Coastal residents have come to expect endless weeks of fog from May through August. A few find it romantic; others mutter about how the apocryphal Mark Twain quotation ("the coldest winter I ever spent ...") is right on the money. In truth, we should be grateful for the blanket of mist around them. Its arrival marks the start of a great annual surge of life just offshore.

Squadrons of Dungeness crabs, million-tentacled congregations of snow-white market squid, herds of barking sea lions—all are powered by the fog-generator known as upwelling, the churning of deep cold waters that recurs each summer off our shores. And though most people are unaware of its existence, upwelling touches our lives in many ways beyond murky summer weather. Fishery closures, salmon populations, mass die-offs of seabirds, and the impacts of global warming on our coastal ecosystems are all intimately tied to variations in this ocean phenomenon.


Upwelling begins, if it can be said to begin anywhere, far from California, in the close and sweaty air at the equator. Heated year-round by a relentless sun, tropical air expands and rises high into the atmosphere. Colder, denser air from the poles rushes south in the exchange.

Off western North America, this chilly air flows from the vicinity of the North Pacific and Alaska, pushing ocean waters before it. But instead of flowing directly south, coastal waters are deflected to the west by the west-to-east rotation of the earth (the Coriolis effect).

Cold waters from 300 to 600 feet deep percolate upward to take the place of the departing warmer water. Though rising just a few meters a day, upwelling can cause surface water temperatures to drop by as much as 20 degrees Fahrenheit within a week or two. The stronger the winds, the more deep water is pulled upward. When chilly upwelled water encounters warmer spring air at the surface, tiny droplets condense in the cooling air-making fog a fine barometer of upwelling.

However, fog—drawn inland through the Golden Gate and other breaks in the Coast Range—is just a minor byproduct of upwelling. "The single biggest thing about upwelling is that it continually resupplies nutrients" to the upper layers of the ocean, where photosynthesis can occur, says oceanographer John Largier of UC Davis's Bodega Marine Laboratory.

Deep ocean waters carry a heady cocktail of nitrogen, phosphorus, and silicates recycled from life at the surface. When they die, animals and plants sink down into a sprawling compost pile on the sea floor, where bacteria break down this detritus into a nutrient-rich mixture tailor-made for plants.

Upwelling pumps this marine fertilizer to the ocean's surface. Its arrival in sunlit waters kicks off a weeks-long fiesta of feeding, breeding, and growth. Suddenly bathed in nutrients, clouds of microscopic algae crank their photosynthetic engines into overdrive, melding sunlight, carbon, and nutrients into living tissue. Diatoms and other tiny marine plants, known collectively as phytoplankton, can double their numbers twice in a day.

Grazers converge on the floating fields like cattle in a meadow. Zooplankton—krill, copepods, and larval fish—gorge themselves to repletion. Now awash in a blizzard of rice-grain-size zooplankton, the bloom attracts ever larger predators, until gangs of mature salmon and seabirds, sea lions and whales, and even great white sharks circle by to claim their share.

Ocean creatures have had plenty of time to adapt to upwelling—it's an ancient fixture of coastal waters. The evidence comes from the fossils of organisms known as forams. Identified by their uniquely patterned, bead-size shells, each species of foram grows only within a narrow range of water temperatures. Cores pulled from Southern California seabeds indicate that cold-loving forams have lived along the coast for tens of thousands of years or more.

Upwelling occurs in many parts of the ocean. But thanks to a confluence of wind and geography, it is especially strong, sustained, and reliable in the waters off California and just a few other places. A glance at the globe helps explain why. All four of the world's major upwelling centers—the California Current, the Humboldt Current off the coast of Chile and Peru, the Canary Current off northwest Africa and the Iberian Peninsula, and the Benguela Current off southern Africa—are located on the western edges of their continents at midlatitude, where the winds that drive upwelling blow. Though upwelling centers make up less than 2 percent of the surface area of the ocean, they have historically supplied half of the world's fish catch...

As with most complex systems, "typical" circumstances are only part of the story. Other cycles that affect the entire Pacific basin alter sea surface temperatures and help determine whether upwelling will inject extra nutrients into coastal ecosystems—or leave animals hungry.

The most famous of these cycles is the El Nino Southern Oscillation. Known for lashing the West Coast with epic winter storms, El Nino's power lies in its tropical energy. Every three to seven years, a blanket of anomalously warm water spreads along the Pacific coast, creating a thicker layer of warm water at the surface and stranding nutrients in deeper layers. "During El Ninos, you can actually have a lot of wind and upwelling," says marine ecologist Bill Sydeman of PRBO Conservation Science in Petaluma, "but it is totally ineffective; it's just turning over warm, low-nutrient water"—leaving marine larders virtually bare. An El Nino year is often followed by a La Nina event, when typical spring and summer upwelling are intensified and species can recover from the previous year's losses.

Another, more recently discovered player in the system is called the Pacific Decadal Oscillation, or PDO. It doesn't appear to amount to much; it raises or lowers the average temperature of the Pacific by just a degree or two. But what it lacks in strength it makes up in staying power. Each warm or cold regime typically lingers for two to three decades at a time, with profound effects on marine ecosystems. Oceanographers Francisco Chavez and John Ryan of the Monterey Bay Aquarium Research Institute found that sardines and anchovies boom and bust in synchrony with the PDO. The pattern explains the overnight disappearance of Monterey's sardine fishery in the 1940s, and it's visible in centuries-old layers of fish scales dredged from the sea floor off Santa Barbara. However, because one go-round of the PDO takes roughly half a century to complete, scientists haven't had much chance to understand how it works.

Scientists are even more baffled by the most recent disruption in the upwelling cycle, which doesn't seem attributable to either El Nino or the PDO. In 2005, strong, sustained upwelling arrived in July instead of late spring. Its absence proved catastrophic to marine animals up and down the Pacific coast.


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