ScienceShot: Why Did Sea Level Drop in 2010?

Sea level has been rising about 3 millimeters per year, on average, since 1993, according to data gathered by space-based radar altimeters. But in mid-2010, scientists noticed a curious trend: For the first time in two decades, global average sea level began dropping. Over the course of 6 months or so, sea level fell about 7 mm, and it didn’t rise back to its previous level until the end of 2011. Researchers looking to solve this mystery found that ocean heat content had remained high, so a sudden chill in ocean waters (which would have caused upper layers of the seas to shrink in volume) wasn’t the answer. Then, they looked at satellite data that measures variations in Earth’s gravitational field to track the long-term and seasonal movements of water between the oceans and the continents. Voilà! About half the drop in sea level during the 18-month period in question could be ascribed to unusually high rainfall in Australia, the researchers will report in a forthcoming issue of Geophysical Research Letters. In late 2010 and early 2011, the continent Down Under received about twice its normal complement of rain, thanks in large part to unusually warm sea-surface temperatures just north of Australia and a particularly strong La Niña—in essence, combining a source of warm humid air with the weather patterns that steered the moisture over the continent where it condensed and fell as precipitation. One big reason the sea level dropped for so long: Only 6% of the rainfall in Australia runs directly back to the sea, and the rest runs inland to lowlands (image depicts a normally dry area in eastern Australia flooded by rains in late November 2011) where it either soaks into the ground or evaporates back into the atmosphere—a process that returns moisture to the sea far more slowly than the rivers draining other continents do.

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