News | July 28, 2016
Neglected effects might influence sea level
A recent sea-level spike along the northeastern coast of the United States was widely interpreted as a troubling sign: It seemed to coincide with an apparent weakening of a major Atlantic current, a possible effect of global climate change.
But two new studies offer a significant revision of that view—or at least an important asterisk.
The first study reveals a much larger role in regional sea level for an often-neglected phenomenon. Known as the “inverted barometer” effect, it pushes sea levels down when air pressure is high, and raises them when air pressure plummets.
“The analogy is imperfect, but imagine a water bed,” said Christopher Piecuch, lead author on both papers and a research scientist who also is a member of NASA’s Sea Level Change Team. “You’ve got this bed full of water, you sit down on it, and it depresses underneath you; the water that used to be beneath you has gone elsewhere.”
In the same way, high-pressure air systems place greater weight on an ocean surface, while low-pressure systems, a far lighter load, allow the ocean surface to rise.
And a persistent low-pressure system over the northwestern Atlantic could account for as much as half of a dramatic spike in sea level from 2008-2010 on the northeastern coast, according to the first study, published in “Geophysical Research Letters” in 2015.
The second study attributes much of the remaining sea level variability along the coast of the northeastern U.S. over the last few decades to wind stress and the near-shore physical context, such as the shape of the shallow ocean bottom. That study was published in July 2016 in the “Journal of Climate.”
Link to weakening current questioned
Those two factors, low pressure and wind stress, could potentially offer a nearly full explanation for the sudden rise in sea level without invoking a weakening AMOC—the Atlantic Meridional Overturning Circulation, a massive ocean current that transports heat from the tropics to the poles.
Initial papers linked the 2008-10 sea-level spike with a slowing of AMOC, a long-dreaded possible consequence of human-induced climate change. But many of those papers failed to consider local meteorological effects, such as the inverted barometer, or failed to give them their due, say Piecuch and Rui Ponte, his colleague at Atmospheric and Environmental Research, Inc., in Lexington, Mass., who is also a member of NASA’s Sea Level Change Team.
A number of studies also made a connection between decadal sea-level variations in the U.S. East Coast and AMOC, a link brought into question by the second study.
“A lot of studies seem to imply that the Meridional Overturning Circulation is causing the sea-level variability,” said Ponte, an author on both of the new studies. “And that is a very confused way of stating things, because correlation is not causation.”
To check the link between the inverted barometer effect and sea level for the first study, Piecuch and Ponte compared tide-gauge records of annual mean sea level with five separate data sets of monthly sea-level pressure, covering the period from 1950 to 2013.
They found not only that the inverted barometer accounted for roughly half of the 2008-10 extreme sea level event along the Atlantic coasts of Canada and New England; they also determined that the IB effect explains about a quarter of year-to-year sea-level variability, and up to a third of recent multi-decadal sea-level rise acceleration over the mid-Atlantic bight and southern New England.
Ocean model explains half of tide-gauge variations
In the second study, Piecuch and Ponte, along with European colleagues, focused on the tide-gauge record for the northeastern coast from 1980 to 2010. After removing the inverted barometer signal from the tide gauges, they compared the record to ocean-reanalysis fields—a kind of recursive combination of general ocean-circulation models with most of the available data for the entire period.
They also compared the tide-gauge record to a simplified ocean model that simulates the effects of wind on the behavior of fluids. Although this model lacked the vertical stratification of the water column required for an overturning circulation, it nevertheless performed far better than ocean reanalyses, explaining about 50 percent of yearly tide-gauge variations north of Cape Hatteras after adjusting for the inverted barometer effect.
The results of the two studies demonstrate that local meteorological effects are central to understanding recently observed year-to-year sea level changes along the northeastern U.S. coast, though they do not completely discount a possible link to AMOC.
“We’ve projected that these local meteorological factors will not make dominant contributions to future regional sea level rise,” Piecuch said. “But they’re not something to totally neglect, either.”
Links to AMOC must be examined more closely, and effects such as the inverted barometer, wind stress and bathymetry—sea-floor shape—must be taken more fully into account, Ponte said.
“There may be an element of sea-level rise in the U.S. East Coast that is related to the (A)MOC,” he said. “But it is almost certainly not the MOC that is causing sea-level rise in a dynamical sense. If the MOC has anything to do with sea level, it is probably sea level causing the MOC to change, not the other way around.”
View the studies:Inverted barometer contributions to recent sea level changes along the northeast coast of North America Annual sea level changes on the North American northeast coast: influence of local winds and barotropic motions