Southern Oscillation
The Southern Oscillation refers to an oscillation in air pressure between the southeastern and southwestern Pacific waters. When the eastern Pacific waters increase in temperature (an El Nino event), atmospheric pressure rises in the western Pacific and drops in the east. This pressure drop is accompanied by a weakening of the easterly Trade Winds. Together with El Nino, this phenomenon is known as ENSO, or El Nino-Southern Oscillation. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI), the normalized surface air pressure difference between Darwin, Australia and Tahiti.