Magazine article Oceanus

To Forecast Rain, Look to the Ocean: Scientists Explore Compelling New Way to Predict Seasonal Rainfall

Magazine article Oceanus

To Forecast Rain, Look to the Ocean: Scientists Explore Compelling New Way to Predict Seasonal Rainfall

Article excerpt

Ever since humans have existed on Earth, they have looked to the heavens to forecast rain. But more reliable clues may lie in the ocean.

New research by scientists at Woods Hole Oceanographic Institution has found clear links between saltier regions in the North Atlantic Ocean in the spring and increased rainfall during the following summer over areas of Africa and the United States. The discovery offers potential breakthroughs to predict rainfall in regions "where even slight variations in rainfall can be a matter of life or death for millions of people," said Laifang Li, a WHOI physical oceanographer.

Li and colleagues suggest that saltier-than-normal areas in the ocean indicate places where evaporation has increased--leaving salt behind and putting more fresh water vapor into the atmosphere. The researchers tracked how water that evaporated over ocean regions eventually rained on certain regions on land.

Each year, an estimated 100,000 cubic miles of water evaporates from the ocean's surface--enough to flood our entire lower 48 states to a depth of 180 feet. About 90 percent of this moisture precipitates right back into the ocean--a vast recycling of moisture that represents the bulk of our planet's water cycle. But about 10 percent of the evaporated water gets carried over land and precipitates there.

Li and WHOI colleagues Ray Schmitt, Caroline Ummenhofer, and Kris Karnauskas analyzed 60 years of data on rainfall and ocean salinity. Schmitt and colleagues used detailed salinity maps generated by NASA's Aquarius satellite to identify a patch of the North Atlantic with the highest salt concentrations anywhere in the open ocean.

Li and Schmitt drew a box around the area and explored the central question: Where did all the evaporated fresh water from that area go? They found that it went to the Sahel region in northern Africa. And across the ocean, they found that evaporation that left saltier areas in the western Atlantic and the Gulf of Mexico led to more rainfall in the United States Midwest.

The water isn't directly or quickly transported from ocean to land, however. Upon closer analysis, they found a more complicated process that creates a three-month lag time.

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The process works like this: Water vapor evaporated from the ocean is transported through the atmosphere to Africa. …

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