Monthly departures from average surface pressure (1981-2010) in Darwin, Australia, relative to average departures in Tahiti. Positive values indicate a strong Walker Circulation, which occurs during La Niña. Negative values indicate a weak or disrupted Walker Circulation, which occurs during El Niño. Data from NOAA CPC.
Sea level pressure patterns during winters when the Southern Oscillation Index was positive (top) or negative (bottom). During La Niña (positive SOI), the pressure is higher than average (red) over the central Pacific, and lower than average (gray) over Australia. During El Niño, the SOI is negative, and the anomalies are reversed. Data from NOAA PSL.