How Do Trade Winds Work? The Belts That Built Sailing Routes

The equator receives far more solar energy per square metre than the poles, so air there is heated, expands, rises and flows away aloft towards higher latitudes. If the earth did not rotate, that would produce one enormous circulation per hemisphere, rising at the equator and sinking at the pole. Rotation breaks it into three. Air rising at the equator cools and loses moisture as it travels poleward aloft, then sinks around twenty-five to thirty degrees of latitude, warming and drying as it descends, and returns towards the equator at the surface. That loop is the Hadley cell, and its surface return flow is the trade wind. The descending branch is why the great subtropical deserts, the Sahara, the Arabian, the Kalahari, the Atacama and the Australian interior, all sit at roughly the same latitudes in both hemispheres, and why the subtropical high pressure belts are permanent features on any climate map.

Air returning towards the equator from thirty degrees is moving into a region where the ground is travelling east faster, because a point on the equator completes a larger circle in the same twenty-four hours than a point further north or south. The air retains its original, slower eastward momentum and therefore falls behind the surface beneath it, appearing to be deflected to the west. The same effect expressed as a rule is the Coriolis effect, deflecting moving air to the right in the northern hemisphere and to the left in the southern, which turns a north-to-south flow into a north-easterly wind above the equator and a south-to-north flow into a south-easterly below it. The mirror-image deflection of poleward-moving air at higher latitudes produces the westerlies, the prevailing winds from the west that dominate temperate zones and that sailing ships used for the return leg.

Sailors mapped the belts long before anyone explained them, and the vocabulary survives:

• •The trade winds, north-easterly in the northern hemisphere and south-easterly in the southern, steady enough to plan a voyage around
• •The doldrums, the intertropical convergence zone where the two trade belts meet, air rises and surface winds are light and variable, which becalmed sailing ships for weeks and produces the heavy convective rainfall of the equatorial belt
• •The horse latitudes, the subtropical high pressure zones at around thirty degrees where the descending air gives clear skies and weak winds, reportedly named for the horses thrown overboard when voyages ran long and water ran short
• •The westerlies of the middle latitudes, and within them the roaring forties, furious fifties and screaming sixties of the southern ocean, where almost no land interrupts the flow
• •The polar easterlies, weaker and less regular, completing the three-cell pattern

The trades do far more than push ships. Dragging steadily westward across tropical oceans, they push surface water with them and drive the great equatorial currents, piling warm water up on the western side of each ocean basin and causing cold water to well up on the eastern side, which is why the sea off Peru and off Namibia is cold and biologically rich while the same latitude in the west Pacific is a warm pool. That arrangement is the baseline that El Nino disturbs: when the trades weaken, the piled-up warm water sloshes back east, upwelling fails, fisheries collapse and rainfall patterns shift across half the planet. The trades also carry dust from the Sahara across the Atlantic in quantities that fertilise the Amazon basin and suppress hurricane formation, and they steer tropical cyclones westward, which is why Atlantic hurricanes form off Africa and travel towards the Caribbean rather than the other way. The belts shift north and south with the seasons as the zone of maximum heating follows the sun, which is the mechanism behind tropical wet and dry seasons.

Trade winds are the surface return flow of the Hadley cell, in which air rises at the equator, travels poleward aloft, sinks at around thirty degrees leaving the subtropical deserts beneath it, and flows back towards the equator. Rotation deflects that return flow westward, making it north-easterly above the equator and south-easterly below, with the doldrums between them and the westerlies beyond. The same winds drive the equatorial ocean currents and the upwelling whose failure is El Nino.