SAN FRANCISCO — After three years of severe drought, California water managers hoping for significant rainfall during the coming winter are unlikely to get the relief they seek from El Niño, a disruption in the normal weather patterns of the tropical Pacific Ocean that changes the course of the jet stream and can have a dramatic impact on temperature and precipitation levels in North America.
One of the primary instruments researchers use to gauge the likelihood of El Niño is the space-based altimeter flying on the Jason-2 Ocean Surface Topography Mission, launched in 2008 by the French space agency, CNES; NASA; the U.S. National Oceanic and Atmospheric Administration; and Europe’s meteorological satellite agency, Eumetsat. Researchers use Jason-2’s altimeter to measure sea levels in the eastern equatorial Pacific Ocean.
Jason-2 data are compared with ocean temperature, current and wind measurements gathered by NOAA’s moored Tropical Atmosphere Ocean buoys. The multiple daily measurements provided by the buoys complement the satellite data, said Michelle L’Heureux, a meteorologist in NOAA’s National Weather Service Climate Prediction Center in College Park, Maryland. “Satellites give you greater coverage and the buoys provide data corrections,” she said.
When water levels in the eastern equatorial Pacific are higher than their long-term average, indicating unusually warm temperatures in the upper ocean, forecasters begin to look for El Niño.
“El Niño has a great signal in sea level height,” said William Patzert, a climatologist at NASA’s Jet Propulsion Laboratory in Pasadena, California.
El Niño occurs every few years when the trade winds that normally blow from east to west across the Pacific Ocean relax or reverse course, moving warm water from the western equatorial Pacific region eastward. Because rainfall tends to occur over the warmest water, El Niño affects precipitation.
Although satellite observations captured in mid-October show water levels in the eastern Pacific Ocean are higher than average, the temperatures are not as warm as those observed during years when El Niño triggered significant increases in precipitation in the southern third of the United States and warmer-than-normal conditions in much of the rest of the country.
Earlier this year, Pacific Ocean water temperatures were similar to those of 1997, when El Niño brought record winter rainfall to California. In the spring and early summer of this year, however, the trade winds resumed their normal pattern and temperatures in the eastern Pacific Ocean moved closer to normal. Patzert now anticipates a weak to moderate El Niño.
“Everybody thinks of El Niño these days as the great wet hope, but there are lots of puny ones,” said Patzert, who has studied El Niño since the 1960s. “For a sustained multi-month soaking, you can’t beat a Godzilla El Niño like 1997-1998.” In comparison, current climate conditions appear more likely to produce a “gecko,” he said.
L’Heureux is not surprised by recent changes in Pacific Ocean sea surface temperatures. NOAA officials refrain from making any El Niño forecasts early in the year because sea surface temperature data obtained in March and April “are historically unreliable,” she said.
NOAA officials prefer to wait until June or July when they can see whether warming ocean temperatures are producing an atmospheric response, such as changes in trade winds. Even when that occurs, the agency couches forecasts for what it refers to as the El Niño Southern Oscillation in terms of probability. As of Oct. 9, the consensus opinion of NOAA forecasters was a 65 percent chance of El Niño occurring between November 2014 and January 2015.
Southern Oscillation refers to changes in air pressure often associated with the ocean conditions named El Niño by South American fishermen who noticed warm currents that sometimes appeared around Christmas. El Niño means “little boy” in Spanish and is a term often used to refer to Jesus.
NOAA forecasters also avoid predicting the strength of El Niño because “it’s a moving target,” L’Heureux said. When pressed, she said that as of early October, the most likely outcome for the coming winter is a weak El Niño.
Throughout the year, meteorologists monitor sea surface temperatures using Jason-2, which, like its predecessors — the Topex-Poseidon mission and Jason-1, launched in 1992 and 2001 respectively — carries an altimeter to bounce microwaves off the ocean’s surface and measure their travel time to create a detailed map of ocean height around the world every 10 days. Because warm water has greater volume than cold water, researchers can track changes in water temperature by noting variations in sea surface height.
The Jason-2 altimeter gathers data at two frequencies to negate the affect of Earth’s ionosphere on the microwave signal. In addition, the sea level measurement missions employ microwave radiometers to determine the water content of the atmosphere, which can have a significant impact on the speed microwaves travel.
By gathering data from with Topex-Poseidon, Jason-1 and Jason-2, researchers have compiled a 22-year series of global observations. That series is expected to continue with the scheduled March 2015 launch of Jason-3 on a Space Exploration Technologies Corp. Falcon 9 version 1.1 rocket, NASA spokesman Stephen Cole said by email.
“NASA, NOAA and CNES have maintained the continuity of these missions because they are so powerful,” Patzert said. “We have seen overall sea level rise of nearly three inches as well as regional changes and, of course, these missions are great and powerful El Niño monitors.”