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 ri