Friday, October 20, 2017
 
Sea level spiked for two years from New York to Newfoundland

Sea level spiked for two years from New York to Newfoundland

University of Arizona scientists collaborate with NOAA scientists, using NOAA climate models

This web story was written and provided by Mari N. Jensen of the University of Arizona with contributions from NOAA. The University of Arizona story may be read online.  

Sea levels from New York to Newfoundland jumped up about four inches in 2009 and 2010 because ocean circulation changed, a University of Arizona-led team, in collaboration with NOAA scientists, reports in an upcoming issue of Nature Communications.  

The team was the first to document that the extreme increase in sea level lasted two years, not just a few months. 

“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 University of Arizona doctoral candidate in geosciences. 

Independent of any hurricanes or winter storms, the event caused flooding along the northeast coast of North America. Some of the sea level rise and the resulting flooding extended as far south as Cape Hatteras.

The paper is also the first to show that the unusual spike in sea level was a result of changes in ocean circulation.

Co-author Jianjun Yin, UA assistant professor of geosciences, said, “We are the first to establish the extreme sea level rise event and its connection with ocean circulation.” 

Sea level spikes

Sea level spikes

UA geoscientist Paul Goddard and his colleagues determined that sea level rose four inches from New York to Newfoundland (red dots) in 2009 and 2010. Gauges from New York south to Cape Hatteras (pink dots) showed a smaller spike in sea level for the same time period. No sea level spike was recorded on the gauges (white dots) south of Cape Hatteras. (Paul Goddard/UA Department of Geosciences)
Goddard detected the two-year-long spike in sea level by reviewing monthly tide-gauge records, some of which went back to the early 1900s, for the entire Eastern Seaboard. No other two-year period from those records showed such a marked increase. 

The team linked the spike to a change in the ocean’s Atlantic Meridional Overturning Circulation and also a change in part of the climate system known as the North Atlantic Oscillation. 

The researchers then used computer climate models from the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J., to project the probability of future spikes in sea level. 

The team found that, at the current rate that atmospheric carbon dioxide is increasing, such extreme events are likely to occur more frequently, Goddard said. 

Stephen Griffies of NOAA's GFDL, a co-author to the study, noted that "these extreme sea level events will be further exacerbated by rising global mean sea level associated with ocean warming and melting land ice. Such rising sea levels, as well as changes to the Atlantic overturning circulation, are projected for the 21st century climate by the NOAA climate models used in this study, as well as the broader suite of models produced by climate centers worldwide."  

The research paper entitled, “An Extreme Event of Sea Level Rise along the Northeast Coast of North America in 2009-2010,” was published today, February 24, online in Nature Communications. In addition to lead authors from UA and NOAA's Stephen Griffies, Shaoqing Zhang of NOAA's GFDL was also a co-author. NOAA funded the research. 

Yin’s previous work on climate models suggests that weakening of the Atlantic Meridional Overturning Circulation could cause sea levels to rise faster along the northeast coast of North America. 

Yin wondered whether such sea level rise had actually been observed, so he asked Goddard to compile the tide-gauge records for the east coast of North America. The 40 gauges, spanning the coast from Key West, Florida, north to Newfoundland, have been recording sea levels as far back as the 1920s.  

Goddard’s work revealed a surprise – that during 2009 and 2010, sea level between New York and Newfoundland rose an average of four inches (100 mm).  Sea level from Cape Hatteras to New York also had a notable spike, though not as dramatic. 

“The sea level rise of 2009-2010 sticks out like a sore thumb for the Northeast,” Goddard said. His research also confirmed that, as others have reported, sea level has been gradually rising since the 1920s and that there is some year-to-year variation. 

About the time Goddard finished analyzing the tide-gauge records, another group of researchers reported that the Atlantic Meridional Overturning Circulation, or AMOC, had a 30 percent decline in strength in 2009-2010. Those researchers reported the decline started just two months before the tide gauges started recording the spike in sea level. 

“To me, it was like putting together a puzzle,” Goddard said. The more he and his colleagues examined the timing of the AMOC downturn and the subsequent increase in sea level, the more it fit together, he said. 

Graph of extreme sea level rise

Graph of extreme sea level rise

This graph shows yearly sea level rise along the northeast North American coast in millimeters per year. The red bar shows how extreme the spike was in 2009. The extreme spike lasted into 2010. (Paul Goddard/University of Arizona Department of Geosciences)
The Atlantic Meridional Overturning Circulation brings warm water from the tropics and the southern Atlantic Ocean to the North Atlantic and the polar regions. The water then cools and sinks, eventually flowing south in the deep ocean. Yin’s climate model predicted that when the AMOC weakened, sea level in northeastern North America would rise.  

In addition to the weakening AMOC, during 2009-2010 the region’s atmosphere was in a very negative phase of the climate mode called the North Atlantic Oscillation. The NAO flip-flops between negative and positive phases. 

“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 said. 

Although the NAO has resumed flipping between positive and negative states, observations show that the AMOC, while somewhat stronger, has still not recovered its previous strength. 

ven now, sea level is still higher than before 2009, Yin said. He’s not surprised, because most of the climate models predict a weakening of the AMOC over the 21st century. 

Yin said that at the current rate of increase in greenhouse gases, most climate models predict a weakening of the AMOC over the 21st century. Therefore, such extreme sea level rise events and coastal flooding are quite likely to occur along the densely populated northeast coast of North America more often. 

University of Arizona media contact: 

Mari N. Jensen 520-626-9635 mnjensen@email.arizona.edu 

NOAA Research media contact: 

Monica Allen 301-734-1123 monica.allen@noaa.gov 

NOAA's Geophysical Fluid Dynamics Laboratory has also posted online a research highlight on the paper appearing in Nature Communications.  

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