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Ocean Surface Topography from Space
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Putting Ocean Altimetry to Work
August 01, 2008

Diablo Canyon Power Plant
Diablo Canyon Power Plant

Ocean conditions have a big effect on operations at the Diablo Canyon Power Plant.

As the newest ocean-observing satellite begins its job surveying the sea surface from space, here on Earth many people will put what it learns into everyday practical use. Take Walter Reil, for example, and his employer, Pacific Gas and Electric.

Reil works at the Pacific Gas and Electric's Diablo Canyon Power Plant on the coast near San Luis Obispo, Calif. Part of his job is to report on the plant's current operating performance and forecast future performance. He keeps a close eye on the factors that affect that performance. One of these is the temperature of the seawater the plant uses in its cooling system.

"The colder the water, the better," said Reil. "Cold water usually means increased power production. It improves the plant's thermal efficiencies, resulting in slightly more power output." When he noted this spring that the seawater in the plant's area has been colder than usual, he wondered why.

"From 1977 through 2007, the mean seawater temperatures for April is 52.1 degrees Fahrenheit along the Diablo Canyon coastline," said Pacific Gas and Electric meteorologist John Lindsey. He closely monitors ocean conditions that impact power plant operations, including temperature, winds and waves. "This April the mean was 49.5 degrees, that's the coldest on record."

The big chill in the water off the coast of California this spring was largely due to a huge ocean feature called the Pacific Decadal Oscillation (PDO).
The big chill in the water off the coast of California this spring was largely due to a huge ocean feature called the Pacific Decadal Oscillation (PDO). The signature of its "cool phase" is a horseshoe pattern of higher than normal sea-surface heights, shown in yellow, caused by warm water that connects the north, west and southern Pacific, with cool water in the middle. Nestled within the PDO is a much smaller La Niña, seen as blue band of lower-than-normal water near the equator.

Reil wanted to know what ocean conditions were responsible for the unusually cool seawater and what the outlook was for the future. "I wondered if these cold temperatures are a result of changes occurring because of effects of global warming and whether models suggested that the Pacific will be seeing increased La Niña conditions in the future," he said.

These are exactly the kinds of the questions that the Ocean Surface Topography Mission on the Jason-2 satellite (OSTM/Jason-2), launched on June 20, 2008, and its two predecessors, Jason-1 and Topex/Poseidon, are helping to answer. Together, they have been using radar altimetry to measure the ocean surface since 1992. Their long record of sea surface topography observations provides a unique insight into how the ocean changes over time and how those changes are linked to Earth's climate.

Reil's queries about unusual ocean temperatures found their way to the email in-box of JPL oceanographer Josh Willis. Willis is a member of the Ocean Surface Topography science team, a group of NASA-sponsored scientists who use altimetry data to study the oceans.

"The cool waters along the coast right now are part of a larger climate phenomenon known as the Pacific Decadal Oscillation, or PDO for short," Willis replied. "The PDO refers to a huge horseshoe-shaped pattern in sea surface temperature that involves pretty much the entire Pacific Ocean. Unlike La Niña and El Niño, which last only a year or so, the PDO only changes once every 5 to 20 years."

"Right now, we are in a cool phase of the PDO, in which warm waters in the North, West and South Pacific are separated by a cool wedge along the equator and eastern Pacific. Along the California coast, this means winds from the north, which bring deep cool waters to the surface, are more common," he said. The weak La Niña that ended earlier this year, he added, didn't have much effect on local temperatures in California. Its cooler-than-normal waters were limited to the middle of the Pacific.

Global warming enters the picture as a small, but significant warming of local surface temperatures of about 1 degree Centigrade (1.8 degrees Fahrenheit) over the past 100 years. "This is important," said Willis, "because it is beginning to swamp the cooling we get from the PDO." Features like the PDO and El Niño continue to create big regional warm and cool patches like the cold water at Diablo Canyon. But these cycles happen against a backdrop of gradual warming. "In other words," Willis says, "the cool phase of the PDO ain't what it used to be."

Looking beyond today's ocean condition into the future gets a lot trickier. "Questions about how global warming might affect the PDO and El Niño are still really up in the air," Willis said. "Some climate models predict permanent El Niño-like states, while some predict permanent La Niña-like states. The truth is, we really don't know."

One of OSTM/Jason-2 's goals is to improve the ability to forecast the PDO, El Niño and other ocean-driven climatic events.

These events have a big impact on power supply and demand, and the more utility companies can plan ahead the better, said Lindsey. "Increased rainfall, often associated with El Niños, means there's more water available to run hydroelectric plants," he explained. Drought and scant mountain snow have the opposite effect, increasing the need for power from other sources such as Diablo Canyon.

Lindsey already routinely uses altimetry data from OSTM/Jason-2's predecessors and will put the new mission's findings right to work. "What happens in the ocean has dramatic consequences for utility industries such as Pacific Gas and Electric," he said. "These are very real-time, real-world cases of how important ocean monitoring and forecasting are," said Reil.

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