El Nino 1997-98 in the coastal waters of Southern California: A timeline of events

Timely detection of changes in the tropical Pacific and subsequent predicti ons that the west coast of North America would be impacted by a strong El N ino event in 1997 enabled CalCOFI to augment its time-series sampling to pr ovide monthly coverage of a subset of the sample grid. The 1997-98 El Nino event in California waters is described here ill terms of a preliminary tim eline of events. This improved description of the timing of changes in the CalCOFI region provides new insights into the types of changes that might b e expected during future events, and it also constitutes a natural experime nt which can be used to test hypotheses about the cause-and-effect mechanis ms linking changing physical structure with ecosystem impacts. Although som e effects attributed to El Nino were evident in the summer and fall of 1997 , the strongest occurred in winter 1997-98 and spring 1998. Advection and c hanges in ranges of mobile populations were consistent with the northward e xtension of the range of southern warm-water species of migratory game fish and plankton. However, the timeline observations were not consistent with predictions based upon a simple model of bottom-up trophic processes (chang es in physical structure, nutrients, phytoplankton, zooplankton, upper trop hic levels) as the mechanism linking changing physical and ecosystem struct ure. While not surprising, since few simple models have much skill in predi cting structure in natural ecosystems, this suggests that ecosystems manage rs will have to await a better understanding of cause-and-effect linkages b efore improved skill in observing and predicting the physical effects of El Nino upon California waters can be directly translated into improved skill in predicting ecosystem effects.