Sea level rise along the eastern mid-Atlantic coast of the U.S. is approximately 2 times greater than the average global rate. Sustained regional sea level rise has implications for coastal communities, increasing the risk of flooding and the local effect of storm surge. Professor Tal Ezer from the Center for Coastal Physical Oceanography at Old Dominion University addressed two fundamental questions related to the spatial and temporal variations in sea level rise along the US East coast: (i) how do changes in ocean dynamics affect the spatial pattern of sea level rise, and (ii) how do decadal and multidecadal variations in ocean density and currents affect local and global sea level rise?
Monthly mean sea level records from 10 stations between Boston and Key West, Florida, and one station in Bermuda were analyzed to determine the effect of ocean dynamics on the spatial and temporal variations in East coast sea level rise. Changes in ocean density and currents were analyzed by empirical mode decomposition analysis using 60 years of sea level records compared against global sea level rise measurements obtained by altimeter and tide gauge level. Data for the ocean transport from the Florida current and semi daily Atlantic Meridional Overturning Circulation transport were also analyzed.
Recent studies indicate that sea level rise has accelerated along the U.S. mid-Atlantic coast in recent decades, probably owing to a slowdown of the Atlantic Meridional Overturning Circulation and specifically its upper branch, the Gulf Stream. A slowing of the current is compensated by sea level rise; the water must go somewhere.
In this study, Tal Ezer found a latitudinal pattern in sea level rise consistent with a weakening Gulf Stream flow, which achieves a maximum just north of Cape Hatteras, and decreases northward. Multidecadal sea level rise was highest in the high latitudes and decreased southward; a pattern explained by (i) multidecadal changes in water column density structure and (ii) ocean circulation of the subpolar North Atlantic.
While decadal and multidecadal variations in sea level were coherent along the coast, they were in opposite phase to sea level in Bermuda (high along the East coast, low along Bermuda and vice versa). Sea level differences between the East coast and Bermuda are consistent with the concept of geostrophy, which relates elevation gradient to a current’s velocity. Within the past 5 years, Gulf Stream transport has decreased, possibly related to climate-induced weakening of the Atlantic Meridional Overturning Circulation. As a result, sea level has increased along the East coast.
Ezer, T. (2013), Sea level rise, spatially uneven and temporally unsteady: Why the U.S. East Coast, the global tide gauge record, and the global altimeter data show different trends, Geophys. Res. Lett. 40, 5439–5444, doi:10.1002/2013GL057952.