Scientists seeking an oceanic counterpart to tree rings that record past weather on land found one in Dry Tortugas National Park, where corals contain chemical signals of past water temperatures. Researchers found coral core evidence that the 60- to 85-year-long cycle called the Atlantic Multidecadal Oscillation (AMO) has taken place since the 1730s, influencing mainland rainfall, droughts, hurricanes and Gulf Stream flows.