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Repeated hydrobiological surveys over the period 1988–2002 perpendicular to the central California coast indicate strong coupling between physical circulation and biological production. An equatorward-flowing jet about 100–200 km from shore marked the inshore edge of the California Current (CC). This “CC Jet” had its highest velocities during late winter and spring. The jet divided inshore, biologically productive waters from offshore, low-production waters. Mean flow in the inshore waters is poleward. However, this flow is interrupted in late spring and summer by a surface-enhanced, equatorward-flowing, coastal upwelling jet. The upwelling jet coincides with maxima of nutrients, chlorophyll-a and primary production. Annual variability in the inshore zone is related to (1) vertical pycnocline movements associated with geostrophic adjustments to accelerations of the California Current system, and (2) coastal upwelling. In offshore waters, the annual cycle accounted for a small fraction of the variability, indicating the dominance of eddies and meanders in this zone (J. Geophys. Res. 92 (1987) 12 947). The offshore regime was mesotrophic to oligotrophic, with a subsurface chlorophyll-a maximum above the nutricline. Considerable subduction may occur under the California Current jet and be an important process in the export of biogenic material to the deep sea. [Copyright &y& Elsevier]

The physical, chemical and biological perturbations in central California waters associated with the strong 1997–1998 El Nin˜o are described and explained on the basis of time series collected from ships, moorings, tide gauges and satellites. The evolution of El Nin˜o 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 Nin˜o 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 Nin˜o 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 Nin˜o. The nitrate in surface waters was less than 20% of normal and new production was reduced by close to 70%. The La Nin˜a 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 Nin˜o was estimated to be 50 million metric tons of carbon (5×1013 g C). This reduction certainly had deleterious effects on zooplankton, fish, and marine mammals. The 1992–1993 El Nin˜o 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 Nin˜o appears related to the longer-term background climatic state of the Pacific Ocean. The 1982–1983 and 1992–1993 El Nin˜os occurred during the warm phase of the Pacific Decadal Oscillation (PDO). The PDO may have changed sign during the 1997–1998 El Nin˜o, resulting in weaker ecological effects than would otherwise have been predicted based on the strength of the temperature anomaly. [Copyright &y& Elsevier]