Walker circulation

The Walker circulation is a simplified model of tropical air movement in the lower atmosphere, often called the Walker cell. It describes how air parcels move in a closed loop along east-west (zonal) and vertical paths. This pattern, which generally matches real-world observations, arises from uneven heating between oceans and landmasses. While the model focuses on zonal and vertical motion, the tropical atmosphere also experiences significant north-south (meridional) flow, such as in the Hadley circulation.

This circulation is driven by a pressure gradient: high pressure sits over the eastern Pacific Ocean, while low pressure dominates near Indonesia. In the tropical Indian, Pacific, and Atlantic basins, the Walker circulation produces westerly surface winds during the Northern Hemisphere summer in the Indian Ocean, and easterly winds in the Pacific and Atlantic. These wind patterns create stark temperature differences across the three oceans. In northern summer, the eastern equatorial Pacific and Atlantic have cool surface waters, while the western Indian Ocean is the only region with cooler surface temperatures. These temperature variations reflect changes in the depth of the thermocline.

The Walker circulation changes over time alongside shifts in surface temperature. Some changes are externally driven, like the seasonal movement of the sun into the Northern Hemisphere. Others stem from coupled ocean-atmosphere feedback: for instance, easterly winds cool the sea surface in the east, strengthening the east-west temperature contrast and intensifying the easterlies. These stronger easterlies then boost equatorial upwelling and lift the thermocline in the east, amplifying the initial cooling. From an ocean perspective, easterly winds create the equatorial cold tongue. If Earth’s climate were symmetric around the equator, cross-equatorial winds would disappear, and the cold tongue would be much weaker and have a very different east-west structure.

The term "Walker circulation" honors Gilbert Walker, who discovered the Southern Oscillation in the early 1900s. It was first used in 1969 by meteorologist Jacob Bjerknes to describe his proposed east-west overturning circulation near the equator in the Pacific. Bjerknes identified a region of sinking air, which he called the arid zone, bordered to the north and south by the cloud bands of the Inter-Tropical Convergence Zones. He suggested these bands were the equatorial edges of the Northern and Southern Hemisphere Hadley circulations.

As originally described, the Walker circulation weakens during El Niño events, when the Pacific temperature gradient and easterly winds both diminish, and strengthens during La Niña events, when the opposite occurs. In modern usage, the term may cover a much broader range of latitudes and longitudes, and can include more than one overturning cell.

Gilbert Walker was an accomplished applied mathematician at Cambridge University when he became director-general of observatories in India in 1904. There, he studied the Indian Ocean monsoon, whose failure in 1899 had caused severe famine. Analyzing vast weather data from India and worldwide over fifteen years, he published the first descriptions of the large-scale seesaw in atmospheric pressure between the Indian and Pacific Oceans, and its links to temperature and rainfall across much of the tropics, including India. He also collaborated with the Indian Meteorological Department, especially in connecting the monsoon to the Southern Oscillation. He was made a Companion of the Order of the Star of India in 1911.

Walker found that a one-year time scale was unsuitable because geographic relationships changed with the seasons. So he divided his analysis into four periods: December–February, March–May, June–August, and September–November. He then selected "centers of action"—areas like the Indian Peninsula—located in the heart of regions with permanent or seasonal high and low pressure. He also added points where rainfall, wind, or temperature were key controls. He examined relationships between summer and winter pressure and rainfall, later extending this to spring and autumn.

He concluded that temperature variations are generally governed by pressure and rainfall changes. Earlier, some had suggested sunspots caused temperature shifts, but Walker argued against this by showing monthly correlations between sunspots and temperature, winds, cloud cover, and rain that were inconsistent. He made a point of publishing all his correlation findings—both significant and insignificant—to discourage researchers from chasing nonexistent relationships.