The modern California Current system has well-known physical dynamics, chemical structure, biological standing stocks, and biogeochemical fluxes, but on paleoceanographic time scales (102-107 yr) its dynamics are poorly understood. Drilling during Ocean Drilling Program (ODP) Leg 167 was designed to explore the evolution of the California Current system and to study how the North Pacific Ocean has interacted with the global climate system from about 12 Ma to the present (Fig. 1). Many of the Leg 167 scientific party are also studying the effect of northeast Pacific paleoceanographic change on the regional climate of western North America, and how productivity along the California margin has evolved with changes in the California Current system.
Other Leg 167 priorities are to improve regional stratigraphy of the sediment column, to understand how paleoceanographic signals can be preserved or modified by diagenesis in a highly productive sedimentary system, and to understand how paleomagnetic signals are preserved on a rapidly sedimenting continental margin.
In this synthesis we will describe briefly what we have accomplished toward the Leg 167 objectives and then describe further work that should prove fruitful. With the immediate postcruise studies we have already profoundly improved our understanding of the California Current system and its variability on paleoceanographic time scales. The true synthesis work is just beginning, however. We are only just beginning to compare data between drill sites to study water column structure and are only just beginning detailed studies of the sedimentary record prior to the last 150 k.y. We have noted major changes in sediment deposition since the end of the middle Miocene but so far have only done reconnaissance-scale study of Neogene events (Ravelo et al., 1997). The prospects for future study are bright, however. The high quality of the Leg 167 sedimentary records will make it possible to study important climate transitions since 12 Ma in very high resolution.
The California Current system is the eastern expression of the North Pacific subtropical gyre (Fig. 2). It carries cold, relatively fresh surface water out of the subarctic North Pacific and mixes it into warmer more saline water of the subtropical and tropical regions. The northern edge of the Leg 167 California margin drilling is roughly aligned with the modern position of the northern edge of the North Pacific subtropical gyre. Here the North Pacific Current flows across the Pacific, separating the cold nutrient-rich waters of the subarctic Alaska Gyre from the warm nutrient-depleted waters to the south. The southern Leg 167 drill sites reach into the southern reaches of the California current to permit latitudinal study of changes in strength of the California Current through time.
Because the California Current is an eastern boundary current, the region of southward flow is diffuse, especially when compared to the northward flow in the Kuroshio, the cognate western boundary current. Embedded within this diffuse flow are southward jets that separate nutrient-rich upwelled waters from the relatively barren offshore waters (e.g., Huyer et al., 1991). The core of the offshore California Current flow is located approximately 250-350 km from the coast of Oregon (~45°N) to as far south as Point Conception (~35°N; Hickey, 1979; Lynn and Simpson, 1987).
The California Current system has strong seasonal and interannual cycles. The pattern of winds along the coast controls seasonal variations in the current strength and position (Fig. 3), whereas changes in the dynamic topography of the North Pacific Gyre produce interannual variability in the current (Fig. 4, after Roessler and Chelton, 1987). The importance of both local and remote forcing in California Current flow has been emphasized by modeling efforts like that of Pares-Sierra and O'Brien (1989), who found that the local wind field in the northeastern Pacific is adequate to drive the annual cycle of the current system and to create the general features of its structure. They could model interannual variations of the California Current only by coupling the local model with one driven by equatorial winds. Kelvin waves generated during El Niño events in the equatorial Pacific propagate up the western coast of North America and strongly affect the California Current. California Current structure thus reflects both the local winds along the west coast of North America and basinwide events within the north and equatorial Pacific Ocean.
In the much longer climatic cycles that are observable by paleoceanographic studies, the location and strength both of trade winds and of westerlies should probably have a major impact on mean transport in the California Current. Shifts of the mean wind patterns (e.g., a shift in the position of the North Pacific high at 18 ka; Kutzbach, 1987; Kutzbach et al., 1993a) should also strongly affect the structure of the California Current flow as well as the locations of maximum coastal upwelling.
Coastal California Upwelling: Upwelling along coastal California is driven by equatorward winds that roughly parallel the coast (Huyer, 1983). Ekman transport of surface waters by these winds moves them away from the coast and causes upwelling of nutrient-rich waters from below. The upwelling waters are restocked by shallow flow inward toward the shelf beneath the surface ocean layer. The seasonality of northerly winds in Northern California causes strong upwelling in the summer but downwelling in the winter (Fig. 5, after Huyer, 1983). South of San Francisco there is coastal upwelling the year around. Because of the coastal embayment of the Southern California Bight, coastal upwelling is low all year round (Fig. 5). Instead upwelling occurs offshore near Sites 1012, 1013, and 1014, driven by the curl of wind stress (Bray et al., 1999).
The seasonal cycle of winds and upwelling is a direct result of the annual migration of the North Pacific high-pressure regime. The North Pacific high migrates between its southerly limit at 28°N in February and its most northerly limit, 38°N, in July (Fig. 3; Huyer, 1983). Using paleoceanographic data to monitor the strength and the seasonality of coastal upwelling along coastal California also is an indirect monitor of wind strength and seasonality along the margin. This in turn gives information about the strength and position of the North Pacific high.