|December 11, 2017|
Second La Niña Winter Could Extend Drought Across the West
|WINTER IS OFF to a dry start across the West, raising the specter of ongoing drought in many locations. The culprit could be La Niña -- a periodic cooling of Pacific Ocean waters near the equator that often brings drought. And not just any La Niña, but a "double whammy" effect, which latest research concludes may cause even worse water shortages.|
La Niña is popularly known as the opposite of El Niño, a warming of equatorial Pacific waters that can result in wet winters. But that is a misconception, according to Yuko Okumura, a research associate at the University of Texas Institute for Geophysics.
In reality, La Niña is unpredictable. As we found out last winter in California, for example, it can also produce wetter than average conditions.
"They are not quite mirror images," Okumura said. "There are important differences."
One of those is that a strong El Niño one winter is often followed by two or three winters of La Niña. And because La Niña often means drought in the Southwest and Texas, Okumura set out to understand this pattern better.
In two new studies, Okumura and her colleagues figured out how to predict this multiyear pattern more than 12 months in advance. Using computer models, they forecast the current second-year La Niña in the fall of 2015.
The researchers also found that the second La Niña year is often drier than the first -- even though the second La Niña may be weaker -- and the drought-affected area may grow larger. That's what the West is facing now: a second consecutive La Niña year, and the prospect of reduced precipitation across California -- even though Gov. Jerry Brown declared the drought over only eight months ago.
"We found the precipitation anomaly over the United States can be stronger in the second year. That was a surprise," she said. "Also, the pattern of the precipitation deficit will be slightly different. The region where we experience less precipitation in winter might expand to the North, compared to the first winter."
The La Niñas both this winter and last are considered weak, meaning the temperature change toward cooler Pacific waters is relatively small.
Jan Null, a consulting meteorologist based in the San Francisco Bay Area, said weak La Niña events were the most difficult to forecast.
"There's no such thing as a typical La Niña," he said. "There's a huge range of solutions of what happens in a La Niña -- from very wet ones to very dry ones. I don't think we have the skill to say what's going to happen."
Null points out on his website, for instance, that of the six California winters that caused at least $1 billion in flood damage, two were La Niña winters. The point is that precipitation can be a wild card during La Niña.
Temperature is another matter. It is fairly easy to assume La Niña will be warmer, thanks to climate change, he said.
"Pretty much we are warmer than it was over that average period from 1970 to 2000," Null said. "So forecasting above-normal from that is not rocket science, like forecasting the precipitation."
Higher temperatures do make it easier to predict other results, however, including a smaller snowpack and a greater risk of storm-induced flooding.
"The average temperature in California for December through February has increased 2.5F in the last century," said Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in Pasadena, California. "So that means the snowpack comes later and it leaves earlier. That in itself contributes to the drought."
What makes the difference in weak La Niña years is atmospheric rivers. Sometimes called "horizontal hurricanes," these events funnel tropical moisture to the West Coast in huge doses, like a fire hose. A series of atmospheric rivers was blamed for the heavy rainfall that led to the damage last winter at California's Oroville Dam.
"Last winter, atmospheric rivers played a huge role, both in the North and South, and the situation was very similar to what we're seeing now," Patzert said. "But December, January and February -- those are our wet months. So if something's going to happen, it better happen soon."
Unfortunately, December looks like it will be completely dry across most of the West. A dome of high pressure has parked itself off the West Coast, diverting storms far to the North, into Canada. It is very similar to the "ridiculously resilient ridge" that contributed to California's severe drought.
The latest pattern, dubbed a "Godzilla Ridge" by University of Washington atmospheric scientist Cliff Mass, appears likely to keep the West dry until at least mid-December. One new long-range forecasting tool, developed by the National Center for Atmospheric Research in Boulder, Colorado, indicates dry conditions are likely for the entire month.
Looking further out, a three-month forecast from the Climate Prediction Center at the National Weather Service shows dry conditions are likely across the southern half of the U.S. through February. This fits the "typical" La Niña pattern, but is much more uncertain given the long forecast time frame.
For California, La Niña creates unique challenges. Its impacts are usually sharply different on either side of a line crossing the middle of the state, causing dry conditions to the South and wet conditions to the North. Often this line falls near Interstate 80, which crosses the Sierra Nevada from West to East near Lake Tahoe. But it can shift and swing, depending on a variety of factors, such as atmospheric rivers.
"The impacts of La Niña can be very different from event to event in Northern California," Okumura said. "According to my study, this winter the precipitation will be reduced across the whole state of California."