Biological and chemical consequences of the 1997-1998 El Nino in central California waters

The physical, chemical and biological perturbations in central California waters associated with the strong 19971998 El Nino are described and explained on the basis of time series collected from ships, moorings, tide gauges and satellites. The evolution of El Nino off California closely followed the pattern observed in the tropical Pacific. In June 1997 an anomalous influx of warm southerly waters, with weak signatures on coastal sea level and thermocline depth, marked the onset of El Nino in central California. The timing was consistent with propagation from the tropics via the equatorial and coastal wave-guide. By late 1997, the classical stratified ocean condition with a deep thermocline, high sea level, and warm sea surface temperature (SST) commonly associated with El Nino dominated the coastal zone. During the first half of 1998 the core of the California Current, which is normally detected several hundred kilometers from shore as a river of low salinity, low nutrient water, was hugging the coast. High nutrient, productive waters that occur in a north-south band from the coast to approximately 200 km offshore during cool years disappeared during El Nino. The nitrate in surface waters was less than 20% of normal and new production was reduced by close to 70%. The La Nina recovery phase began in the fall of 1998 when SSTs dropped below normal, and ocean productivity rebounded to higher than normal levels. The reduction in coastal California primary productivity associated with El Nino was estimated to be 50 million metric tons of carbon (5 X 10(13)gC). This reduction certainly had deleterious effects on zooplankton, fish, and marine mammals. The 1992-1993 El Nino was more moderate than the 1997-1998 event, but because its duration was longer, its overall chemical and biological impact may have been comparable. How strongly the ecosystem responds to El Nino appears related to the longer-term background climatic state of the Pacific Ocean. The 1982-1983 and 1992-1993 El Ninos occurred during the warm phase of the Pacific Decadal Oscillation (PDO). The PDO may have changed sign during the 1997-1998 El Nino, resulting in weaker ecological effects than would otherwise have been predicted based on the strength of the temperature anomaly. (C) 2002 Elsevier Science Ltd. All rights reserved.