This explanation of The California Current is an excerpt from the book California Marine Life by Marty Snyderman. This is a great explanation; it’s easy to read and understand.
“The California Current is a major surface current that brings cold water from the Gulf of Alaska into California waters. It maintains a generally slow southward flow throughout the year, though the intensity of the flow varies with the seasons. During late summer and fall, the current tends to hug the coastline of northern and central California more closely than in winter and spring. Another, smaller current called the Davidson Current brings warm water northward along the coast between November and February.
If you examine the map of California… you will notice that the entire mainland coast south of Point Conception indents sharply to the east. This indentation is referred to as the Great California Bight. The California Current more or less hugs the state’s coastline until it reaches the bight; south of this point the current does not meet the coast again until just about 100 miles south of the Mexican border, along the shores of Baja. Because all eight California Channel Islands lie roughly in the lee of this bight, they, along with the mainland coast south of Point Conception are less immediately influenced by the California Current than are waters to the north of the bight.
During spring and early summer, north-northwest winds (which blow from the north toward the south) cause upwelling as they drive surface waters from the coastline, bringing colder, nutrient-rich water up from below. The upwellings, which are generally more prominent in central and northern California than they are in the southern end of the state, are the reason that coastal waters are colder than surface waters of the open sea. The nutrient-rich upwellings play a major role in the life cycle of many species and are crucial for the high productivity of California waters. Water conditions, especially visibility, are also affected by current flow and by the concentration of nutrients.”