By Richard Yablonsky | June 23, 2016

The globe that you may have at home presents the world as a perfectly smooth, spherical planet, which of course it isn't. In reality, Earth is an oblate spheroid, bulging out in the middle and flattening at the poles.  Further, our land masses aren't smooth; they are covered in mountains, hills, and valleys. But the sea? Surely, except for its waves, it is uniformly level?

In reality it isn't, as a couple of news articles that garnered a great deal of attention highlighted.

Early last year, BBC News Environment Correspondent Helen Briggs published a news article entitled "US sea level north of New York City 'jumped by 128mm'." The same day, University of Arizona Media Contact Mari N. Jensen issued a press release article entitled "Sea Level Spiked for Two Years Along NE North America." Both of these articles report on a Nature Communications article by Paul Goddard and his coauthors at the University of Arizona and NOAA's Geophysical Fluid Dynamics Laboratory.

The basic premise of the original journal article is that the sea level along the coast from New York City to Newfoundland rose by about 4–5 inches during the two-year period from 2009–2010 and then returned to more typical levels.

The main reasons for this unusual—and localized—sea level rise, Goddard and his co-authors argue, are a temporary downturn of the Atlantic Meridional Overturning Circulation (AMOC) and a negative North Atlantic Oscillation (NAO) index.

So how does a slowing of the AMOC increase sea levels on the northeastern coastlines of the U.S. and Canada?

The part of the AMOC at the sea surface is the northeastward-flowing Gulf Stream and North Atlantic Current, a relatively thin "river" of water in perpetual motion.  That motion is associated with a sea level gradient, in which the sea level is lower on the left (northwest) side, toward the coastline, and higher on the right (southeast) side, further offshore. 

AIR staffer volunteer
A schematic diagram of the North Atlantic section of the AMOC, in which the northward-flowing Gulf Stream is the upper branch, and water cooled down in polar and sub-polar regions becomes denser and sinks down to form the southward-flowing part of the circulation. (Source: T. Ezer and L. Atkinson, Old Dominion University)

If the Earth were to suddenly stop spinning and the wind were to stop blowing (don't worry – it won't), then the sea level would flatten, causing a rise in the sea level toward the coastline and a drop well offshore.

But on our rotating earth, the sea level gradient across the Gulf Stream is closely associated with the speed of the Gulf Stream currents, locked in what scientists call geostrophic balance. Slowing the forward speed of the Gulf Stream has a similar effect to slowing the rotation rate of the Earth: partially flattening the sea level gradient across the Gulf Stream.

It was this temporary slowdown of the Gulf Stream (and all of the AMOC) from 2009 to 2010 that was a major contributor to the temporary sea level "jump" north of New York City.

It is important, however, to distinguish between a regional sea level rise that is balanced by a sea level drop elsewhere, as we have described here, and the global sea level rise of about 6–9 inches over the last 100 years caused primarily by melting ice sheets and thermal expansion due to global ocean warming. That is something else entirely.

Categories: Climate Change

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