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Connection Between Gulf Stream and Sea Level Rise Confirmed by ODU Study

Tal Ezer

Recent studies have identified a "hot spot" of accelerated sea level rise from Cape Hatteras, N.C., to Boston, and researchers have speculated that a diminished flow of the Gulf Stream might be partly to blame. Now, data analysis by a team of oceanographers led by Tal Ezer of Old Dominion University has given that hypothesis new scientific footing.

The team's findings were published in January by the Journal of Geophysical Research: Oceans, and have been reported by the websites of numerous media that cover climate and weather, such as Climate Central (www.climatecentral.org), Discover Magazine and the Weather Channel.

"There have been several papers showing the acceleration of sea level rise in the mid-Atlantic. This new paper confirms the hypothesis for why it's happening," Ezer said.

"The importance of the study is that in addition to the well-known causes for coastal sea level rise - global sea level rise and land subsidence in some places - the study points to a new source of sea level rise that is not yet fully understood: changes in ocean currents," Ezer added.

Findings of the researchers can explain why in some regions, such as Hampton Roads, the sea level has been rising two to three times faster than the average global sea level, and why the rate of the rise has increased in recent years.

Additionally, Ezer said, the findings can explain why unusually high sea levels that cause floods during high tides can persist for months in areas such as Norfolk, with no apparent weather event to cause them, and can help to improve projections of sea level rise for flood-prone regions.

Ezer, professor of ocean, earth and atmospheric sciences at ODU 's Center for Coastal Physical Oceanography (CCPO), has research specialties in numerical ocean modeling and coastal circulation and has been active in the 3-year-old ODU Climate Change and Sea Level Rise Initiative (CCSLRI). His co-authors on the latest publication are Larry Atkinson, Slover Professor of Oceanography and director of ODU's CCSLRI; William Bryce Corlett, a former ODU undergraduate student and now a graduate student at the University of Southampton in England; and José Blanco, a Chilean scientist who has worked extensively with ODU's CCPO.

The Gulf Stream flows like a huge river - a half-mile deep and more than 50 miles across - at about 100-200 miles off the eastern coast of the United States. It transports about 500 times the volume of water of the Amazon River. After flowing along the Florida eastern coast, the Gulf Stream separates from the coast at Cape Hatteras, N.C., and then turns northeastward, bringing large amounts of warm tropical waters into the cold North Atlantic Ocean. This causes it to play an important role in Earth's climate and weather.

When the Gulf Stream turns eastward it pulls water away from the mid-Atlantic shore, and the water level on the inshore side of the Gulf Stream, along the mid-Atlantic coast, is kept about 3-5 feet lower than the water on the other side of the Gulf Stream. Basic dynamic oceanography implies that this water elevation difference across the Gulf Stream is proportional to the speed of the stream, so the theory is that changes in the flow of the Gulf Stream would affect the coastal sea level in the mid-Atlantic more than any other coast.

"But does it really happen? And can observations confirm this theory? Those were the challenging issues tackled in this latest research," Ezer said.

Based on computer climate models, scientists have hypothesized for some time that a warming climate in the Arctic will slow the Atlantic Ocean circulation and reduce the Gulf Stream transport. The new study analyzed data that suggest that slowing of the Gulf Stream may have started already, and the accelerated sea level rise that has been measured in the area over the last few years is related to the changing Gulf Stream.

The research involves a new data analysis method that was developed by Ezer and Corlett, and published in Geophysical Research Letters in October 2012. The method can separate between long-term variations that may relate to climatic changes and faster changes such as seasonal cycles.

To obtain the Gulf Stream flow, the researchers pulled together available data on the total water transport of the Gulf Stream taken by a sea-floor cable across the Florida Strait, where the Gulf Stream starts, as well as observations of sea level obtained from satellite data further upstream. These data were compared with observations obtained from 10 tide gauges, some of which have continuously measured water levels for more than 100 years.

Somewhat to the surprise of the researchers, long-term sea level changes from far-apart stations, from Atlantic City, N.J., to Duck, N.C., on the Atlantic coast, and from Baltimore to Norfolk on the Chesapeake Bay, all seem to fluctuate up and down together within a period of about six to eight years.

"These results indicate that sea levels at all those stations are driven by the same force, and the candidate for that force was clearly the Gulf Stream," Ezer said. "Even more surprising was the very high statistical correlation found between changes in the Gulf Stream strength and the coastal sea level. The result was exactly as predicted by the theory, but real data rarely show such clear results.

"Another finding was that about 10 years ago the pattern of both sea level and the Gulf Stream suddenly changed from a fluctuating cycle to a continuously weakening Gulf Stream, and at the same time sea level rise rates seemed to increase."

These findings open the door for more studies that will try to better understand the causes of regional sea level changes and how best to predict future changes. "These scientific studies will help places like Norfolk to prepare for the increasing frequency of flooding that the city has seen in recent years," Ezer said.