Long-term variability in the Southern California Current System

Abstract

Abstract We summarize 50 years of physical oceanographic data in the southern portion of the California Current System (CCS) based on the California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrographic record, 1950–1999. The long-term mean water property and circulation patterns are described, and the local signature of the large-scale 1976–1977 North Pacific climate regime shift is characterized. Changes associated with the climate shift include (1) significant warming in the upper 200– 400 m of the water column, a decrease in salinity in near-surface coastal waters, and an increase in salinity in subsurface offshore waters; (2) deeper density surfaces and increased stratification throughout the region, particularly within the Southern California Bight; (3) a subtle reorganization of the geostrophic flow structure, including an offshore shift of the California Current and increased nearshore poleward flow; and (4) a cross-shore dichotomy, with the nearshore regime changes occurring primarily during the upwelling season and the offshore variability being of lower frequency than nearer the coast. Changes in the vertical structure of the water column are of particular significance, as they have likely rendered upwelling less biologically effective. This is evident in the temperature and salinity changes, which were greatest (warming and freshening) just above the peak stability changes. There is evidence to suggest that another North Pacific-wide regime shift occurred following the 1997–1998 El Nino event, and large ecosystem changes are anticipated. The CalCOFI record is ideally suited for revealing the mechanisms of interdecadal physical–biological interactions in the coastal ocean.