California Current

The California Current is a Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Peninsula. It is considered an Eastern boundary current due to the influence of the North American coastline on its course. It is also one of five major coastal currents affiliated with upwelling zones, the others being the Humboldt Current, the Canary Current, the Benguela Current, and the Somali Current. The California Current 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 results in coastal waters cooler than coastal areas of comparable latitude on the east coast of the United States, along with cool summer air temperatures for the immediate coastline areas. For example, Half Moon Bay at 37 degrees latitude has no month with an average high above 67 °F (19 °C) and San Francisco often stays below 70 °F (21 °C) in summer. Additionally, extensive upwelling of colder sub-surface waters occurs, caused by the prevailing northwesterly 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 measured in California's coastal waters during summer. This translates into cold coastal waters during the summer, stretching from Oregon to Baja California. This does not include the coastal water surrounding San Diego, where a warm water anomaly occurs.

...
Wikipedia