Hydrographic conditions near the coast of northwestern Baja California : 1997–2004

Abstract

The effects of the 1997-98 and 2002-04 El Ni˜no on the upper waters in the con- tinental shelf and slope regions off northwestern Baja California are explored with data from eight cruises taken in late spring from 1998 to 2004 and the summers of 1997 and 1998. Geostrophic velocities were calculated referenced to a specific vol- ume anomaly surface separating the southward flowing California Current waters from the waters advected to the north by the California Undercurrent. The result- ing fields show equatorward flow near the surface except in the summer of 1997, when a poleward jet was found in the upper 40 dbars. This shallow jet advected anomalously warm and salty waters characteristic of the 1997-98 El Ni˜no, with its core found within 20-30 kms from the coast. By spring of 1998, the waters brought into the region by the jet had mixed across the pycnoline with the salty California Undercurrent waters below, resulting in high salinity levels on the density surfaces corresponding to the otherwise fresh California Current waters (25-26¾t). By con- trast, the 2002-04 El Ni˜no stands out for the very fresh and cold waters found on the same density surfaces in late spring of 2003 and 2004, marking a pronounced presence of subarctic waters. The fresh conditions found on the latter years repre- sent a nearshore expresion of the anomalous intrusion of subarctic waters observed 50-150 km from the coast of Southern California and Punta Eugenia, reported from July 2002 until April 2003. Our results suggest that the presence of this intrusion has continued to influence the region at least until May 2004.