The rapid transition from strong El Ni¿o to strong La Ni¿a conditions in the equatorial Pacific in 1998 was accompanied by considerable environmental variability in the southern California Current System (CCS). The evolution of this transition in the CCS is investigated based on hydrographic and biological data collected on 25 cruises over a 45-month period (February 1996 to October 1999). The El Ni¿o period was characterized by high steric heights and a deep nutricline in the coastal regions, which effectively limited biological production. This was followed by a period of increased cross-shore dynamic height gradients, a significant shoaling of the nutricline, and a dramatic rebound in primary and secondary production. The observed physical and biological response in the CCS is remarkably similar to that observed in the tropical Pacific, but with a lag of several months. ¿ 2001 American Geophysical Union