Another x-post from the BSR...
For those of us living in California, one of life's great tragedies is that the Pacific ocean is both so close to us, and so poor for actual swimming. Just to our west lies miles and miles of beautiful California coast and beaches, but spending more than five minutes in their waters sounds like a recipe for pain and thermal shock, rather than the leisurely fun that summer is supposed to bring.
So why are the California waters so cold, anyway? As always, the answer is a combination of several factors, all of which highlight the intricate complexity of our global ecosystem, and how the effects that we feel locally often originate from hundreds of miles away.
Perhaps the first, most obvious answer for California's chilly waters lies in the ocean currents that carry water from up north. The dominant current that flows past California is part of the "North Pacific Gyre", a giant spiraling circle of water that takes up most of the Pacific Ocean.
It's basically an "aquatic highway", transporting water, marine life, and nutrients from one part of the ocean to another. In our case, this water comes from the frigid north, cooling down the water significantly. However, these currents don't tell the whole story about our unusually chilly waters. For the complete picture, we need to turn to a set of phenomena known as the Coriolis effect, Ekman spirals, and upwelling.
As we all know, our earth rotates in a west-to-east direction. However, our atmosphere and waters don't necessarily rotate at the same rate as Earth itself. Instead, they tend to move independently. Whenever something is thrown through the air, the earth continues to rotate underneath the object, causing it to be slightly "off-target" when it arrives. This is called the "Coriolis Effect", and applies to anything passing through the air. From our vantage point, the time of flight is too short to notice any real difference, but when you're talking about an air current flowing from the north pole to the equator, the earth has a long time to rotate and offset the final destination of the current.
So how does this alter ocean temperatures? Notice how the Northern Pacific Gyre in the picture above flows clockwise, so that the waters flowing past California move from North to South. As the earth rotates, it causes the top layer of the ocean to press up against this northern breeze, creating a force that pushes it to the right (Westward). The top layer of water pulls off to the west, causing an Ekman spiral. This basically serves to shear away the top layer of the coastal ocean.
With the top level of the ocean pushed away, space is available for deeper, colder waters to rise to the surface. This is a process known as "upwelling" (here's an animation describing what it is), and is the reason that the Pacific coast is known for its cold waters. Interestingly, it's also responsible for the infamous San Francisco fog, as well as for the rich ecosystem that exists off of our coast (those deep waters are also filled with nutrients, as described in this video).
So the next time that you're on the beach and curse the ocean for not giving you the warm summer waters that you dream of, remember that these properties are actually the result of an incredibly complicated system of ocean currents, air currents, and good old fluid dynamics. Even if it's uncomfortable, it can still be impressive, right?