Walker circulation

The Walker circulation arises from the east-west sea surface temperature (SST) gradient across the tropical Pacific, with warmer waters in the west and cooler waters in the east. This gradient drives deep atmospheric convection over the warm western Pacific, where air rises, and subsidence over the cooler eastern Pacific, completing a zonal overturning cell. The low-level flow is manifested as the trade winds, which blow from east to west, while the upper-level return flow is westerly. The circulation is closely coupled to the ocean through the thermocline and upwelling, forming the basis of the El Niño–Southern Oscillation (ENSO) system.¹

Fluctuations in the Walker circulation are central to ENSO. During El Niño events, the SST gradient weakens, causing the rising branch to shift eastward and the trade winds to slacken, leading to widespread changes in precipitation and atmospheric circulation. Conversely, La Niña strengthens the gradient and the circulation. These variations affect weather patterns globally, including monsoons, tropical cyclones, and mid-latitude storm tracks. The Walker circulation also interacts with the Indian Ocean and Atlantic Ocean through atmospheric teleconnections.

The circulation is named after Sir Gilbert Walker, a British meteorologist who, in the early 20th century, identified the Southern Oscillation—a seesaw in atmospheric pressure between the eastern and western Pacific—while studying the Indian monsoon. The three-dimensional structure was later clarified by Jacob Bjerknes in the 1960s, who linked the circulation to ENSO. Subsequent research has used satellite data and climate models to quantify its variability and its response to global warming.²

Beyond the Pacific, the Walker circulation has counterparts in other ocean basins, such as the Indian Ocean dipole and the Atlantic zonal circulation. The circulation's strength has varied over geological timescales, affecting paleoclimate. In recent decades, some studies suggest a weakening of the Walker circulation under global warming, though projections remain uncertain. The circulation also influences the distribution of marine ecosystems and the transport of atmospheric pollutants.³

El Niño–Southern Oscillation (ENSO)

A coupled ocean-atmosphere phenomenon involving periodic changes in Pacific SST and atmospheric pressure.

Trade winds

Persistent easterly surface winds in the tropics that drive the low-level branch of the Walker circulation.

Thermocline

The layer in the ocean where temperature changes rapidly with depth, separating warm surface water from colder deep water.