Sahara Desert and the Sahel of Africa
Jet Locations
Jet stream locations
greatly affect
local and global
climate.
Jet Formation
Sharply varying pressure and
temperature differences create the
exaggerated situation shown.
Tightly packed isobars create stronger
winds aloft in the frontal region.
Polar Jet
Winds are Westerly
and parallel to the
frontal boundary.
This creates the
polar jet stream.
It is strongest in the
winter and weakest
in the summer.
Seasonal Polar Jet Changes
• Location and Velocity variations
What cause the jets?
•L = m v r
– r = distance from
rotational axis
– m = mass
– v = velocity
Fig. 4, p. 270
Jupiter’s Bands
• Higher Angular Momentum, Yields more zones?
Wind Jets
• Other jet formation mechanisms are less well known.
The Dishpan Experiment
• Uneven heating of the equator and poles of the earth.
• Rotation.
• Viscosity and turbulence.
Rossby Waves
Kinking in the jet
stream occurs on
a cyclic basis.
Weather patterns
are also somewhat
cyclic.
Rossby Cycle
A complete
Rossby cycle
observed over
~ 6 weeks.
Ocean Currents
Gulf Stream
Coastal Upwelling
Ekman spiral, Ekman layer, and Ekman transport
Normal South Pacific Condition
• Easterly “trade-winds” usually prevail and upwelling occurs
• When exceptionally strong this cooling is called a La Nina.
El Nino
• During an El Nino, pressure conditions (and winds) reverse.
• Extremely warm water and wind reversal affects weather.
Sea Surface
Temperatures
A warm water wave
migrates eastward
during and El Nino
as upwelling and
cooling is severely
diminished along
the western coast
of S. America.
Fig. 10-21b, p. 276
El Nino and La Nina events, and ENSO Cycle
• La Nina versus El Nino conditions over the past 60 years.
• The Y-axis is a parameter calculated from a combination of:
–
–
–
–
–
air temperature
water temperature
air pressure (sea-level)
wind speed and direction
cloud cover
Weather pattern changes during El Nino condition
Weather pattern changes during La Nina condition
Global hydrological impacts of El Nino
Pacific Decadal Oscillation (PDO)
North Atlantic Oscillation (NAO)
Fig. 10-26a, p. 280
Fig. 10-26b, p. 280