Figure 6 – Walker circulation during La Niña and El Niño

During La Niña, or reverse El Niño, the Walker Circulation is enhanced and became very pronounced, with well-defined rising and sinking branches (figure 6b). While the cells in Pacific and Atlantic Oceans intensify, the Indian Ocean cell weakens.

Impacts on world climate

The Walker Circulation regulates global exchange of momentum, heat, and water vapor within the tropics via massive overturning motions. In doing so, it plays an important role in the balance of atmospheric energy in the equatorial region and in determining the characteristics of weather and climate in the tropics.

The strongest atmospheric impacts associated with the fluctuations of the Walker Circulation are found over tropical and subtropical regions around the Pacific Rim. During an El Niño, the weakening Walker Circulation causes widespread drought in Indonesia/maritime continent, drought in northeastern Brazil, severe floods in Peru and Ecuador, and in south-eastern Brazil and northern Argentina. During a La Niña, the Walker Circulation intensifies and leads to rainfall anomalies with reverse sign compared to El Niño.

The Walker Circulation also represents the fundamental link between the changes in sea surface temperature in the eastern Pacific and the variability of the Asian–Australian monsoon. The mechanisms that are responsible for the interactions between the monsoon and El Niño Southern Oscillation have been attributed, in part, to the changes in the Walker Circulation. During El Niño, Walker Circulation is weakened and shifted eastward owing to reduced east– west sea surface temperature gradient across the Pacific Ocean. This suppresses broad-scale convection over the western Pacific and eastern Indian Ocean and leads to weaker South Asian monsoon.

Walker circulation interacts with the Hadley cell3 in the form of an inverse variation between the two circulations. When the cold water belt along the Equator is well developed, the air above it will be too cold and heavy to join the ascending motion in the Hadley circulations. Instead, the equatorial air flows westward between the Hadley circulations of the two hemispheres to the warm west Pacific. These changes are the causes for severe weather and climate anomalies in the Asian–Pacific–American regions.