General circulation

General circulation of the atmosphere is the pattern of planetary winds. It depends on how heating varies with latitude, the pressure belts, the migration of belts with the sun, the spread of continents and oceans, and the earth's rotation. It also drives slow ocean currents, and the oceans in turn feed energy and water vapour back to the air.

The three cells:

• Hadley cell (tropics): at the Inter Tropical Convergence Zone (ITCZ) near the equator, strong heating makes air rise by convection, creating low pressure. The air climbs to about 14 km, moves poleward, piles up around 30° N and S, cools and sinks, forming the subtropical highs. At the surface, air flows back towards the equator as the easterlies (trade winds), which meet at the ITCZ.
• Ferrel cell (middle latitudes): sinking cold air from the poles and rising warm air from the subtropical highs; surface winds are the westerlies.
• Polar cell: cold, dense air sinks at the poles and flows towards middle latitudes as the polar easterlies.
• Together they move heat from low to high latitudes, which keeps the circulation going.

El Niño and ENSO. The warming and cooling of the Pacific matter most.

• Sometimes warm water from the central Pacific drifts towards South America and replaces the cool Peruvian current. Warm water off Peru is called El Niño.
• It goes with pressure changes between the central Pacific and Australia, called the southern oscillation. Together they are ENSO.
• In strong ENSO years: heavy rain on the normally arid west coast of South America, drought in Australia and sometimes in India, and floods in China.
• ENSO is closely watched and used for long-range forecasting.

Example (new): when meteorologists announce that an El Niño is developing in the Pacific, farmers in India may be advised to plan for the risk of a weaker monsoon, even though the event is thousands of kilometres away.