The movement of Alaskan and northern ocean currents southward down the west coast results in much cooler ocean temperatures than at comparable latitudes on the east coast of the United States, where ocean currents come from the Caribbean and tropical Atlantic. The cooler ocean current along the on the west coast also makes summer temperatures cooler on the west coast compared to the east coast. 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, while Virginia Beach, VA close to the same latitude has six months when high temperatures are above 70 °F (21 °C).
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 cool ocean waters. As a result, ocean surf temperatures are much colder along the Pacific coast than the Atlantic coast. For example, the average July SST (sea surface temperature) at New York City at 40.7 degrees latitude is 73 °F (23 °C), while at the same latitude in Eureka, CA is 57 °F (14 °C). As such, ocean surf temperatures are rarely above 66 °F (19 °C) during the summer along the California coast south to San Diego, while they are often above 80 °F (27 °C) on the east coast from North Carolina southward.
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. 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. 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.
The "Bakun upwelling index" is based on a 20-year average of the monthly mean Ekman transport for different regions off the California coast. It ranges from 300 meters-cubed/second (in the offshore direction) to −212 meters-cubed/second (toward the coast, or onshore direction). There is year-round upwelling off Southern California's coast, but it is strongest in the summer months. 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.
