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/m 2 ) 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 temperature PV=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 view What’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 and subpolar low Subtropical highs

ITCZ Africa Westerlies and subpolar low 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