Physical and biological effects of Los Niños in the eastern tropical Pacific, 1986–1989

Abstract

The eastern tropical Pacific Ocean was surveyed in August–November 1986, 1987, 1988 and 1989 as part of a long-term program to monitor dolphin stocks. Temperature, salinity, chlorophyll and nutrients were monitored to help interpret variability in dolphin stock estimates. The four surveys reveal major environmental changes during the moderate 1987 El Niño and the cold La Niña episode that followed in 1988. During the “onset” phase of El Niño in fall 1986, surface temperatures were up to 1.5°C above normal in equatorial water, but near normal in tropical water north of the equator. The equatorial thermocline ridge was deepened by 10–30 m. During the “mature” phase of El Niño in fall 1987, surface temperatures anomalies were up to +2.5°C in equatorial water and about +1°C in tropical water. Thermocline topography was anomalously flat. Surface chlorophyll and nutrient concentrations declined by 11–48% compared to 1986, with the greatest declines occurring in coastal and equatorial upwelling systems and along the countercurrent thermocline ridge. During La Niña 1988, equatorial surface temperatures were up to 2.5°C below normal and the equatorial thermocline ridge was 10–50 m more shallow than normal. Chlorophyll and nitrate concentrations increased by 58–65% compared to 1987. In 1989, surface temperature was within ±1°C of normal throughout the study area and chlorophyll concentrations were similar to those observed in 1986. Changes in nutrient availability and biological productivity during the El Niño/La Niña cycle were caused by variations in both the rate of wind-driven upwelling and in the nutrient content of subsurface water entrained by upwelling.