California Current

The California Current is a Pacific Ocean current that moves south along the western coast of North America, beginning off southern British Columbia, and ending off southern Baja California. There are five major coastal currents affiliated with upwelling zones. These are the California Current (located off the coast of Oregon and California), the Humboldt Current (located off the coast of Chile and Peru), the Canary Current (located off the coast of northwest Africa), the Benguela Current (located off the coast of southwest Africa), and the Somali Current (located in the western Indian Ocean) (Mann and Lazier, 2006). The five major coastal currents are parts of the global ocean gyre system and as such, these currents are driven by wind and deflected by landmasses. Each of the major ocean basins has both a western boundary current and an eastern boundary current. The western boundary currents tend to be deep and fast and the eastern boundary currents are mainly shallow, broad, and less-defined (Mann and Lazier, 2006). The California Current is an Eastern boundary current and is part of the North Pacific Gyre, a large swirling current that occupies the northern basin of the Pacific. The movement of northern waters southward makes the coastal waters cooler than the coastal areas of comparable latitude on the east coast of the United States. Additionally, extensive upwelling of colder sub-surface waters occurs, caused by the prevailing northeasterly winds acting through the Ekman Effect. The winds drive surface water to the right of the wind flow, that is offshore, which draws water up from below to replace it. The upwelling further cools the already cool California Current. This is the mechanism that produces California's characteristic coastal fog and the negative temperature anomaly we measure in California's coastal waters during summer (Mann and Lazier, 2006). This translates into cold coastal waters during the summer, stretching from Oregon to Baja California. Note, this does not include the coastal water surrounding San Diego. There is a warm water anomaly off San Diego (Mann and Lazier, 2006).

The cold water is highly productive due to the upwelling, which brings to the surface nutrient-rich sediments, supporting large populations of whales, seabirds and important fisheries. Winds of the appropriate direction and strength to induce upwelling are more prevalent in the presence of Eastern boundary currents, such as the California Current (Mann and Lazier, 2006). Phytoplankton production is dramatically increased in these areas because the nutrient-rich water lying below the pycnocline is relatively close to the surface and is thus easily upwelled (Mann and Lazier, 2006). Scientists at Scripps Institution of Oceanography said in 2011 that the average surface temperature of the water at Scripps Pier has increased by almost 3 degrees since 1950.

A narrower, weaker counter current, the Davidson Current, occasionally moves somewhat warmer water northwards during the winter months. During El Niño events, the California Current is disrupted, leading to declines in phytoplankton, resulting in cascading effects up the food chain, such as declines in fisheries, seabird breeding failures and marine mammal mortality (Schwing et al., 2003). In 2005, a failure in the otherwise predictable upwelling events, unassociated with El Niño, caused a collapse in krill in the current, leading to similar effects (Schwing et al., 2003).

Bakun (1973) calculated a 20-year average of the monthly mean Ekman transport for different regions off the California coast. His 'Bakun upwelling index' ranges from 300 meters-cubed/second (in the offshore direction) to -212 meters-cubed/second (toward the coast, or onshore direction) (Mann and Lazier,2006). Bakun's index showed there is year-round upwelling off Southern California's coast, but it is strongest in the summer months. Bakun's work also shows that off the coast of Oregon and Washington, there is forceful downwelling in the winter months, and upwelling in the region is restricted to the months of April through September (Mann and Lazier, 2006).