Wednesday, February 03, 2016

Why the U.S. East Coast Could Be a Major ‘Hotspot’ for Rising Seas

Average SST anomaly for Jan. 22-23, 2016 relative to the long-term average from 1981 to present. (Credit: Vincent Saba, NOAA) Click to Enlarge.
New research published Monday adds to a body of evidence suggesting that a warming climate may have particularly marked effects for some citizens of the country most responsible for global warming in the first place — namely, U.S. East Coasters.

Writing in Nature Geoscience, John Krasting and three colleagues from the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration find that “Atlantic coastal areas may be particularly vulnerable to near-future sea-level rise from present-day high greenhouse gas emission rates.”  The research adds to recent studies that have found strong warming of ocean waters in the U.S. Gulf of Maine, a phenomenon that is not only upending fisheries but could be worsening the risk of extreme weather in storms like Winter Storm Jonas.

“When carbon emission rates are at present day levels and higher, we see greater basin average sea level rise in the Atlantic relative to the Pacific,” says Krasting.  “This also means that single global average measures of sea level rise become less representative of the regional scale changes that we show in the study.”

In the new research, the scientists used a high powered climate change model based at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., that simulates the ocean, the atmosphere and the cycling of carbon throughout the Earth system.  The goal was to determine how much sea level rise would occur in the Atlantic, versus the Pacific, under a variety of global carbon emissions scenarios.

And the simulation found that at high emissions scenarios similar to current rates, the Atlantic sea levels rise considerably faster than the Pacific, with particularly noteworthy impacts for the U.S. East Coast.  (Other recent research by scientists with the U.S. Geological Survey has suggested this increased rate of sea level rise is already happening — finding sea level rise rates “~ 3–4 times higher than the global average” along a large stretch of the U.S. East Coast, which the researchers dubbed a sea level rise “hotspot.”)

The reason for the difference, the researchers say, is that the Atlantic, more than the Pacific, is characterized by a strong “overturning” ocean circulation — technically known as the Atlantic Meridional Overturning Circulation, or AMOC — that spans the north-south length of the globe and ultimately connects waters off New York with those at the tip of Antarctica.  This means that waters circulate through the entire Atlantic much faster than they do throughout the Pacific:  A “parcel” of water that sinks beneath the surface in the Atlantic will generally make it back to the surface again in 200 to 300 years, versus about three times as long for the Pacific, Krasting explains.
Another way of putting it is that the Atlantic waters “ventilate” more, plunging from the surface to great depths before eventually making their way back to the surface again.  But if this circulation slows due to climate change, the study finds, less cold water will dive to ocean depths in the North and far South Atlantic (technically called “deep water formation”), leading to warmer water pooling below the surface and, ultimately, greater warming overall.

“The average temperature of the basin actually goes up, because you’re not bringing that cool water,” says Krasting.  Warm water expands, and that’s the cause of the sea level rise expected in the study.

Read more at Why the U.S. East Coast Could Be a Major ‘Hotspot’ for Rising Seas

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