AS 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
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EAS 100 Global atmospheric circulation Key points global distribution of solar energy buoyancy of air convection convergence and divergence Hadley circulation.
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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