GEU 0027: MeteorologyLecture 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.

• Because of earth’s rotation we have several circulation cells not just one per hemisphere

Global Circulation

• With the 3 cell structure of wind circulation and the combination of PGF and Coriolis, semi-permanent circulation patterns are established.

Three-cell Model

Semi-permanent structures in the atmosphere provide consistent wind patterns and breeding grounds for air masses.

Semi-permanent Pressure and Winds

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

• 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.

Intertropical Convergence Zone

• Equatorial Cumulus and Thunderstorms

ITCZ

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 FormationSharply varying pressure and

temperature differences create the

exaggerated situation shown.

Tightly packed isobars create stronger

winds aloft in the frontal region.

Polar JetWinds 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

• Higher Angular Momentum, Yields more zones?

Jupiter’s Bands

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 WavesKinking in the jet

stream occurs on

a cyclic basis.

Weather patterns

are also somewhat

cyclic.

Rossby CycleA 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