RAIN AND TERROR
What makes a storm a killer? Scientists are searching for the early warning signs, say Jeremy Manier and E.A. Torrier
The two hurricanes that roared into the Gulf of Mexico earlier this year were identical in nearly every way. Born in the same region near Haiti, the storms called Katrina and Rita reached monster status in the warm waters off Florida and swirled toward major cities along the coast.
But before they struck, the two hurricanes underwent subtly different yet fateful changes deep within them that resulted in Katrina reaching land with considerably more destructive power – and a far greater death toll – than Rita would nearly four weeks later.
That divergence is stirring ardent debate among experts eager to build better theories of what separates less intense storms from those that become historic killers. The battle of ideas will help shape how experts study hurricanes and prepare for the next big one.
One explanation in this case may be the movement of deep, warm currents in the Gulf that fed Katrina but slipped to the side of Rita days before that storm reached land. Some researchers believe a Gulf system called the loop current played a major role in the evolution of Katrina and Rita.
During both hurricanes, government scientists deployed a battery of experimental tools to measure deep ocean temperatures and currents where the storms passed through the Gulf. Experts hope the new information will improve forecasters’ ability to predict the intensity of future hurricanes.
“We’re looking at what we did with these storms as a poster child for techniques we might use in the future to get better observations on the interaction between hurricanes and the ocean”, said Peter Black, a meteorologist with the Hurricane Research Division of the United States’ National Oceanic and Atmospheric Administration.
Hurricanes are among the most complex weather systems that bedevil meteorologists, in part because of the peculiar way the storms can change their nearby ocean environment, which in turn can affect the power of the hurricane.
One way to think of a hurricane is as a vast engine that converts ocean heat – its fuel – into high winds. A shortage of fuel or other glitches in the engine can reduce the storm’s strength.
An example of this is when a hurricane’s winds churn up cold water from the ocean depths, robbing the storm of the warm water it needs to sustain high winds. Deep, warm currents such as the loop current in the Gulf can reduce that effect. They provide more fuel for the storm to rage without picking up colder water from below.
Both Katrina and Rita strengthened as they passed over the loop current, experts said. Katrina headed straight from the current to the shore, where it unleashed destruction across a heavily populated region. Rita was just as powerful at its peak, but it took longer to reach shore after it moved off the deep current, losing energy along the way.
“Rita peaked early”, said Kerry Emanuel, a professor of atmospheric sciences at the Massachusetts Institute of Technology. “It was on its way out when it hit the coast.”
Researchers have recognized the importance of that interaction between hurricanes and the ocean only in the last 10 years or so, Emanuel said. In fact, some experts at the National Hurricane Center in Miami still doubt that deep temperatures played a decisive role in building up the two storms.
“That stuff about the loop current – it doesn’t hold water, so to speak”, said Stacy Stewart, a hurricane specialist at the Hurricane Center. “You have to have a lot of other conditions right to allow the storm to extract energy from the water.”
She pointed out that other factors also affected Rita’s decline, including a lack of moisture in the hurricane’s middle levels. As it hit land, the storm also was undergoing eye wall replacement, a poorly understood phenomenon that happens in cycles with the most powerful hurricanes and often saps their strength.
Katrina and Rita were unusual from the start, in that they were “Bahama busters” that took shape in the Caribbean rather than off the coast of Africa, which spawns most of the storms that become hurricanes. Hugh Willoughby, a hurricane researcher at Florida International University, said the wind shear – a change in wind speed at different altitudes – was too great for large storms to develop near Africa.
That wasn’t the case in the Caribbean, where Katrina and Rita formed within a few hundred miles of each other.
“They were almost like twins,” Willoughby said.
At 11 a.m. on Aug. 24, the National Hurricane Center announced the formation of Tropical Depression 12, the storm that became Katrina, about 200 miles southeast of Miami.
Actually, it was an energizing small squall that started off the coast of Africa but never formed into a storm because of the wind shear. Some of the formation came from a different tropical depression that ran out of gas.
Tropical Depression 14 was spotted on Sept. 17 at 11 p.m., about 500 miles southeast of Miami. This was the birth of Rita.
The storms were nourished by the exceptionally warm waters of the Atlantic, a pattern since 1995. But in both cases, high pressure across much of the United States blocked the storms from turning northward, a trend for much of the last two years. Instead, they headed west over the open ocean.
“Both would have turned otherwise,” said Keith Blackwell, a hurricane researcher at the University of South Alabama, “and we would have heard from them no more.”
In the Gulf of Mexico, both hurricanes moved over the loop current, which moves around the Gulf and exits south of Florida into the Atlantic, becoming part of the Gulf Stream current.
Black of NOAA’s Hurricane Research Division said the data his team gathered this year should help improve computer models used to predict hurricane intensity. Forecasting intensity remains a glaring weak spot in hurricane models, experts say, even as the ability to anticipate where a hurricane will go has improved greatly.
The workhorses of Black’s research are small, disposable probes called AXBT devices, which are dropped from planes and measure the temperature of the ocean at depths up to 1,000 feet. Black got his probes as Navy surplus, leftover from Cold War efforts to track enemy submarines using sonar.
He said it would help attempts to gauge hurricane intensity if the US government would buy more temperature probes and make their deployment a routine part of hurricane tracking.
“We’re just about out of these hand-me-downs,” Black said.