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Ancient shipwrecks, tree rings provide clues into Caribbean's hurricane history

Driven by the pursuit of wealth in distant lands, the Spanish flag adorned many maritime vessels as they traversed the turbulent waters of the icy Atlantic, beginning in the late 1400s.

While many riches were gained in this new era of exploration and conquest, many Spanish vessels also perished into the briny abyss beneath chaotic, storm-driven waves. Today, these ancient shipwrecks, along with the history imbued in the rings of Caribbean pine trees, are providing new data for more than 300 years of the region's hurricane activity.

"When a hurricane hits, the [trees] lose their needles, and when salt water, or storm surge hits, it slows growth," University of Arizona Associate Professor of Dendrochronology Valerie Trouet said regarding her findings in a recent study published in PNAS, which found a 75 percent decrease in Caribbean hurricane activity between 1645-1715.

The slower growth of the trees is marked by narrowing of the bands in the tree rings up to a few years following a hurricane impact, Trouet added, stating data had been collected by University of Southern Mississippi researcher Grant Harley when she met with him and fellow dendrochronologist, Marta Domínguez Delmás, at a conference in 2013.

Following a discussion at the conference, the three researchers were motivated to conduct the first study of its kind to include both ancient shipwrecks and tree-ring data for hurricane research.

"We looked at the record from 1495-1825," Trouet said.

The team filtered through a shipwreck database and narrowed their search to include only Spanish shipwrecks, 657 in total.

The shipwrecks included in the study were those documented to be caused by storms, or unknown causes, and not by other factors including pirates, as the timeframe spanned the "Golden Age of Piracy," Trouet said.

Following a year and a half of research, the team discovered that the narrowing tree-rings corresponded to years where ships were lost to storms, allowing them to reconstruct cyclone activity dating back to 1495.

Another discovery the team made included evidence of a lull in cyclone activity in a period of reduced temperatures in the northern Atlantic and decreased sunspot activity in the late 17th century known as the Maunder Minimum.

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"This period of solar inactivity also corresponds to a climatic period called the 'Little Ice Age' when rivers that are normally ice-free froze and snow fields remained year-round at lower altitudes. The connection between solar activity and terrestrial climate is an area of on-going research," according to a NASA report.

According to a report by Trouet's team, lower sea surface temperatures during the Maunder Minimum, because of decreased solar irradiance, resulted in fewer tropical cyclones and fewer shipwrecks.

The researchers also looked at factors like the El Nino-Southern Oscillation and North Atlantic Oscillation (NAO), which likely converged during the Maunder Minimum to enhance the decrease in hurricane activity further.

"We know from 20th-century data that these climate patterns play an important role in Caribbean tropical cyclones, with fewer cyclones occurring during positive El Nino phases and negative NAO phases. For instance, the continued hurricane drought in 2015 has been attributed to this year's strong El Nino," according to the report on TheConversation.com.

For a decade, Florida, a state which has been hit by seven of the 10 most costly and damaging hurricanes in U.S. history, has not had a major landfall.

"It's very unusual," AccuWeather.com Expert Hurricane Meteorologist Dan Kottlowski said last year. "This is by far the longest that state has gone without a hurricane hit going back to 1851."

However, unlike the period of the Maunder Minimum, Kottlowski said the Atlantic has not necessarily had a drastic decrease in tropical storm development, but rather a deflection of storms tracking directly into Florida.

"They're out there," he said.

A combination of factors may be contributing to Florida's decade-long respite from a major landfall, according to Kottlowski.

Frequent, multi-decade drought in northern Africa sends dust and dry air across the Atlantic, and both are inhibiting factors for tropical systems.

In addition, drier air moving from Spain and Portugal, pushing across the Atlantic and dipping down the Caribbean might also be a factor, he added. This influx of drier air, which has been above normal in recent years, has impacted thunderstorm development and in turn tropical storm development, Kottlowski said.

Perhaps the most predominant factor in storms tracking away from Florida over the past decade is the influence of the Bermuda-Azores High, an area of high pressure that has deflected storms farther south in the Caribbean and into areas like Central America.

The orientation and strength of the Bermuda High have consistently changed, even weakening at points, but the Beruda High has been one of the major factors in diverting storms away from the Sunshine State, which Kottlowski likened to winning hockey games.

"If you have a good goalie, you're going to win more games," he said. "Our goalie is that high pressure."