EAS 100 Global atmospheric circulation

Key points• global distribution of solar energy • buoyancy of air• convection• convergence and divergence• Hadley circulation• Coriolis effect• Intertropical Convergence Zone• westerlies and trades• subtropical highs

• Incoming solar energy flux (W/m2) decreases with latitude

• Energy is continuously redistributed from regions of surplus energy to ones of net deficit; this is the origin of atmospheric circulation

• Heated air tends to rise (buoyancy), since volume (and hence density) are related to temperaturePV=nRT (Ideal gas Law)

• Greatest heating occurs in the tropics, leading to rising air masses

• The decreased mass of this air results in low atmospheric pressure

• Air flows from zones of high to low pressure

Hadley circulation: convective cells ultimately fuelled by the excess solar energy received in the tropics (InterTropical Convergence Zone - ITCZ)

Idealized viewWhat’s missing?

The Earth rotates

Where drag and friction are minimal, and over large distances, the Earth’s rotation imparts a deviation from linear trajectories of movement: the Coriolis effect

Putting it all together

ITCZ

North America

Westerlies andsubpolar low

Subtropical highs

ITCZ

Africa

Westerlies andsubpolar low

Subtropical highs

Subtropical highs

A final consideration: seasonal migration of the ITCZ due to the tilt of the Earth’s axis (23.5˚ from vertical)

The ITCZ migrates 10-20˚ north and south of the Equator on a seasonal basis, closely reflecting the zone of maximum solar energy receipt, and dictating important weather phenomena (monsoons, etc.)

SUMMARY

• The driving force of atmospheric circulation is the global distribution of solar energy

• Due to the incident angle of incoming solar radiation, there is more solar energy in the tropics, resulting in an equator-to-pole temperature gradient

• High temperatures produce buoyant air and hence low pressure

• Air flows from high to low pressure, resulting in winds

• Winds are subjected to the Coriolis effect from Earth’s rotation

• Global circulation redistributes available thermal energy from hot to cold areas, thus providing a negative feedback against runaway heating of the tropics, and cooling of the poles