Reaching down into the muck below a lagoon off Puerto Rico, two geologists at Woods Hole Oceanographic Institution reached back 5,000 years to compile the longest record of strong hurricanes in the Atlantic region.
The record showed that the dominant forces spawning heightened hurricane activity appeared to be atmospheric conditions generated by weak El Niños in the tropical Pacific and strong West African monsoons, Jeff Donnelly and Jon Woodruff reported May 24, 2007, in the journal Nature.
Somewhat to their surprise, they also found extensive periods of intense hurricanes in the past, when ocean temperatures were cooler than they are now. Today, concerns about global warming have focused public attention on warmer ocean waters as a prime suspect for increased hurricane activity.
“Warm sea surface temperatures are clearly important in fueling intense hurricanes,” Donnelly said. “Over the past several thousand years, ocean temperatures have never been as warm as they are now, so we have no analog to help predict how they will affect hurricane activity.”
“But our research demonstrates that the El Niño/Southern Oscillation and the West African monsoon are certainly important,” he said. “Understanding how they will change in a warming world could be extremely important in determining the kind of hurricane activity we will see in the future.”
Donnelly, an associate scientist at WHOI, and Woodruff, a doctoral student in the MIT/WHOI Joint Graduate Program, began reconstructing the history of Caribbean hurricanes in 2003, by driving 13-foot (4-meter) cores through the bottom of Laguna Playa Grande on Vieques, Puerto Rico. The lagoon is protected and separated from the ocean by dunes during all but the most severe tropical storms. But storm surges from intense hurricanes carry sand and bits of shells from the ocean beach over the dunes and into the lagoon, leaving telltale layers in the typically black, organic-rich silt on its bottom.
The interspersed layers of silt and sand recorded long stretches of frequent hurricane strikes punctuated by lulls that lasted many centuries. The team then compared their new hurricane record with other climate influences, such as El Niño, the periodic diminishing winds and buildup of warm waters in the eastern tropical Pacific. Other researchers have established that El Niño can stunt hurricane activity by causing strong high-altitude winds that shear the tops off hurricanes or tip them over as they form.
The researchers also examined precipitation records from Lake Ossa, Cameroon, and discovered that when monsoon rains increased, intense hurricanes occurred more often on the other side of the Atlantic. Researchers have theorized that storms over western Africa generate atmospheric waves that move into the Atlantic and provide “seedlings” for hurricane development.
“If we have few El Niño events and a strong West African monsoon, combined with exceedingly high sea surface temperatures, we could experience an active hurricane period that is unprecedented in the last 5,000 years,” Donnelly said. “Conversely, if we have more steady-state El Niño conditions, it may reduce—but not stop—intense hurricane activity in a warmer world.”
The research was funded by the National Science Foundation, the Risk Prediction Initiative, the National Geographic Society, the WHOI Coastal Ocean Institute, and the Andrew W. Mellon Foundation.