Harbinger of hurricanes to come

PENN STATE (US)—In an effort to foresee future hurricane activity, a team of scientists looked to the past and reconstructed 1,500 years of hurricanes using two independent methods.

The findings, featured in the Aug. 13 issue of Nature, suggest the Atlantic Ocean’s most active hurricane period occurred during the Medieval Climate Anomaly about a thousand years ago when conditions created a perfect storm of La Niña–like conditions combined with warm tropical Atlantic waters.

“Hurricane activity since the mid-1990s is the highest in the historical record, but that only goes back a little more than a century and is most accurate since the advent of air travel and satellites in recent decades,” says Michael Mann, professor of meteorology at Penn State.

“It is therefore difficult to assess if the recent increase in hurricane activity is in fact unusual. La Niña conditions are favorable for hurricanes because they lead to less wind shear in the tropical Atlantic.”

One method based the estimate of hurricane numbers on sediment deposited during landfall hurricanes. The researchers looked for coastal areas where water breached the normal boundaries of the beaches and overwashed into protected basins. Samples from Puerto Rico, the U.S. Gulf Coast, the southern U.S. coast, the mid-Atlantic coast, and the southeastern New England coast were radiocarbon dated and combined to form a history of landfall hurricanes.

The other method used a previously developed statistical model for predicting hurricane activity based on climate variables. They applied the model to paleoclimate reconstructions of tropical Atlantic sea surface temperature, the history of El Niño Southern Oscillation (ENSO) and another climate pattern called the North Atlantic Oscillation (NAO), which is related to the year-to-year fluctuations of the jet stream.

Warm waters are necessary for hurricane development, ENSO influences the wind shear and the NAO controls the path of storms, determining whether or not they encounter favorable conditions for development.

“We are at levels now that are about as high as anything we have seen in the past 1,000 years,” Mann says.

The researchers found that while both hurricane reconstructions indicate similar overall patterns and both indicate a high period of hurricane activity during the Medieval Climate Anomaly around A.D. 900 to 1100, they are not an identical match.

The exact force of a storm that will breach the beach area and deposit sediments is not known, they say, and the relationship between landfall hurricanes and those that remain at sea is not uniform through all time periods.

They believe, however, that key features like the medieval peak and subsequent lull are real and help validate current understanding of the factors governing long-term changes in Atlantic hurricane activity.

One thing the estimates show is that long periods of warm Atlantic Ocean conditions produce greater Atlantic hurricane activity.

“It seems that the paleodata support the contention that greenhouse warming may increase the frequency of Atlantic tropical storms,” says Mann. “It may not be just that the storms are stronger, but that there may be more of them as well.”

Researchers from the University of Massachusetts and Woods Hole Oceanographic Institute contributed to the study, which was supported by the National Science Foundation and the Bermuda Institute for Ocean Sciences.

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