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Gilbert Walker used correlations among atmospheric observations to identify large-scale relationships, notably the opposed tropical pressure variations called the Southern Oscillation.^[ref]

The Walker circulation is named in his honour, but a pressure index and a circulation mechanism are different objects. Correlation identifies a recurring pattern; interpreting it requires the coupled physics of trade winds, ocean temperature and atmospheric motion. That distinction remains useful when a climate index is used for prediction.

The Southern Oscillation is a large-scale variation of tropical atmospheric pressure and circulation, associated with the coupled El Niño–Southern Oscillation. Its pressure pattern is commonly monitored through the contrast between the eastern and western tropical Pacific.

Changes in trade winds affect ocean upwelling and surface temperatures; those temperature changes can feed back on convection and pressure. The atmospheric oscillation and the oceanic El Niño pattern are therefore connected components of a coupled system. A pressure index is an observational summary, not a complete dynamical model of the phenomenon.