Scientists looking for an oceanic counterpart to tree rings that record past weather have found one in the Dry Tortugas National Park . Corals here contain chemical signals of past water temps. Coral core evidence shows a 60-85yr long cycle called the Atlantic Multidecadal Oscillation (AMO) tha that has taken place since the 1730’s, influencing mainland rainfall, droughts,hurricanes, and Gulf Stream flows.
The AMO has a large impact on humans and the economy through it’s influence on rainfall patterns. Climate scientists suspect that the AMO is a natural climate cycle that has existed for more than 1,000 yrs. Until recently most of the evidence of it came from ships at sea, and only went back 150yrs.
The Dry Tortugas core samples show several complete AMO cycles going back 278 yrs. This give climate modelers much new info to work with as they try to understand the past and try to predict future weather. The Dry Tortuga samples accurately track major climate phenomena like the Little Ice Age that ended in the early 1800’s , and the tough Dust Bowl drought of the 1930’s.
Dry Torrugas National Park is a group of small remote islands at an important marine cross roads; the Florida Straits. where the gulf of Mexico and the Caribbean Sea flow into the Atlantic Ocean. A large zone of sea water surrounds these islands called the Atlantic Warm Pool, which can heat up to 28.5 Celsius or more. The heat stored here appears to influence rainfall in the Caribbean and parts of North America, and the formation and intensity of hurricanes.
The Dry Tortugas also lie near the origin of the Gulf Stream, the current that carries warm seawater north to Greenland, where it chills, descends deep into the water column , then heads back to the equator. This north/south movement is called the Atlantic Meridional Overturning Circulation(AMOC), which affects weather in the North Atlantic,and much of Europe. The AMOC has been understood for years, but parts of it like the AMO are only recent discoveries.
The coral core samples taken from the Dry Tortugas show growth rings that preserve evidence of past weather conditions. While alive corals take up strontium and calcium from seawater in ratios that vary with water temperature. By measuring the ratios in the coral we can reconstruct past sea surface temperatures going back 278 years.
The corals showed that after a cool spell in the 1960’s sea surface temps have risen by .8 degrees Celsius between 1970 and 2012. They also show two sets of oscillations in sea surface temps: a shorter cycle lasting 28-30 yrs, and a longer cycle of 80-90 yrs, consistent with the AMO.
By looking at the sea surface temps in the Dry Tortugas, climatologists may be able to predict weather changes that would affect the entire North Atlantic basin.