WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Thanks to all the readers who attended the Meteorologists' Forum at the Mercury News on Wednesday night. I think
everyone had an interesting and fun time, and I will address some of your questions in upcoming columns.

Q. While surfing (no pun intended) the Net this afternoon, I observed these sea temperatures recorded by the deep moored buoys: Gulf of Alaska, 53 degrees Fahrenheit; Astoria, Ore., 62 degrees; San Francisco, 57 degrees; and Catalina Island, 68 degrees. So why is the sea temperature off San Francisco significantly colder than off Oregon or Catalina? Also, in the past few weeks, the Gulf of Alaska sea temperature has been warmer than waters off San Francisco. Very strange.  Jack Forristel - San Jose

A  If you were surfing (pun intended) along the West Coast, you would find these water temperatures fairly typical for summer. The relatively cooler water along the Northern and central California coast is a key component of our summer climate. Technically known as ``upwelling,'' it's the result of the interaction of atmospheric pressure, ocean currents and the geography of California.

Let's look at these elements.

The first is a semipermanent area of atmospheric high pressure known as the Pacific High, which during the summer is the dominant weather feature in the northeast Pacific. It deflects the storm track away from the California and the Pacific Northwest. Winds flow away from this high-pressure center, toward California out of the Northwest.

The Pacific High and other global wind systems push across the sea surface waters, creating expansive current systems in the oceans' upper layers. Along the West Coast, the Pacific high drives the California current southward, bringing cool water from the north.

The next element in this long-winded (yep, another pun) explanation is the geography of California. The Coast Range separates the cool Pacific from the warm Central Valley. In summer, heat in the Sacramento and San Joaquin valleys creates an inland area of low pressure known as a ``thermal low.'' Wind is simply the movement of air from areas of high pressure to lower pressure. The thermal low helps draw the northwest winds toward California.

These factors combine to produce persistent summertime winds from the northwest that parallel the coast and are somewhat blocked by the Coast Range. These winds interact with the top layers of the ocean as they blow across the Northern California coastal areas.

Although the winds are pushing the ocean water south, the rotation of Earth causes the surface water to deflect toward the west -- away from the coast. This curving motion, called the Coriolis Effect, affects ocean currents as well as winds. Its effects are greatest in the uppermost layer of the ocean, but can be felt a couple hundred feet below the surface. As this surface water is pushed offshore, colder waters from the ocean's depth replace it. Along the west coast of North America, this phenomenon, known as upwelling, is greatest between San Francisco and the Oregon state line.

These cold waters affect Northern and central California in a number of ways.

The northwest winds sweep over this cold water, chilling the air before blowing through gaps in the Coast Range. The chilled air cools inland areas. If cooled enough, the water vapor in the air condenses into low clouds and fog that is so prevalent along the coast in summer.

The cold, upwelled water is rich in nutrients -- the bottom of the food chain in the oceans -- creating great habitat for the abundant sea life along our coast.

Other parts of the world also feel significant effects from upwelling.

In Southern California, upwelling can occur during prolonged Santa Ana events, when hot dry air from inland blows over the ocean. After several days of a Santa Ana, the winds have blown enough surface water away from the coast to allow cooler waters to rise, cooling Southern California beaches.

Significant upwelling of cold, nutrient-rich water along the west coast of South America is responsible for that region's prominent fishing industry. But the fishing suffers when warmer than normal tropical waters push into the region and cut off the upwelling. We know that as El Niņo.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them weathercorner@ggweather.com.