Researchers trying to mathematically describe all the geographical and atmospheric variations that produce the weather find themselves working on some of the most complex equations in the world. "That's one of the reasons the weather is so awfully difficult to understand," Namias pointed out.
Nevertheless, some of the ocean's influences on San Diego weather can be clearly traced. Cool sea water welling up off the coast comes into contact with the warmer air above, for instance, forming the low clouds that hug the coastline much of the year.
In addition, as temperatures rise inland each day, the air above the county rises too, drawing in cool air from over the ocean and creating the sea breeze that gives coastal San Diego such a mild climate.
"I live out east of Rancho Bernardo," said Cayan, "and with pretty fair regularity, at about 11 in the morning every day, we get this sea breeze coming in."
Nevertheless, most of the county's weather is not generated locally, said Namias. "It's generated by things going on thousands of miles away, from the poles to the tropics, from the western Pacific to the Atlantic Ocean." And much of it can be traced to differences in temperature between the ocean and the continents.
Cayan explained that because the ocean is cool and the continents are warm, air tends to heat up and rise over the continents. It spreads out in the upper atmosphere and then descends over the ocean, creating high-pressure zones as it pushes down on the air beneath it.
San Diego owes its typically low rainfall to one of these high-pressure zones, known as the North Pacific High, which is usually centered about 1,000 miles off the coast of Northern California and Oregon. The polar jet stream, bringing storms with it as it angles across the Pacific Ocean toward California, is deflected northward as it hits the North Pacific High. The storms swing northward with the jet stream, dumping rain on Washington and Canada instead of California.
But the ocean also helps to spawn the storms that bring most of our rain. In the winter, arctic air moving south across Alaska flows out over the relatively warm water of the Gulf of Alaska, "setting up a form of instability in the air which conspires with a lot of things to form a low-pressure area" over the gulf, Namias said. Storms are generated in this area of low pressure and head south toward California, borne by the polar jet stream.
If the North Pacific High is strong, it will deflect these storms to the north. But the North Pacific High tends to weaken in the winter--sometimes it disappears altogether--allowing the storms to roar southward across Oregon and California. Thus, the storms that often turn San Diego's skies chilly and gray in December, January and February are a direct result of the interaction between ocean and atmosphere thousands of miles away.
The huge "pools" of relatively warm or cool water that form in the Pacific Ocean affect the weather in a different way.
As anyone who has ever spent the night camping outdoors knows, air temperatures can change rapidly within a few hours. But the ocean is a different story.
As Namias puts it, it is "sluggish" compared with the atmosphere; the ocean's temperature changes over a period of weeks or months, not hours. And it stores vast quantities of heat.
"The top three meters of the ocean contain as much heat as the entire overlying atmosphere," said Namias. "And these pools may go down in the cold season to a depth of 300 meters. If you look at the overall dimensions of the pools--some of them have areas of millions of square miles--you can see you've got a terrific source for (heating or cooling) out there."
Although the temperature of the pools is usually only two or three degrees different from that of the water surrounding them, because of their vast size they can influence such things as the temperature and humidity of the air above them--and thus the weather.
The formation and growth of the pools is tracked each month by Namias and his associates, based on reports from merchant ships crossing the Pacific that drop sensors to measure the temperature of the water around them daily. These measurements--some 20,000 of them each month, on the average--are radioed to government weather stations, compiled in computers and forwarded electronically to the researchers at Scripps.
One of the most dramatic developments documented by the scientists occurred in January, 1969. A pool of relatively cold water stretching from Alaska to the Hawaiian Islands formed in the ocean northwest of California, while another expansive pool of warm water appeared west and south of San Diego.