Most climate models predict that climate change will cause rising sea levels and frequent flooding for the states bordering the Atlantic Ocean. From 2009 to 2010 the area from New York to Newfoundland got a small taste of that when sea levels rose an average of four inches.

A paper published by a University of Arizona led team in Nature Communications is the first to document the extended sea level rise.

“The thing that stands out is the time extent of this event as well as the spatial extent of the event,” said first author Paul Goddard, a UA doctoral candidate in geosciences in a statement.

According to the researchers the sea level rise was caused by ocean currents and was unrelated to specific weather events such as hurricanes or winter storms. Unusually high amounts of flooding occurred as far south as Cape Hatteras.

The researchers detected the spike in sea levels by reviewing tide-gauge records, some of which extend back into the early 20th century. No other two-year period during that stretch matched the substantial increase seen during the 2009-10 period.

The increase in sea levels was primarily due to a change in the Atlantic Meridional Overturning Circulation (AMOC) as well as a change in the North Atlantic Oscillation (NAO) according to the team.

The AMOC is a major current that normally drives warm, salty water in the upper layers of the Atlantic and a southward flow of colder water in the deep atlantic. Along with the water it moves a considerable amount of heat energy from the tropics to the North Atlantic.

The NAO is a more complicated climactic phenomenon, driven by fluctuations in atmospheric pressure at sea level. It controls the strength and direction of westerly winds and storm tracks in the North Atlantic and can impact weather patterns from the eastern U.S. to Southern Europe and the Middle East.

In addition to documenting the two year sea level change, the team used computer modeling to predict the probability of future sea level spikes in the region. They found that, with the current level of increase in atmospheric carbon dioxide, such events are likely to become much more frequent.

Co-author Jianjun Yin, University of Arizona assistant professor of geosciences, has done previous work which suggests that weakening of the AMOC could cause more rapid sea level rises in the northeastern U.S.

In addition to finding the two-year spike Goddard found, as others have, that the sea level in the area has been gradually rising since the 1920s with some year-to-year variation.

At about the same time Goddard finished his analysis of tide-gauge records, another group reported that for the 2009-10 period, the AMOC had a 30 percent decline in strength. That decline appeared just two months before the tide-gauges in the northeast began showing a spike. At the same time, the NAO went into a “very negative phase”.

“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,” said Goddard.

While the NAO has resumed normal changes between positive and negative states, the AMOC has gained some strength but still hasn’t returned to normal.

According to Yin most climate models predict a long term weakening of the AMOC during the 21st century. That likely means that extended periods of sea level rise and flooding are in store for the northeastern U.S. in addition to the predicted permanent rise in sea level due to climate change.