OCEANUS
A Rainfall Forecast Worth its Salt
WHOI scientist Ray Schmitt and sons take top prize in rainfall forecasting competition
| March 25, 2019
Seven-to-ten day forecasts help us plan our daily lives, but when it comes to sub-seasonal to seasonal forecasts—the 3-week to 3-month weather predictions we don’t rely on the evening news for—there may be a lot more at stake. Droughts, for example, can put millions at risk for food insecurity and water management. Extreme floods and hurricane seasons can destroy neighborhoods, businesses, and infrastructure.
When WHOI scientist Ray Schmitt heard about a U.S. government-sponsored weather forecasting competition aimed at improving sub-seasonal forecasts, he wanted in. As a climate scientist, he had been on his own winning streak in predicting seasonal rainfall in regions ranging from the African Sahel to the U.S. Midwest.
“A few days before Christmas in 2016, I got an email from my WHOI colleague Steve Elgar that pointed me to a weather prediction contest that had just been announced by the U.S. Bureau of Reclamation (USBR),” said Schmitt. “They were offering big prize money if you could provide the most accurate rainfall predictions over a whole year for the U.S. West in their ‘Climate Forecast Rodeo’.”
Getting into the game, for Schmitt, was a way to leverage a unique rainfall prediction technique he’d been honing at WHOI for decades. Rather than basing seasonal forecasts on the El Nino phenomena, which is what most forecasts focus on, his method uses salinity in the ocean as a natural rain gauge.
Ray’s Rule
The idea struck him in 1993 after the Mississippi and Missouri River floods. After the seven-month dousing of the region, he noticed reports of abnormally low salinity in the Gulf of Mexico and Gulf Stream.
“Since all that flood water was freshening the ocean, conservation of water and salt requires that some part of the ocean had to get saltier before the floods. A lot of freshwater had to leave the ocean ahead of time to supply the extra rainfall on land.” he said. Using “Ray’s Rule”, that areas of water export must equal areas of water import, he realized that a large region of the ocean had to lose fresh water to evaporation in order to supply the flood on land. That evaporation would have to make the surface ocean saltier than normal. “With that basic concept in mind, I began looking at high-salinity areas of the ocean to see if variations in salinity could be used to predict rainfall.”
By May, 2016, the line of investigation took a fruitful turn. Schmitt and Laifang Li, a postdoctoral scholar in physical oceanography at WHOI, with co-authors Caroline Ummenhofer and Kris Karnauskas, published a study showing that high springtime salinity levels in the subtropical Atlantic Ocean correlated closely with increased summer-season rainfall in the African Sahel, where even small shifts in rainfall patterns can be a matter of life and death for millions of people. “It was a remarkable result,” said Schmitt.
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