By Abby Metzger
Posted March 9, 2018
In the fall of 2015, California was bracing for unbelievable rain. Ominous headlines warned of a “Godzilla” El Niño event that promised a punishingly wet winter. The California Department of Transportation even increased its maintenance staff by 25 percent, readying sandbags and plows in case of floods, mudslides or worse.
But as winter progressed, the torrential rains never materialized. And in Southern California, where El Niños have historically had the greatest impact, the Godzilla winter turned out to be among the driest on record.
With all our satellites and simulations, what exactly went wrong?
Nick Siler, then a post-doctoral fellow at the Scripps Institution of Oceanography, wondered the same thing.
“The seasonal forecast models all predicted wetter-than-average conditions, so it was a failure of the models and not just some scientists overstating things,” says Siler, a recently hired assistant professor of atmospheric sciences in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State.
Getting seasonal forecasts right in California is surprisingly hard, Siler explains. Rain there can be “flashy,” arriving suddenly and strongly. Water managers have to plan for floods and also ensure enough water is stored in reservoirs to make it through the summer.
“It doesn’t rain often, but when it does, it comes in these very big storms, what we sometimes call atmospheric rivers. So, it’s actually a very tricky place to try and manage water resources,” Siler says.
Siler recalls a scientific meeting where one presenter compared precipitation forecasts in California to observed records over the past ten years. More often than not, the forecast models were wrong. “Most models actually had negative predictive skill in California, meaning you’d be better off betting the opposite of whatever the model predicts,” he says.
To investigate the forecast failings during the 2015-16 winter, Siler and his collaborators compared the El Niño event to previous strong El Niños and found a wide range of differences in the atmospheric responses across the globe. By running a series of experiments with a global climate model, his team found that the key to those differences stemmed in part from sea surface temperatures in the Indian Ocean.
That’s right – California’s left-hook dry winter may have been caused by warmer sea surface temperatures in the tropics.
“The Indian Ocean has been warming faster than the rest of the tropical oceans, and what we found is that a warm Indian Ocean relative to the rest of the tropics tends to push the storm track farther north, which is consistent with a drier California,” he says. “The forecast models seem to have trouble capturing this pattern, for reasons we don’t fully understand.”
Siler hopes that this work will eventually improve the forecast models, which could help water managers plan for the year ahead. And in an agricultural Mecca like California, getting water availability right is critical.
“California water resource managers rely a lot on seasonal forecasts. The climate prediction center at NOAA issues these forecasts every month, and water resource managers pay a lot of attention to them,” he says. “These forecasts will never be perfect, but we can probably do better.”
While his research examines the global ocean and atmosphere, his work ties him to his roots, which are not far from Oregon. Siler grew up in a small farming community in Eastern Washington, and his father still works in the grain industry there.
“In the Palouse, farmers don’t irrigate, so they care a lot about how wet the winter and spring are. With my dad especially, conversations about my research mostly revolve around its implications for farmers,” he says.
Through his undergraduate studies at Harvard, his graduate work at the University of Washington, and his post-doc at Scripps, Siler honed his interest in the water cycle and the physics that influence it. His interest in precipitation spans both personal and professional, whether following snow forecasts in hopes of a good ski season, or studying rain shadows, clouds and mountain precipitation.
At Oregon State, Siler continues to study the water cycle, trying to better understand both its current variability and response to climate change. While he’s covered a lot of ground since his days on the family farm, Siler has hopes of collaborating with local growers and using his research to help them be successful.
“Precipitation matters to everybody, but perhaps no one more than farmers,” he says. “I think it would be great to work with people in the farming communities around here to study aspects of climate variability and climate change that could impact their livelihoods.”
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