The Pacific El Niño Phenomenon and the Atlantic Circulation

Abstract

Atmospheric–oceanic departure patterns in the tropical Atlantic and eastern Pacific associated with the Ecuador/Peru El Niñno and its antithesis are studied on the basis of long-term ship observations during 1911–71. Departure maps of sea level pressure (SLP), wind speed and sea surface temperature (SST) in March/April are presented for composite of ten extreme years of either regime. The evolution of SLP anomalies from the preceding to the event year is traced for large ocean areas. Gaussian and binomial probabilities are calculated as a measure of the importance of SLP departures. El Niño years are characterized by abnormally low SLP over the eastern South Pacific and positive SLP anomalies over the Atlantic; winds are weaker than normal over the Pacific, and stronger over the Atlantic; SST anomalies are positive in the Pacific, but negative in a band of the Atlantic extending from West Africa toward the South American coast. During counter-El Niño years, departure patterns am approximately inverse. Departure patterns during droughts and floods in northeast Brazil are similar to, but not identical with, the ones obtained by stratification with regard to the Ecuador/Peru El Niño and counter-El Niño, respectively. El Niño has a tendency to occur in a series of successive years, as does its antithesis. The seasonal development of departure patterns in initial El Niñ years (a single event or the first in a sequence) and sequential years (the second or later years in a sequence) differs, in that an initial El Niño tends to be heralded by anomalously high SLP and cold waters in the eastern South Pacific. In sequential years, the pre-season exhibits anomalies of the same sign as the peak season. Statistical probabilities indicate the possibility of foreshadowing initial El Niño from SLP anomalies in the Atlantic.