Changes in ocean circulation elevated sea levels on the North American east coast by four inches starting in 2009, and the extreme increase lasted 2 years, researchers have determined.

From New York north to Newfoundland, the rise caused flooding independent of any hurricanes or winter storms, they report.

First thought to have lasted just a few months, the increased sea levels lasted throughout all of 2009 and 2010, a research team led by University of Arizona scientists report in the journal Nature Communications.

"The thing that stands out is the time extent of this event as well as the spatial extent of the event," says study lead author Paul Goddard, a UA geosciences doctoral candidate.

Goddard discovered the 2-year-long sea level spike as he was analyzing monthly tide-gauge records going back as far as the early 20th century.

Covering the east coast from Key West, Florida, north to Newfoundland, some 40 gauges have been recording sea levels as far back as the 1920s.

On the entire Eastern Seaboard, no other 2-year period showed as marked an increase as the two years described in the study, the researchers say.

"The sea level rise of 2009-2010 sticks out like a sore thumb for the Northeast," Goddard says.

They linked the rise to changes in the ocean's Atlantic Meridional Overturning Circulation and part of the climate system dubbed the North Atlantic Oscillation.

"The negative North Atlantic Oscillation changes the wind patterns along the northeast coast, so during the negative NAO the winds push water onto the northeast coast," Goddard explains.

Using computer climate models to predict the chances of future spikes in sea level suggests that, at the current rate of increases in atmospheric carbon dioxide, such extreme events are likely to happen more often, Goddard says.

The work confirms what other studies have found, the researchers say, that sea levels have been rising slowly since the 1920s, but their study of the 2009-2010 event was the first to focus on a particular cause.

"We are the first to establish the extreme sea level rise event and its connection with ocean circulation," says study co-author and UA geosciences Professor Jianjun Yin.

Even now, he says, sea level is still higher than before 2009, something he says is not surprising because the majority of climate models predict a weakening of the Atlantic Meridional  Overturning Circulation through the 21st century.