Hurricane Created Rare 'Trapped Wave'

A schematic of the coastally trapped wave generated by Hurricane Dennis. Water is pushed to the right of the storm winds, which created a high sea level anomaly along the coast, which traveled to Apalachee Bay as a coastally trapped wave. Credit: Steven Morey

A rare phenomenon that ramped up storm surge in the Gulf of Mexico during Hurricane Dennis last year has been identified, and researchers say the same enhancement could bring large surges during future hurricanes following similar paths.

Storm surges are caused by wind pushing water toward the shore and piling it up.

When Dennis made landfall on July 10 as a Category 3 hurricane along the western end of the Florida panhandle, 10-foot storm surges battered the coastline. That was about 3 to 4 feet more than had been predicted or could be explained by the local winds normally behind the surge.

Mystery solved

Researchers at Florida State University and the National Oceanographic and Atmospheric Administration (NOAA) have found that the answer to the mystery lay in a number of factors, including the hurricane's speed and path, influencing a phenomenon called a "trapped wave."

In the Northern Hemisphere, hurricane winds always rotate counterclockwise as seen from above. Earth's rotation pushes ocean water to the right, in the direction of the storm's winds, creating a slight build-up on that side. If the hurricane travels parallel to a coastline on its right side, its winds will force the ocean water against the coast, "creating a bulge of high sea level," explained Steven Morey, a Florida State University oceanographer. This bulge will travel along the coast, forming what is known as a constantly trapped wave.

This was what happened during Dennis: The storm ran alongside the western coast of Florida, allowing the trapped wave to form [ graphic ].

And "because Dennis traveled nearly parallel to the Florida Peninsula coast at the same speed as the wave, winds from Dennis amplified the wave" as it traveled northward, Morey said.

Rare setup

Though trapped waves are common and have been understood for years, "this is the first time it's made a difference in the hurricane storm surge," Morey said.

Storm surge forms in just a few hours as a hurricane approaches land, when its winds directly drive the water onshore. A trapped wave takes at least a day or longer to form.

Dennis was a special case that combined these two phenomena because the hurricane was barreling towards the northern Gulf Coast, with the Florida coast on its right. Timing, however, was the crucial element, because the hurricane brushed the shore just as the trapped wave arrived, amplifying the normal storm surge.

"They just added together," Morey said.

This type of trapped wave could play a part in future storms that travel a path similar to Dennis's or along the Louisiana coastline heading towards Texas. National Hurricane Center forecasters plan to factor this phenomenon into their forecasting models so they can better predict storm surge levels.

The research was reported earlier this month in the journal Geophysical Research Letters .

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