Weather Makers - A Look at El Niño and More

It’s baaaaacck.

On July 9, scientists from the National Oceanic and Atmospheric Administration (NOAA) announced ththat an El Niño is forming in the Pacific Ocean, which can affect weather around the world. The most recent El Niño occurred in 2006.

El Niño is the large-scale climate phenomenon linked to periodic warming in sea surface temperatures in the eastern tropical Pacific Ocean. Its impacts depend on a variety of factors, such as intensity and extent of ocean warming and the time of year. Strong El Niño events can bring damaging winter storms in California and increased storminess - and water supply - across the southern United States, while weaker El Niños can actually result in below-normal rainfall for the same areas.

NOAA forecasters say that the sea surface temperature in June climbed to more than 1.5 degrees Fahrenheit above normal along a narrow band in the eastern equatorial Pacific. Forecasters expect this El Niño to continue developing during the next several months, and to last through winter 2009-10.

There are conflicting opinions on the eventual strength of this El Niño, but according to NOAA, current conditions and recent trends favor the continued development of a weak-to-moderate strength El Niño in the Northern Hemisphere in the fall.

Although NOAA says further strengthening is possible after that, Bill Patzert, a climate scientist at NASA’s Jet Propulsion Laboratory in Pasadena, calls this one “El Wimpo” or “the great wet hope.”

“Not all El Niños are equal,” he said, “they come in all sizes. This one is definitely modest.”

There are many myths surrounding this climate phenomenon. See the box below for some of the more common ones.

So, what’s the difference between El Niño, La Niña, and Pineapple Express?

La Niña (the opposite phase of the Southern Oscillation from El Niño) refers to the periodic cooling of ocean surface temperatures (or below-average sea surface temperatures) in the central and east-central equatorial Pacific.

The Pineapple Express is distinct from El Niño and La Niña activity and not associated strongly with either phase of the Southern Oscillation. The Pineapple Express is a meteorological phenomenon driven by a strong, southern branch of the Polar jet stream and characterized by a strong, persistent flow of atmospheric moisture and heavy rainfall extending from near the Hawaiian Islands to the coast of North America.

“Pineapple Express circulations yield warm-wet storms along the west coast of North America, and are known for the floods that they can generate,” said Mike Dettinger of U.S. Geological Survey in a 2004 Public Interest Energy Research Program (PIER) report. Characterized by their tendency to draw warm, wet air from the tropics near Hawaii, Pineapple Express storms then deliver it in violent, unusually warm and unusually wet storms on the west coast of North America.

In the Sierra Nevada during winter, the storms average about twice as much precipitation as other, non-Pineapple Express storms. The storms also yield warmer minimum temperatures, and produce daily increases in stream flow that are larger than those from other types of storms, the report further said.

The historic flooding and series of California storms at the end of December, 1996 and into early January, 1997 was considered a Pineapple Express. The resulting widespread flooding wasn’t due to a single storm, but a series of successive storms spanning just over a week that resulted in increased runoff until a final New Year’s Day storm pushed rivers and flood protection projects beyond their natural or designated capacities.

A New Form of El Niño

The late William H. Quinn, an early expert on the El Niño Southern Oscillation (ENSO) phenomenon was often quoted as saying, “The atmosphere gives you weather, but the oceans produce climate.” We continue to learn more about the ENSO phenomenon and climate. For example, El Niño now is known to come in two forms with different impacts according to a new study from climatologists at the Georgia Institute of Technology

This second form of El Niño, known as El Niño Modoki (from the Japanese meaning similar, but different), forms in the Central Pacific rather than the Eastern Pacific as the typical El Niño events that reach the South American coast. Warming in the Central Pacific is associated with a higher storm frequency and a greater potential for hurricanes making landfall along the Gulf Coast and the coast of Central America. The traditional El Niño tends to reduce the number of Atlantic hurricanes, but El Niño Modoki can lead to more hurricanes than usual in the Atlantic Ocean.

“It’s not clear why this new form is occurring more commonly,” said Peter J. Webster, a professor at Georgia Tech’s School of Earth and Atmospheric Sciences and a co-author of a report on the finding that was published in the July 3, 2009 edition of the journal Science.

“This could be part of a natural oscillation of El Niño,” Webster said. “Or it could be El Niño’s response to a warming atmosphere. There are hints that the trade winds of the Pacific have become weaker with time and this may lead to the warming occurring further to the west. We need more data before we know for sure.”

Whether you call it El Niño, El Wimpo, or El Niño Modoki, it’s here and could have an impact on California’s weather.

But Patzert cautioned against breaking out the umbrellas and tossing out water conservation measures. “Don’t turn the sprinklers back on to five days a week, or leave the water running when you brush your teeth,” he said. “There have been a lot of false starts on El Niño in the past. A couple years ago when NOAA started talking about El Niño, we had the lowest recorded rainfall in Los Angeles that year.”