Sandy, Irene, Katrina, Andrew, and Bob are all retired hurricane names that read like a list of exes whose wake of destruction was not anticipated and whom we hope to never see again.
These storms were some of the costliest and deadliest storms in U.S. history, catching communities off-guard with severe winds, storm surge, and heavy rain.
Although advanced satellite and computer technology have allowed us to better predict and track the path of tropical cyclones, or hurricanes, we’re still not as able to predict their strength.
Part of this knowledge gap is the result of a rapidly changing climate that is altering how the ocean interacts with the atmosphere.
“It’s important to be able to predict how damaging these storms will be in order to create appropriate infrastructure,” says Nyla Husain, a Ph.D. candidate at the University of Rhode Island’s Graduate School of Oceanography (GSO), during the Bay Informed monthly public lecture series held on July 20.
Husain, who has been investigating the role of ocean waves on hurricane intensity with hurricane expert Isaac Ginis and other GSO oceanographers, discussed how understanding air-sea interactions can improve hurricane forecasting to help communities avoid such devastation.
Just Add Hot Water
Tropical cyclones are fueled by heat and moisture. For these storms to form and thrive, sea surface temperatures need to be at or above 79°F. The deeper this warm surface layer is, the stronger the storm. This is why the majority of storms occur during the summer and at high-temperature regions near the equator and mid-latitudes.
And since the ocean has absorbed over 90 percent of the heat trapped by greenhouse gases since the 1970s (Scientific American), becoming “a heat and moisture reservoir,” said Husain, we can expect to see storms with more intensity as the climate continues to warm.
“The atmosphere will be warmer and more humid, giving more energy to the storms,” she said, explaining that heat and moisture transfer from the ocean to the atmosphere through evaporation. As moisture from warm surface waters rises with air warmed at the ocean’s surface, it releases that heat as it condenses at higher altitudes to form clouds.
This process doesn’t necessarily mean there will be more storms, she said, but that the storms that do form will have more fuel. According to the Intergovernmental Panel on Climate Change, tropical cyclone intensity will increase 2 to 11 percent by the end of the century. Higher rainfall rates are also expected during these storms, which could mean more intense flooding.
In order to get a better handle on storm intensity for adequate forecasting, all the variables, including ocean waves, need to be considered.
“The ocean’s surface is similar to an airplane wing. There’s a drag force that slows down the wind. When waves get bigger they can grab onto more wind and absorb the momentum of the wind,” said Husain, explaining how this dynamic can cause the storm to lose energy mechanically.
She also pointed out that as the wind passes over the surface of the ocean to create waves, it also creates eddies and currents that churn the warm upper layer, allowing colder waters from below to come to the surface. This colder water would reduce the heat source for the storm and dampen its strength.
However, the depth of this warm upper layer (which can range from 200 to 1,000 meters) varies by season and location. As the oceans continue to warm, this layer may go deeper. This will mean more fuel for the storm as more warm water is being churned up to the surface instead of cold water.
This mixing will also enhance moisture and gas exchange through air bubbles in the water and sea-spray droplets in the air, which are thought to contribute to heat transfer in the atmosphere and intensify storms.
“Breaking waves cause sea spray, which injects aerosols like sea salt into the atmosphere. Sea salt can act as condensation nuclei to produce clouds,” said Husain, noting this as an example of feedback between the ocean and atmosphere.
Understanding all of the physics where the ocean and atmosphere meet is a vital piece, she added, to enhance large-scale hurricane computer models that will enable more accurate forecasting.
Hopefully, we’ll be better prepared for when the next Sandy or Katrina arrives.
The Bay Informed Discussion Series is sponsored in part by Rhode Island Sea Grant in partnership with URI’s Graduate School of Oceanography. This series is held every third Thursday of the month at 7 p.m. at GSO’s Bay campus in Narragansett. These events are designed for the community to get involved and learn more about ongoing research.
The next event will be on August 17: The Carbon Cycle and Climate
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Meredith Haas | Rhode Island Sea Grant Research Communications Specialist