What is the Southern Oscillation?

It was the atmospheric part of ENSO - (called the "Southern Oscillation" or "SO"), that first attracted the attention of scientists.

Sir Gilbert Walker documented and named the SO in the 1930s. It is one of several persistent patterns of high and low pressures around the globe.

The clearest sign of the Southern Oscillation is the inverse relationship between surface air pressure at two sites: Darwin, Australia, and the South Pacific island of Tahiti. Over periods of a month or longer, higher pressure than normal at one site is almost always concurrent with lower pressure at the other, and vice versa. The pattern reverses every few years. It represents a "see-saw", a mass of air oscillating back and forth across the International Date Line in the tropics and subtropics.

Monthly values of the Southern Oscillation Index (SOI), a measure of changes in atmospheric pressures across the Pacific, and the 3-month mean (black line). SOI mean values: May SOI -0.3; March to May average -0.3.
What is plotted is the 5-month running mean SOI, from Jan 1950 (ie, Nov 1949 to Mar 1950 average) to Jan 2014 (Nov 2013 to Mar 2014).