Walker circulation

Walker circulation
Name	Walker circulation
Caption	Conceptual diagram of the Walker circulation in the equatorial Pacific.
Namedafter	Sir Gilbert Walker
Yearnamed	1920s
Relatedphenomena	El Niño–Southern Oscillation, Madden–Julian oscillation, Pacific decadal oscillation

Walker circulation. It is a conceptual model of the large-scale atmospheric circulation of air in the tropics over the Pacific Ocean. This east-west overturning circulation is driven by differential heating and is a fundamental component of the global climate system. Its strength and position are primary drivers of the El Niño–Southern Oscillation, making it critical for seasonal climate prediction worldwide.

Overview

The Walker circulation is characterized by rising air over the warm maritime continent of the western Pacific, near Indonesia and New Guinea. This ascending branch flows eastward at high altitudes before sinking over the cooler eastern Pacific, near the coast of South America and the Galápagos Islands. The completed loop involves surface westerly winds, known as the Pacific trade winds, flowing back across the ocean basin. This circulation is a key part of the tropical atmospheric circulation and interacts closely with ocean currents in a coupled system. Its behavior influences rainfall patterns from Australia to the Andes and sea surface temperatures across the Pacific basin.

Discovery and historical context

The circulation is named for Sir Gilbert Walker, a British physicist and statistician who served as director-general of observatories in India. In the 1920s, while attempting to predict the Indian monsoon, Walker identified a vast, seesawing pattern of atmospheric pressure between the Pacific and Indian Oceans, which he termed the "Southern Oscillation." Although he described the statistical connections, the physical three-dimensional circulation cell was later elucidated by Jacob Bjerknes in 1969. Bjerknes, a prominent meteorologist from the University of California, Los Angeles, linked Walker's Southern Oscillation to ocean temperature anomalies, formally connecting it to the El Niño phenomenon and coining the term "Walker circulation." This work built upon earlier oceanographic observations by scientists like Alexander von Humboldt and Charles Darwin.

Mechanism and dynamics

The primary engine of the Walker circulation is the stark contrast in sea surface temperature between the warm western Pacific warm pool and the cooler eastern Pacific cold tongue. The warm pool heats the air above it, causing it to rise through convection, creating a region of low pressure, abundant clouds, and heavy precipitation. Aloft, this air diverges and travels eastward. Upon reaching the cooler eastern Pacific, the air cools, becomes denser, and subsides, creating a region of high pressure and arid conditions, such as those found in the Atacama Desert. The pressure gradient between the eastern high and western low drives the surface trade winds from east to west, which in turn push warm water westward, reinforcing the temperature gradient. This tight ocean-atmosphere coupling is a classic example of a positive feedback loop studied at institutions like the National Oceanic and Atmospheric Administration and the Met Office.

Influence on global climate

Variations in the Walker circulation have profound global climate impacts. Its periodic breakdown and reversal are the essence of the El Niño phase of ENSO, which redistributes heat and rainfall worldwide. During El Niño, weakened trade winds allow warm water to slosh eastward, shifting convection to the central Pacific and causing drought in Southeast Asia and Australia while bringing floods to Peru and California. The complementary La Niña phase features an intensified circulation. These shifts affect the Asian monsoon, the frequency of Atlantic hurricanes, and even winter weather patterns over North America through teleconnections. Events like the 1997–98 El Niño event and the 2010–12 La Niña event demonstrated its power to disrupt global agriculture, fisheries, and economies.

Related climate patterns

The Walker circulation is intrinsically linked to several other major climate modes. It is the atmospheric component of the coupled El Niño–Southern Oscillation system. Its variability also interacts with the Indian Ocean Dipole, which features a similar east-west atmospheric circulation over the Indian Ocean. On shorter timescales, it modulates and is modulated by the intraseasonal Madden–Julian oscillation, a traveling pattern of convection. Longer-term fluctuations in Pacific background state are tied to the Pacific decadal oscillation and the Interdecadal Pacific Oscillation. Furthermore, the Atlantic Ocean has its own analogous, though weaker, zonal circulation sometimes referred to as the Atlantic Niño.

Variability and long-term changes

The Walker circulation exhibits significant variability on interannual, decadal, and longer timescales. While its interannual changes are dominated by ENSO, longer-term trends are a major focus of contemporary climate research. Climate models from the Intergovernmental Panel on Climate Change project a weakening and potential eastward shift of the circulation under anthropogenic global warming, though observed 20th-century trends show strengthening in some analyses. These changes could alter the frequency and intensity of El Niño events, with cascading effects on extreme weather. The interaction of this trend with natural cycles like the Pacific decadal oscillation remains an active area of study at research centers like the Scripps Institution of Oceanography and the Commonwealth Scientific and Industrial Research Organisation.

Category:Atmospheric dynamics Category:Climatology Category:Oceanography Category:Meteorology