When a hurricane comes around, it's best to get out of the way -- unless you have a thing for risky behavior! Or, perhaps, you're in the business of studying them. If that's the case, a good way to get data out of a hurricane is to head straight into one. That could involve a white-knuckled flight into the eye of a storm in a specially designed aircraft called a "Hurricane Hunter."
But when geologist Jeff Donnelly of the Woods Hole Oceanographic Institution (WHOI) hunts for hurricanes, he does it safely at ground level, or just slightly below. He is even able to do it without having to encounter so much as a drop of rain or a gust of wind.
Donnelly leads a team from the WHOI and the University of Massachusetts that studies long-term global hurricane patterns. For the research, the team travels to coastal areas where a hurricane's storm surge has washed over beaches and left sediment deposits, such as sand, in areas where there normally wouldn't be any.
"It's usually very muddy, but during those extreme events, sand is washed in and it leaves a geological record," says Donnelly. By drilling and studying cores from different points around the Atlantic Ocean, he and his team have been able to paint a picture of long-term global hurricane activity. They've unearthed some interesting findings about past hurricane activity which might give us a hint about what we can expect in the future.
On this particular sunny and frigid December day, Donnelly is armed with three assistants and tools for digging--most notably, an aluminum tube measuring about six feet in length. The researchers carefully make their way into the icy cold, ankle deep, muddy marshes of Cape Cod, Mass. The low winter sun angle does little to warm the team, but no one seems to mind. Donnelly is confident that buried in this cold mud are the echoes of hurricanes long gone--layers of sand, violently deposited here from the nearby Atlantic Ocean. He just needs to find a good spot where he can literally suck the dirt out of the marsh and into the tube.
"We're going to put a plug in the top of the tube and it's like basically putting your finger over the top of a straw in a milkshake, and you pull it out and the milkshake--or the mud, in this case--will still be in the tube," explains Donnelly. As the dig concludes, Donnelly takes note of the marsh's tranquility. The sample of marsh mud being collected will reveal just how often the quiet here is disturbed by hurricanes, he adds.
It's a 40-minute drive back to the lab at WHOI where the aluminum tube housing the mud sample is sliced right through the center, from the top to the bottom. The newly separated halves of the six-foot-long tube now reveal the mud housed in a long "U" shaped aluminum container.
The mud's color is not consistent. There are bands of lighter and darker deposits of sand at different parts of the tube. Donnelly points to very muddy grass at the end of the sliced tube.
"This grass is what we were standing on when we took the core. As we move down, we are basically moving back through time. Down here, around 30 centimeters, is probably where the industrial revolution is. Earlier on, you see we have this white layer at around 40 centimeters and if you feel it, you can actually feel all the course grains of sand that were transported from the beach into the marsh during the storm event," explains Donnelly. "This probably dates from the early 1800s or late 1700s and likely reflects a hurricane that occurred in that time. As we go further back down the core, there is another light layer here, around 80, 88 centimeters or so, probably 800 or 900 years ago, and you can feel how it's got some sand in it, as well, providing evidence of a storm we knew nothing about before we started doing this work."
After visual inspection, the tubes are X-rayed and chemically scanned to see subtle differences in the cores not detected by the naked eye, and also to help accurately date them. By analyzing hundreds of core samples from dozens of locations, the team has begun to paint a picture of hurricane activity in the Atlantic dating back nearly 1,500 years. Research funded by NSF is also allowing the team to develop global reconstructions dating back several millennia. Studies covering these longer periods of time reveal that the Atlantic and Pacific Oceans are out of sync. "When the Pacific is active, the Atlantic is inactive and so they're basically out of phase," notes Donnelly.
When looking specifically at the study that covers Atlantic activity over 1,500 years, the team found distinct periods of activity. "About 1,500 years ago to maybe 800 years ago, there was a very active interval and then it wasn't so active for several hundred years. Then, it became active again around 300 years ago," Donnelly explains.
One clue as to what might have fueled that activity was a rise in sea surface temperatures. "We've been able to establish there was a very active hurricane period for several hundred years around 1,000 years ago. That was a period when sea surface temperatures tended to be higher and El Niño activity was reduced," he adds.
As the Earth's climate warms from human-induced climate change, so are the planet's oceans. Could that affect hurricane activity? "What's going to happen in the next hundred years is still an open question, but it's clear climate change can modulate hurricane activity. We're clearly influencing climate. How we're influencing hurricane activity remains to be seen," says Donnelly.