Scientists seeking an oceanic counterpart to the tree rings that document past weather patterns on land have found one in the subtropical waters of Dry Tortugas National Park near the Florida Keys, where long-lived boulder corals contain the chemical signals of past water temperatures. By analyzing coral samples, USGS researchers and their colleagues have found evidence that an important 60- to 85-year-long cycle of ocean warming and cooling has been taking place in the region as far back as the 1730s.
The cycle called the Atlantic Multidecadal Oscillation, or AMO, is linked to rainfall over most of the US, Midwestern droughts, hurricane intensification and landfalls, and the transfer of ocean heat from the tropical Caribbean Sea to the North Atlantic Ocean by way of the Gulf Stream. It interacts with ongoing climate change in poorly understood ways, and it is very hard to spot in pre-20th century records.
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