Bjerknes feedback

The tropical Pacific normally has warmer surface water in the west and cooler water in the east. Easterly trade winds push surface water westward, while upwelling brings cooler subsurface water toward the surface in the eastern Pacific. This east–west temperature contrast is linked to the Walker circulation, a large-scale circulation with rising air and rainfall over the warmer western Pacific and sinking air over the cooler east. The Bjerknes feedback describes how shifts in this coupled system can reinforce one another.

During El Niño development, surface waters in the central or eastern equatorial Pacific become unusually warm, or the trade winds weaken, or both. The warmer water shifts convection and rainfall eastward, changing the atmospheric pressure pattern and further weakening the easterly trades. Weaker trades reduce the westward transport of warm surface water and suppress some of the normal upwelling of cold water in the east. That allows eastern and central Pacific surface temperatures to rise further, reinforcing the initial change. The feedback is positive in the physical sense: it amplifies a perturbation rather than indicating that the resulting conditions are beneficial.

During La Niña, the broad sequence runs in the opposite direction: stronger trade winds tend to increase the contrast between warm western and cool eastern Pacific waters, which supports stronger east-to-west circulation and further cooling in the central and eastern Pacific. The feedback helps explain growth of warm and cool ENSO anomalies, but it is not a complete account of the cycle. Ocean processes operating with delays, including equatorial waves and changes in ocean heat content, can eventually weaken or reverse an event. Individual episodes vary in strength, duration, and spatial pattern, and the relative roles of the processes are not identical in every event.

Jacob Bjerknes connected the Southern Oscillation—an atmospheric pressure pattern across the tropical Pacific—with El Niño and changes in Pacific ocean temperatures. His 1969 work proposed that ocean and atmosphere interact as parts of one system. The feedback named for him has since become a central idea in explaining ENSO, while research continues to examine the processes that initiate events and bring them to an end.