Drilling along the California continental margin should provide important new information on north Pacific climate because of the sensitivity of the region to changes in the wind field. Understanding the dynamics of the currents and upwelling will enable us to understand how the average wind fields in the north Pacific have changed and where rainfall has been directed on North America.
The modern California Current system is probably the best-studied of the world's eastern boundary currents, and because of these modern measurements it is possible to reconstruct the past behavior of this eastern boundary current system with much greater confidence than for any other in the world. It is probably the best place in the world to understand how eastern boundary currents have responded to climate change. The California Cooperative Fisheries Program (CalCOFI) has taken seasonal measurements of hydrography and biology in transects across the current for more than 30 yr. Major physical oceanography experiments such as Coastal Upwelling Experiment and Analysis (CUEA), Coastal Dynamics Experiment (CODE), and the West Coast Satellite Time Series and Coastal Tranzition Zone program (Brink and Cowles, 1991, and references therein) are linking the dynamics of California Current flow and coastal upwelling to climate.
More long- and short-term studies of biogeochemical flux are available from this region than anywhere else in the world, including the VERTEX study (Knauer et al., 1979; Knauer and Martin, 1981), MULTITRACERS (Lyle et al., 1992; Dymond and Lyle, in press); Low Level Waste Disposal Project (Dymond and Lyle, in press, Fischer et al., 1983), long-term sediment trap deployments off Monterey (C. Pilskaln, pers. comm.), and short-term sediment trap deployments in the California Borderlands region (Dymond et al., 1981; Sautter and Thunell, 1991; Kennett, unpubl. data).
Dynamics of the California Current
The California Current combines diffuse flow extending hundreds of kilometers offshore with local high-velocity zones of southward flow. The southward jets 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 at the border of Oregon and California and is about 300 km from the coast at Point Conception (~35°N; Hickey, 1979; Lynn and Simpson, 1987).
It is well known that the California Current is subject to both seasonal and interannual cycles. The pattern of winds along the coast controls seasonal variations (Fig. 3), whereas changes in the dynamic topography of the North Pacific Gyre produce interannual variability in the current (Fig. 4). 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. 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 and to create the general features of its structure. They could only model interannual variations of the California Current 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.
The modeling suggests that 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. 1993) should also strongly affect the structure of the California Current flow as well as the locations of maximum coastal upwelling. The available data support these interpretations.
Coastal California Upwelling
Upwelling along coastal California is driven by equator-ward winds that roughly parallel the coast (Figs. 3, 5, 6; Huyer, 1983). Ekman transport of surface waters by these winds causes transport of surface waters away from the coastline and 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 winds are seasonally to the south in northern California but always blow toward the equator south of San Francisco. In addition, upwelling-favorable winds are strongest in the north during the seasonal upwelling period. This wind pattern today causes the strongest coastal upwelling in July to be located between Cape Blanco and San Francisco in the north, but in the winter months causes the strongest upwelling to be located south of San Diego (Fig. 6; Huyer, 1983).
The seasonal cycle of winds and upwelling described above is a direct result of the seasonal 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). Thus, by monitoring the strength and the seasonality of coastal upwelling along coastal California with paleoceanographic data, we will be able to track the latitudinal position and strength of the north Pacific high as climate changes.
The California margin should be strongly affected by changes in formation of bottom water in the Antarctic, because it is directly upon a flow path for this bottom water into the north Pacific (Fig. 5; Gordon and Gerard, 1970; Mantyla, 1975). Newly formed bottom waters enter into the Pacific near New Zealand and move up along the western boundary of the Pacific Ocean. They split and travel around the Hawaiian Islands before the addition of buoyancy consumes the water in the Alaska Gyre region. One of these paths flows directly into the California margin. The California margin should be a good location to monitor changes in Pacific deep-water properties. For this reason, in the Leg 167 drilling program, we emphasize depth transects, both in the north and in the south of the study region.
To 167 Scientific Objectives and Methodology
167 Table of Contents