Trade winds pile the Pacific about 60 centimetres higher on its western side
A giant loop of air spans the equatorial Pacific: rising over warm, wet Indonesia, sinking over the cool eastern ocean, and returning as trade winds along the surface. It pushes enough warm water west to raise the sea there by some 60 centimetres. When the loop falters, El Niño follows.
The Walker circulation is a model of east-west air flow in the tropical lower atmosphere, driven by uneven heating of land and sea. In the Pacific, high pressure over the eastern ocean and low pressure over Indonesia push surface easterlies westward. Those winds drag sun-warmed water with them, so the western Pacific is warm, humid and stormy, full of typhoons and thunderstorms, while cold, nutrient-rich water wells up off Peru and Ecuador and feeds rich fisheries.
The system reinforces itself. Easterly winds cool the eastern sea surface, which sharpens the temperature contrast across the basin and strengthens the winds further; the stronger winds lift the thermocline and bring up still more cold water. Jacob Bjerknes proposed this ocean-atmosphere feedback. When the circulation weakens or stalls, the eastern Pacific warms above normal and El Niño sets in; when it strengthens, extra upwelling chills the surface into La Niña. The Atlantic also has surface easterlies, while the Indian Ocean gets westerlies in northern summer, so its cool water lies in the west instead.
The name honours Gilbert Walker, a Cambridge applied mathematician who became director-general of observatories in India in 1904, after monsoon failure caused severe famine in 1899. Over fifteen years of analysing weather data he described a great seesaw of air pressure between the Indian and Pacific Oceans, the Southern Oscillation. He split the year into four seasons because relationships changed across them, argued against the idea that sunspots drove temperature, and published even his negative results to steer others away from false leads. Bjerknes coined the term Walker Circulation in 1969.
Whether the circulation is changing is contested. A study published in Nature in May 2006 concluded it had been slowing since the mid-19th century, likely because of global warming. A 2011 analysis from the Twentieth Century Reanalysis Project found instead that, apart from El Niño and La Niña swings, its speed and direction held steady from 1871 to 2008.
Source: Walker circulation
