Figure 2. Two indices of oceanic and atmospheric conditions in the tropical Pacific that reveal the incidence of El Niño and La Niña conditions throughout the last 120 years. The Southern Oscillation Index (SOI), in blue, is a measure of the difference in barometric pressure between Tahiti and Australia. The NIÑO3 index, in black, indicates changes in sea-surface temperature in the eastern Pacific Ocean. As plotted here, peaks in either index indicate El Niño (i.e., warm) episodes; deep valleys are times of (cold) La Niñas. The near-perfect correspondence between the two indices is a reflection of how closely the tropical atmosphere and ocean are tied together in these ENSO events. (From Learning to Predict Climate Variations Associated with El Niño and the Southern Oscillation. National Research Council, Washington, DC, 1996.)