Walker Circulation

The Walker circulation is an atmospheric system of air flow in the equatorial Pacific Ocean. The trade winds across the tropical Pacific flow from east to west: Air rises above the warm waters of the western Pacific, flows eastward at high altitudes, and descends over the eastern Pacific. A weaker Walker circulation (in the reverse direction) occurs over the Indian Ocean.

Sir Gilbert Walker assumed the post of director-general of the observatory in India following catastrophic famines in the late 1800s, that resulted from a general failure of the South Asian monsoon. In an effort to predict the monsoons, Walker undertook an investigation into the regional climate system. Over time, he recognized that the monsoonal system extended to a panoceanic scale. Walker observed that an inverse relation of atmospheric pressures at sea level generally existed between the two sides of the Pacific Ocean.

A high-pressure phase in South America was usually accompanied by low pressure in the western Pacific and vice versa—the Southern Oscillation (SO). The generally accepted measure of the SO is the inverse relationship between surface air pressure at Darwin, Australia, and Tahiti (stations used by Walker); the SO is normally identified by the SO Index (SOI), that is, the difference in atmospheric pressure at sea level between these stations (Tahiti minus Darwin). The greater the SOI, the greater the intensity of the trade winds. Historically, the SO has exhibited a more or less cyclical pattern, in that it weakens or reverses every few years.

Jacob Bjerknes: Explaining the Mechanism On the basis of data collection initiated during the International Geophysical Year in 1957 to 1958, in the 1960s, Jacob Bjerknes of the University of California described the general nature of the mechanism linking the system. He extended the horizontal picture of the SO vertically by theorizing that to complete the system of the trade winds and atmospheric air pressure, there needed to be a countercirculation of air from west to east at high altitudes, descending over the eastern Pacific.

Atmospheric circulation is intimately coupled with the movement of water in the tropical Pacific. At the surface, the trade winds initiate a westward flow of surface water across the Pacific Ocean, producing an increase of sea level in the western Pacific of approximately 40 cm. The equatorial heating of this water produces high seasurface temperatures (SSTs) in oceanic waters near Indonesia. The resulting low atmospheric pressure and evaporation fuels the pan-Pacific upper-atmospheric circulation characterized by convection (low atmospheric pressure) in the west and subsidence (high atmospheric pressure) in the east. This is termed the Walker circulation in honor of Sir Gilbert.

The Walker circulation is closely connected to oceanic upwelling off the coast of South America. Fluctuations in the circulation are closely linked to El Niño and La Niña events—together termed the El Niño/SO system (ENSO). A weakening or reversal of the Walker circulation is closely linked with the El Niño phenomenon, with warmer-than-average SSTs in the eastern Pacific as upwelling diminishes. In contrast, the opposite phase, a particularly strong Walker circulation, produces a La Niña event, with cooler SSTs caused by increased upwelling. Interannular switches in the dipole are linked to global-scale changes in patterns of weather. Several explanations for the variation in the Walker circulation have been hypothesized, but the nature of the mechanisms initiating the change in phase has not been fully identified.

Evidence suggests that the Walker circulation may have been weakening since the mid-19th century. However, there is a high degree of uncertainty concerning the potential effects of climate change on the Walker circulation. Transient warming may dominate before the ocean has had a chance to reach equilibrium. In the short term, warming may occur more quickly in the western Pacific, thereby enhancing the circulation. In contrast, atmospheric models have indicated that climate change will, as part of a weakening of the entire tropical circulation, lead to a general decrease in the strength of the Walker circulation. The specific mechanisms involved are not fully known, and projections remain rather speculative and are a focus of intense research.