When meteorologists look at the monthly or annual averages of pressure, wind speed, and temperature taken at observation stations located worldwide, and then subtract the local long-term mean values, they see certain recurrent spatial patterns. These patterns, called modes, are believed to be the signatures of distinctive dynamical interactions.
Modes are generally favored relative to other spatial patterns because they are reinforced by positive feedback. A familiar example is the El Niño-southern oscillation, the signature of the interactions between surface winds and ocean currents in the equatorial Pacific. In that case, abnormally warm, equatorial sea surface temperatures favor weak trade winds, which, in turn, favor warm sea surface temperatures. Notwithstanding that El Niño is a complicated, global pattern describing the deviations of sea surface temperature from their average values, it is well described by an index formed simply by averaging sea surface temperature deviations over the equatorial Pacific: Intervals of...
