In 1904 the British climatologistGilbert Walkerset out to determine the connections between the Asianmonsoonand other climatic fluctuations around the globe in an effort to predict unusual monsoon years that bringdroughtandfamineto the Asian sector. Unaware of any connection to El Niño, he discovered that atmospheric pressure fluctuated over the tropical Indo-Pacific region, which he termed the Southern Oscillation. During years of reduced rainfall over northern Australia and Indonesia, the pressure in that region (e.g., at what are now Darwin and Jakarta) was anomalously high and wind patterns were altered. Simultaneously, in the eastern South Pacific pressures were unusually low, negatively correlated with those at Darwin and Jakarta. A Southern Oscillation Index, based on pressure differences between the two regions (east minus west), showed low, negative values at such times, which were termed the “low phase” of the Southern Oscillation. During more normal “high-phase” years, the pressures were low over Indonesia and high in the eastern Pacific, with high, positive values of the SOI. In papers published during the 1920s and ’30s, Walker gave statistical evidence for widespread climaticanomaliesaround the globe being associated with the Southern Oscillation pressure “seesaw.”

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