“Intra-seasonal” variability in sea-level change
light declining exponentially with depth it could mean added stress to plants at the deeper edges of the grass beds.”
Periods of unusually low water could also affect seagrasses and other marine life, says Brubaker, but the impacts are likely to be less significant because the long-term rise in sea level tempers their effects.
The summer 2009 event
The release notes that Brubaker’s interest in intra-seasonal variability was piqued during summer 2009 — when a prolonged period of high water affected the U.S. East Coast — and again during the shorter period of unusually high water during May 2011, which he experienced first-hand while teaching a course at VIMS’ Eastern Shore Lab in the seaside village of Wachapreague.
“The 2009 event got a lot of attention — eventually,” says Brubaker. “It wasn’t dramatic and it took quite a while to gain much attention, but at some point NOAA posted a notice about it in response to questions and concerns from the public, who had noticed week after week of abnormally high tides.”
Three NOAA scientists, Bill Sweet, Chris Zervas, and Stephen Gill, subsequently issued a technical report to describe and explain the 2009 event. They note that water levels of 0.6 to 2.0 feet above predicted tides persisted for up to 6 weeks in areas from North Carolina to New Jersey, with slightly lower elevations experienced as far south as Florida and as far north as Maine.
If intra-seasonal changes in sea level are not generated by regular tides, storm surge, seasonal heating or cooling, or long-term sea-level rise, what is their cause? Sweet, Zervas, and Gill attributed the 2009 event to the confluence of two factors — persistent winds from the northeast measured far offshore, and a slow-down in the Gulf Stream.
Brubaker says the northeasterly winds contributed to high water along the coast due to “Ekman transport,” a phenomenon in which surface waters begin to move to the right of the prevailing wind because of the Coriolis force. “The Ekman transport associated with these winds would push the water towards the shore,” says Brubaker. Persistent offshore winds from the northeast were also measured during May 2011.
The slow-down in the Gulf Stream contributed to 2009’s persistently high water levels through the re-positioning of what oceanographers call a “geostrophic slope.” “The Gulf Stream creates a geostrophic slope that’s related to the speed of the current,” says Brubaker. “If the current speeds up, the slope gets
