Some Statistical and Synoptic Characteristics Associated with El Niño

A 49-yr time series of sea surface temperatures along the Peruvian coast is analyzed to find antecedent and subsequent indicators of El Nino (abnormally warm water) and its inverse. Precursory signs show up in certain statistics, collected seasons before the event, and these should be useful in forecasting the occurrence or nonoccurrence of this economically important phenomenon. The macroscale processes implied by the above data are explored with the help of Northern Hemisphere pressure patterns and geostrophic wind profiles. There is a strengthening of the North Pacific winter westerlies when El Nino occurs as suggested by Bjerknes, i.e., through momentum transports from a variable Hadley cell; however, El Nino appears to be associated with an appreciably weakened Pacific high over the eastern third of the North Pacific during the preceding year. This lends support to the theory that generation of El Nino is a long-term, large-scale process in which reduced wind stress permits an accelerated equatorial countercurrent and diminished equatorial upwelling.