The coastal ocean off Oregon and northern California during the 1997-8 El Nino

Abstract The evolution and decay of El Nino 1997–8 was observed in coastal waters off Oregon in a sequence of cruises along 44.6°N from the coast to more than 150 km offshore. Hydrographic observations were made during eleven cruises between July 1997 and April 1999 at stations on the Newport Hydrographic Line, which had been occupied regularly from 1961 to 1971. The data from the earlier decade provide a basis for defining ‘normal’ conditions and allow comparisons with the recent El Nino in terms of T , S , spiciness and geostrophic velocity. Independent of El Nino, the ocean in July 1997 was already anomalously warm offshore of 50 km and above 100 m. By September 1997 there were unambiguous indications of El Nino: isotherms and isohalines sloped down toward the coast indicating poleward flow over shelf and slope, and anomalously spicy water was present at the shelf-break. In November 1997 and February 1998 shelf-break waters were even warmer, and there was strong poleward flow inshore of 100 km, extending to depths greater than 200 m. The April 1998 section closely resembled that of April 1983 (another El Nino year) but by June 1998 the anomalies were mostly gone. November 1998 was near normal and the sections from subsequent cruises resemble the mean sections from 1961–1971. Four cruises between November 1997 and November 1998 included sampling at several latitudes between 38° and 45°N. As expected, these sections show significant alongshore gradients, but also a surprising degree of homogeneity in the anomalous features associated with El Nino (in the temperature, salinity, spiciness and geostrophic velocity fields). The anomalous signature of El Nino was stronger at its winter peak in 1998 than in 1983, but the signature in the temperature and spiciness fields, and in coastal sea level, did not persist as long as in 1983. By April 1999, the coastal ocean from 38°N to 45°N was significantly colder than it had been in April 1984.