Water circulation around the Channel Islands is complex and highly dynamic, resulting from the interaction of large-scale ocean currents, local geography, and the unique basin and ridge topography of the ocean bottom in the Southern California Bight (SCB).
The California Current is a major ocean current that moves through the sanctuary region, staying largely to the west of the islands, but influencing the circulation patterns in the region. Year round, this current brings cold water from upwelling centers along the California coast.
At Point Conception, where the coastline turns east, the California Current moves farther offshore as it continues its southward flow. Near the U.S.- Mexican border the California Current turns east and then north, and flows back up along the coast bringing warm water into the Santa Barbara Channel. This directional shift creates a large eddy known as the Southern California Countercurrent or the Southern California Eddy (Hickey 2000a). At the eastern end of the Channel Islands, the Southern California Countercurrent separates into two parts. One part flows northwestward through the Santa Barbara Channel; the other part flows westward south of the Channel Islands. The California Current and Southern California Countercurrent are both strongest in the summer (Hickey 1993). During the spring, the countercurrent disappears and surface flow throughout the SCB tends to be southward (Hickey 1993). The timing, duration and intensity of upwelling events is driven by seasonal variations in wind direction and climatic variability associated with events such as El Nino. In general, upwelling period begins in March, when westerly winds prevail, and continues until September, when the winds die down (California Coastal Commission 1987).
Upwelling (circulation patterns in which deep, cold, nutrient-laden water moves towards the surface) often occurs where these currents meet. Upwelling currents influence circulation in the sanctuary region. These currents are the result of prevailing winds and the orientation of the coastline. Along the north-south oriented coast of California, winds blowing from the north move surface water westward, away from the coastline, and create upwelling currents that bring colder water to the surface.
Point Conception is the southernmost major upwelling center on the west coast of the United States, and marks a transition zone between cool surface waters to the north and warm waters to the south (Love et al. 1999). However, upwelled water from regions north of the SCB appears to enter the western end of the Santa Barbara Channel and move eastward along its southern boundary (Hickey 2000a). Between the islands and the mainland, these currents create a localized cyclonic gyre that can vary in intensity seasonally based on current and wind speed (Hendershot and Winant 1996, Harms and Winant 1998, Winant et al 2003). These varying conditions create alternate states of upwelling, where cool nutrient- rich water is brought from deeper areas to the photic zone at the surface, and relaxation, when upwelling ceases (Winant et al 2003). Regional upwelling is wind-driven and provides the nutrients and conditions for phytoplankton and zooplankton to thrive, with effects seen throughout the food chain.
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