The Walker Circulation is a concept in atmospheric science, defined as a large-scale, thermally driven, east-west (zonal) atmospheric circulation pattern in the tropics, also known as the Walker cell. It is named after Sir Gilbert Walker, a British mathematician and meteorologist who served as Director General of Observatories in India in the early twentieth century. Walker discovered the Southern Oscillation (SO) in the 1920s while studying the Indian monsoon, identifying a relationship between pressure changes across the tropical Pacific and rainfall variations. The term Walker Circulation was first used in 1969 by Jacob Bjerknes to describe the atmospheric component of this system.
The mechanism is a closed loop driven by the temperature contrast between the warm western Pacific and the cooler eastern Pacific. Warm, moist air rises over the western Pacific, particularly near the Maritime Continent, creating a low-pressure area and heavy rainfall. This air travels eastward at high altitudes, cools, and then sinks over the eastern Pacific, forming a high-pressure area and drier conditions. The air completes the loop by flowing back toward the west near the surface as trade winds.
The circulation is intrinsically connected to the El Niño-Southern Oscillation (ENSO) system, of which it is the atmospheric half. A weaker Walker Circulation corresponds to El Niño conditions, while a stronger circulation corresponds to La Niña conditions. It also connects to the Indian Ocean Dipole (IOD) and the north-south Hadley Circulation.
In recent decades, the Walker Circulation has shown an unexpected strengthening since the late 1990s, which is associated with a warming trend in the Atlantic sea surface temperature and a cooling in the eastern Pacific. However, climate models project that global warming will ultimately cause the circulation to become weaker (more El Niño-like) in the long run.