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Ocean Salinity Can Predict Rainfall a Continent Away

Lead author Laifang Li, with co-authors Ray Schmitt and Caroline Ummenhofer.
Jayne Doucette
Woods Hole Oceanographic Institution

We all love to malign the weather man. But it’s worth taking a moment to appreciate the knowledge and technology that enables forecasts for not only next week, but next month and even years to come. For example, New England is expected to see a warmer – and possible wetter – than usual summer. A major factor in those kinds of predictions is typically ocean temperatures. Now, researchers at Woods Hole Oceanographic Institution say they have a better way to make seasonal rainfall predictions, and it’s based on salinity rather than temperature of the ocean.

Each year, 100,000 cubic miles of water evaporates from the surface of the ocean. That's enough to flood the entire continental U.S. up to a depth of 180 feet. Fortunately for us, ninety percent of that water rains back down on the ocean, but not necessarily in the same place.

When water evaporates, it leaves behind salt. So, the places where water is evaporating get saltier and the areas where it's raining get fresher. That tends to vary from season to season, and year to year. With global climate change, both evaporation and precipitation are increasing, making those differences more pronounced.

“We have pretty good salinity records, especially for the North Atlantic," says Ray Schmitt, a physical oceanographer at Woods Hole Oceanographic Institution. "We can see that the salty areas are getting saltier.”

Several years ago, Schmitt and Laifang Li, a postdoctoral researcher in his research group, started wondering if they could see links between patches of high salinity in the ocean, and periods of rainfall on land. Li sifted through sixty years worth of salinity and rainfall data and found two connections: salinity in the northeastern Atlantic is linked to rainfall in the African Sahel, while salinity near Bermuda correlates with rainfall in the American Mid-west.
“What we’re finding is that certain regions in the ocean have these teleconnections with certain regions of higher rainfall on land,” says Schmitt.

They aren't necessarily simple relationships; in the case of the African Sahel, there's a three month lag between high salinity and increased precipitation. But Schmitt says salinity is an integrator - a composite - of not only ocean temperatures but a host of other weather factors, so could produce more accurate seasonal forecasts than temperature, alone. He and his colleagues are currently working with the National Oceanic and Atmospheric Administration to get salinity included in their forecasting models.

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