A 2-year spike in sea level along NE 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. Sea level from Cape Hatteras to New York also had a notable spike, though not as dramatic.
“The sea level rise of 2009-10 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 2009-10. 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.
The AMOC 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 2009-10 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.
Even now, sea level is still higher than before 2009, Yin said. He is 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 twenty-first 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.
— Read more in Paul B. Goddard et al., “An extreme event of sea-level rise along the Northeast coast of North America in 2009–2010,” Nature Communications 6, Article number: 6346 (24 February 2015) (doi:10.1038/ncomms7346)
