Cold water helps protect Delaware from hurricanes

Some 50 feet below the surface of the ocean, a pool of frigid water lurks with the ability to lessen the intensity of hurricanes in the mid-Atlantic.

This cold pool may influence everything from summer sea breezes – which serve as the seashore's natural air conditioner – to near-shore events where water temperatures drop from bathtublike one day to chilly the next.

Scientists are just beginning to understand the cold pool by using underwater robots called gliders. Twice, gliders were deployed as hurricanes tracked north along the coast, first during Hurricane Irene in 2011 and the following year during Hurricane Sandy.

Hurricane season is set to start June 1, and although the National Weather Service's Hurricane Center is projecting a below-average year for named storms and major hurricanes, officials are warning against complacency.

"It only takes one hurricane or tropical storm making landfall in your community to significantly disrupt your life," Federal Emergency Management Agency Deputy Administrator Joseph Nimmich said last week.

The center predicted six to 11 named storms by Nov. 30, short of the annual average of a dozen. This year is likely to produce up to two major storms with winds of 111 mph or higher, one short of average.

Colorado State University's Tropical Meteorology Project said the season "will be one of the least-active seasons since the middle of the 20th century," under the influence of warmer-than-usual southern Pacific sea-surface temperatures and early coolness in tropical and subtropical Atlantic waters.

The probability of an East Coast landfall this year is 28 percent, Colorado researchers concluded, compared with a 52 percent average for the past century.

For Delaware, Colorado researcher Paul Klotzbach said, the chance of a hurricane making landfall is less than 1 percent. By the time storms reach the Mid-Atlantic, Klotzbach said, they are often moving more generally to the north, becoming more of a hazard to New York and New England.

Irene, overpredicted

The textbook case of cold-pool dampening, according to a study, was Hurricane Irene.

The forecast for Delaware on Aug. 25, 2011, was for more than a foot of rain; winds of 85 mph with higher gusts, two high tides and storm surge of 4 to 7 feet along the ocean and 3 to 5 feet in Delaware Bay.

"If this forecast holds true, we are looking at major flooding throughout the state, but especially in the coastal areas," Gov. Jack Markell said before ordering a mandatory coastal evacuation. "We are looking at wind damage throughout the state and especially in southeast Sussex. We are looking at roads becoming impassible, rivers and streams flooding and power outages."

At one point, Irene was forecast to bring 110-mph gusts to the Delaware coast. Instead, the highest gust was 66 mph near Lewes. The state did get plenty of rain and flooding, and a tornado swept through a residential area in Lewes, but the storm didn't pack the intensity that forecasters had predicted.

While Irene did cause significant flooding and damage, especially to the north, it had far less impact on Delaware and Maryland than was forecast.

The possible reason: As the hurricane tracked north into the Mid-Atlantic, its winds mixed the warm, surface waters with that deeper layer of cooler water. The offshore surface water temperature dropped 11 degrees on Aug. 28, 2011. That shift caused surface water temperatures to drop from 77 degrees to 66 degrees in a matter of hours.

The research team, led by scientists at Rutgers University, sent underwater robots out into the storm to learn more about how hurricanes impact the near-shore ocean.

"As the leading edge of Irene tracked along the coast, real-time temperature profiles from an underwater glider documented the mixing" of ocean waters "well before the eye passed over," they found.

When Sandy was forecast the following year, they didn't have the money to launch a full-scale, coast-wide deployment of underwater robots, but they already had one deployed so they turned it back into the approaching storm.

The cold pool didn't have the same dampening influence – probably because the superstorm occurred much later in the season when the ocean was already well mixed.

"The thinner layer of cold bottom water still remaining before Sandy was forced offshore ... resulted in little change in ocean surface temperatures," they found.

The result: Little change in intensity as Sandy came ashore north of Atlantic City.

Heat means trouble

Researchers are just beginning to understand how the cold pool fits into the many factors in judging hurricane intensity, especially for the Mid-Atlantic.

"Heat in the water is what fuels the storm. Less heat, weaker storm. More heat, stronger storm," said Gerhard F. Kuska, executive director of the Mid-Atlantic Regional Association Coastal Ocean Observing System based at the University of Delaware.

"So it is important to understand what is happening in the water as the storm is moving toward land," Kuska said. "When cloud cover comes in ahead of the storm, the satellites can't see what is happening in the ocean. But if you have an asset in the water, like a glider, you get a three-dimensional picture of what is happening in the water column and how the temperature is or is not shifting, thereby telling what the intensity shift will be."

Hurricane forecasters, over the past two decades, have improved the science of forecasting hurricane storm tracks. But determining their intensity has proven more difficult.

That led forecasters to overpredict the intensity of Irene and underpredict the "rapid acceleration and intensification of Sandy" just before landfall, according to a paper by the team of scientists who deployed gliders into the ocean ahead of the two storms.

Lesson from Texas

Dewey Beach Mayor Diane Hanson said her resort community has taken every step to prepare.

"We've had several trials where storms were predicted to be very severe, so we've kind of learned by doing," Hanson said. "At this point, I think we're in as good a shape as we can be. I think there's always a need to keep the public better informed. We have a lot of new property owners here."

In a twist, the recent, catastrophic inland flooding in Texas has sharpened public awareness in Dewey, Hanson added.

"Texas has been kind of an eye opener for a lot of people. People had their homes on stilts think, 'Water will never get here,' but in Texas the water was up 26 feet in some places."

Scientists have been mapping the cold pool for years, measuring the water temperatures from Cape Hatteras, North Carolina, to the Gulf of Maine, said Wendell S. Brown, professor of marine science and technology at the University of Massachusetts' Department of Estuarine and Ocean Sciences School for Marine Science and Technology.

The water is an annual remnant from New England's cold winters that forms each spring.

Brown said the pool of water moves south at a rate of about 3 mph and typically reaches the offshore waters of New Jersey, Delaware and Maryland in July and August. August and September are typically the peak hurricane months for the Mid-Atlantic region.

Fishermen have known about this pool of cold water for about 200 years, he said.

Scientists identified it a century later and are now trying to get a better understanding of the role it plays in the near-shore ocean.

Oscar Schofield, professor of bio-optical oceanography at Rutgers University, said the cold pool forms a unique ocean ecosystem and it probably influences much of the aquatic biology off the coast.

For instance, four to five times each summer, the cold pool contributes some of its cold water to near-shore upwelling events.

When that happens, those upwell areas tend to be great areas for fishing, he said.

The cold pool can't be seen on satellite images of the ocean, said Matthew Oliver, an associate professor at the University of Delaware College of Earth, Ocean and Environment, who oversees the Ocean Exploration, Remote Sensing and Biogeography Laboratory and studied with Schofield.

The cold pool seems to be an influence only during the summer and early fall. In the winter, the ocean waters are well-mixed, but in the spring, those strong winter winds slow and the layer of warm water forms on the surface with the cold water from New England settling in a layer below it, he said.

As oceanographers look toward the cold pool for answers to hurricane intensity, atmospheric scientists are beginning to use drones to see what is happening in and around storms, looking at everything from relative humidity to temperatures.

The goal: to improve the data that will ultimately go into the computer models used to determine hurricane intensity, said Joe Cione of the National Oceanic and Atmospheric Administration's Hurricane Research Division.

Cione dropped a 3-foot-long, 7-pound Coyote drone into Hurricane Edouard last summer for a closer look at how the storm impacted the physics of the atmosphere.

"This is cutting-edge stuff that could give us ... tremendous improvements in forecasting intensity," he said. "If you get a better model, then you should have a better forecast."

So with oceanographers using underwater robots to look at changes in water temperatures and the atmospheric scientists looking at the conditions surrounding the storm, the understanding of storm intensity will improve, he said.

"It's what I like to call a 'now cast,' " Cione said.

Reach Molly Murray at (302) 463-3334 or mmurray@delawareonline.com. Follow her on Twitter @MollyMurraytnj. Reach Jeff Montgomery at (302) 463-3344 or jmontgomery@delawareonline.com. Follow him on Twitter @JMontgomery_TNJ.