Changes in the hydrography of Central California waters associated with the 1997-1998 El Niño

Abstract

During 1997-1998, oceanographic conditions off Central California were monitored by means of a series of thirteen cruises which measured water properties along an oceanographic section perpendicular to the California Coast. Data were analyzed by utilizing time series plots on isobaric and isopycnal surfaces and by principal component analysis. The following conditions were observed: (a) in June-July 1997, the strong poleward flow at the coast was associated with coastal (within 100 km), subsurface (200-500 dbar) warming of 0. 5 deg C and increased salinity (0.07) on isopycnal surfaces and offshore waters appeared cooler, fresher with stronger equatorward flow; (b)in September 1997, a relaxation of El Nino conditions occurred, with coastal, subsurface waters cooling by 0.3 deg C the band of poleward flow at the coast narrowed; (c) in January 1998, maximum interannual temperature and sea level anomalies were observed with nearsurface (80 dbar), nearshore (within 100 km) warming of 2.5 deg C, subsurface warming comparable to that observed in June-July 1997, and equatorward flow at the coast; and (d) in March-April 1998, coastal waters freshened greatly, both due to the onshore flow of Subarctic water and to river runoff from winter storms. By summer 1998, hydrographic conditions were near normal. The observed warming in late 1997 was not caused by decreased offshore Ekman transport but does appear to be remotely forced by poleward propagation from the Equatorial Pacific along the Eastern Boundary, possibly by Kelvin waves. The subsequent onshore transport and freshening that took place during Spring 1998 could have been related to onshore Ekman transport associated with winter storms. The observed change in heat content associated with the 1997- 1998 El Nino was the same as that observed during a normal seasonal cycle.