Southern Oscillation

FIRST DESCRIBED EXTENSIVELY by British meteorologist Sir Gilbert T. Walker in the 1920s, the Southern Oscillation refers to the periodic exchange of mass across the equatorial Pacific that is recorded in sea level pressure fluctuations between the eastern and western Pacific. Under normal conditions in the tropical Pacific, surface high (low) pressure prevails in the eastern (western) Pacific, with the easterly trade winds dominating surface wind and ocean flow.

This pressure pattern, also known as the Walker circulation, tends to support rising air motions and convectional precipitation near eastern Australia, as well as sinking air motions and dry conditions near coastal northern Peru. Every two to seven years, this generalized atmospheric surface pressure pattern weakens as equatorial Pacific air pressure rises in the west and lowers in the east. This shift in the pressure field considerably weakens the trade winds and promotes the eastward movement of warm surface water across the tropical Pacific. The associated abnormal warming in the eastern Pacific is known as El Nino. Because the reversals in pressure and associated ocean temperature fluctuations are often simultaneous, this coupled climate variability between the tropical Pacific Ocean and atmosphere is often collectively referred to as the El Nino/Southern Oscillation (ENSO).