"Our results stress that in addition to sea surface temperature, which almost certainly plays some role in governing Western Atlantic hurricane intensity, understanding how the El Niño/Southern Oscillation (ENSO) and the West African Monsoon will respond to future predicted climate changes is also very important when projecting future variability in hurricane activity," Jonathan Woodruff of the Woods Hole Oceanographic Institution told environmentalresearchweb.
Woodruff and colleague Jeffrey Donnelly took sediment cores from the Laguna Playa Grande on the island of Vieques, Puerto Rico, in the north-eastern Caribbean Sea. A 2–3 m high sandy barrier separates this lagoon from the sea.
"[Marshes and shallow lagoons] contain coarse-grained sand layers which can be dated to historical hurricane strikes," said Woodruff. "In addition to historical hurricane deposits, these sediments also contain coarse-grain layers from a time well before the first settlers and mariners started noting the occurrence of these storm events in their diaries and log books."
The team correlated the earlier sediment deposits with hurricane records before looking further back in time.
The sediment data revealed that the lagoon experienced relatively frequent intense hurricane strikes from 5,400 to 3,600 years before present (BP, using 1950 as a baseline). The area then had relatively few extreme events until roughly 2,500 years ago, when hurricane activity stepped up. The period from 1000 to 250 years BP again saw relatively few hurricanes: since then hurricanes have been more active.
"Similar work being conducted further to the north in New York tidal marshes and in coastal ponds in the Gulf of Mexico observes very similar trends," said Woodruff.
What's more, some of the periods of intense hurricane activity occurred when sea surface temperatures were cooler than today, indicating that sea surface temperatures are not the only factor behind hurricane activity.
"Palaeo-reconstructions for the ENSO and the West African Monsoon show very similar fluctuations to those observed in our palaeo-hurricane record," said Woodruff. "Specifically, these reconstructions suggest that the Western Atlantic exhibited periods of low hurricane activity spanning many centuries during a time when El Niño conditions were occurring more frequently and fewer storms were being shed off of Western Africa."
Woodruff says that the jury is still out as to what might have driven this climate variability in ENSO and African storminess, but it appears also to have played a dominant role in governing hurricane activity.
Storms in the Western Atlantic make up a relatively small percentage of tropical cyclones around the globe. "On average over a third of all intense tropical cyclones occur within the Western North Pacific basin, yet almost nothing is known about how typhoon activity for this region has varied prior to the documented record," said Woodruff. "We have performed similar work to that in the Vieques study in southern Japan with quite a bit of success."
However, the researchers say that with only one record for the area it is difficult to make any definitive conclusions about how typhoon variability may have fluctuated. "We are therefore preparing to expand these efforts to other sites within the Western Pacific to improve our understanding of what has driven tropical cyclone variability within this very active tropical cyclone basin," said Woodruff.
The researchers reported their work in Nature.