Patches of unusually fresh water on an ocean’s surface can rev up hurricanes and typhoons, a new study reveals. Researchers first noticed the phenomenon for 2008’s Hurricane Omar (shown as a Category 1 storm on 15 October), which surged to Category 4 strength when it crossed a thick layer of fresher-than-normal surface waters east of Puerto Rico (island outlined north of the cyclone). Then, the team analyzed strength changes of almost 600 other tropical cyclones from 1998 through 2007, including hurricanes in the Caribbean and western Atlantic and typhoons in the western Pacific and northern Indian Oceans. In general, slow-moving cyclones that passed over swaths of ocean with low-salinity surface waters gained strength almost 50% more quickly than when they passed over waters of normal salinity, the researchers report online today in the Proceedings of the National Academy of Sciences. When an ocean is covered with a thick layer of low-salinity water (such as the fresh water that spills into the sea from a river), wave-induced mixing that brings cool, deep waters to the surface can’t operate as efficiently, the researchers explain. That, in turn, causes the sea surface to heat up, providing fuel for a growing storm. Because the salinity of the ocean surface isn’t taken into account in current models used to predict storm behavior, the new findings may help scientists better forecast changes in cyclone intensity.
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