Back to Spring 2010 Newsletter
by Eric Betz, Department of Physics and Astronomy and the Department of Communication, Northern Arizona University
A surge of activity in the Pacific Ocean coupled with agreements between multiple computer weather pattern simulation models, led climatologists to suspect a rash of winter and spring storms was headed towards Arizona in 2010; storms born from a massive and warm ocean wave.
In April 2009, information relayed to the Climate Prediction Center in Washington, D.C. from satellites, buoys and ships in the eastern Pacific Ocean showed a marked increase in water temperature, a classical indication of a pending El Niño. To be categorized as an El Niño, there must be at least a half degree Celsius rise in ocean temperatures and when the CPC declared the winter of 2009 2010 as such, this one barely made the grade.
El Niño historically has been a mixed blessing for Arizona, while it’s known to cut short summer monsoons, it can also bring an increase in fall and winter precipitation. Nothing is certain in a weak El Niño year though; the last 50 years worth of rainfall data shows that a weak or average El Niño is just as likely to bring less precipitation to the state as it is to bring more.
“If El Niño is weak to moderate,” said Nick Petro, “I wouldn’t hang my hat on it.” Petro is the senior science advisor at the National Weather Service office in Bellemont, AZ and has compiled Arizona precipitation data from El Niño years over the last half century.
By fall, the year was fast becoming one of the driest ever recorded in many parts of the state largely thanks to the lackluster monsoon. In fact, the last decade has brought a succession of years with very little wet weather. Since 1999, the Colorado River, crucial to the water supplies of 30 million westerners, has run at above average levels only once.
The lack of persistent precipitation this decade has left Lake Powell with a “bathtub ring,” showing how far the reservoir level has plummeted in recent years and pushing questions about the water supply of a state peppered with fairways, swimming pools and desert waterscapes.
A massive warm ocean wave appeared late in fall though that has given confidence to forecasters that the winter of 2009-2010 will extinguish the dry weather in Arizona, if only temporarily.
Under normal conditions in the Pacific, fierce tradewinds force surface water from east to west. As this warmer surface water is pushed into the western Paccific, it forms what is known as the “warm pool”and cold, nutrient rich water upwells off the coast of the Americas to fill the warm water’s place. This cycle creates the unstable atmospheric conditions in the western Pacific that fuel rainfall and influence weather throughout the world. El Niño reverses that pattern.
For unknown reasons, tradewinds weaken in an El Niño and shutdown the movement of warm water from east to west. The warmer waters that normally persist in the west spread back to the Pacific coasts of the Americas and rework weather in the United States and across the Earth. El Niño can carry perilous floods to Peru, Chile and California, spread ruining droughts throughout Australia, Indonesia and India, and intensify fall and winter snowfall in the Southwest.
El Niño is no guarantee of an increase in wet weather though. The last two El Niño years have brought snowfalls less than half of average to many northern parts of the state.
“Are we gonna break through this dry weather pattern?” said Petro, “it’s looking like it.”
Ocean temperatures off the Pacific coast of the Americas remained steady through summer and by fall a weak El Niño looked imminent. Then, throughout October ocean temperatures across the eastern Pacific increased nearly a degree, with some areas seeing as much as a 5 degree increase. The sudden rise in temperature came as a result of a dramatic El Niño event known as a Kelvin wave.
A Kelvin wave is a vast redistribution of warm water from the west caused by diminishing tradewinds. Kelvin waves stretch hundreds to thousands of miles and are indicative of El Niño activity. This particular wave has given NWS forecasters confidence that the coming winter will bring strong snowfalls to Northern Arizona. “That wave helped strengthen El Niño” said Petro. “The temperatures in the eastern pacific are now 1.5 to 1.7 degrees above normal.”
That temperature increase has reclassified this winter as at least a moderate to strong El Niño and that gives scientists at the CPC historical justification to suspect an increase in precipitation through spring and possibly into early summer.
Petro differentiates Kelvin waves and even El Niño as individual factors in the much bigger picture of weather patterns though and reasons that the future is uncertain.
“It’s hard enough to predict the next seven days, let alone anything beyond that,” said Petro.
Even if Arizona does receive a succession of strong winter and spring storms, the state still won’t be able to overcome years of drought conditions. A decade of drought as devastating as the Southwest has seen would take many years to pull out of.
What’s more, climatologists only expect the pattern to get worse. Intensive studies of likely impacts from climate change show that the Southwest is likely to be among the worst hit places in the world. Conditions are likely to progressively worsen for the foreseeable future, with the Colorado river decreasing its flow to levels unseen for centuries by 2050 and dramatic die-offs of native species like Pinon and Ponderosa Pines.
In the meantime, several seasons of increased precipitation could bring a momentary breath of life to parched and precarious desert climes.