The Gulf Stream

Warmest year ever, but northeast, and the ocean just off of Greenland coast hit the coldest ever

A recent study shows that the Gulf Stream flow has decreased more in the past 100 years than at any time in the past 1000+ years:

Possible changes in Atlantic meridional overturning circulation (AMOC) provide a key source of uncertainty regarding future climate change. Maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic. Here we present multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970. Since 1990 the AMOC seems to have partly recovered. This time evolution is consistently suggested by an AMOC index based on sea surface temperatures, by the hemispheric temperature difference, by coral-based proxies and by oceanic measurements. We discuss a possible contribution of the melting of the Greenland Ice Sheet to the slowdown. Using a multi-proxy temperature reconstruction for the AMOC index suggests that the AMOC weakness after 1975 is an unprecedented event in the past millennium (p > 0.99). Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC.

All I can see is the abstract, the article requires cash, but a number of locations have written about in a somewhat more accessible format:

Welcome to this week’s installment of “Don’t Mess with Geophysics.”

Last week, we learned about the possible destabilization of the Totten Glacier of East Antarctica, which could unleash over 11 feet of sea level rise in coming centuries.

And now this week brings news of another potential mega-scale perturbation. According to a new study just out in Nature Climate Change by Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research and a group of co-authors, we’re now seeing a slowdown of the great ocean circulation that, among other planetary roles, helps to partly drive the Gulf Stream off the U.S. east coast. The consequences could be dire – including significant extra sea level rise for coastal cities like New York and Boston.

A vast, powerful, and warm current, the Gulf Stream transports more water than “all the world’s rivers combined,” according to the National Oceanic and Atmospheric Administration. But it’s just one part of a larger regional ocean conveyor system – scientists technically call it the “Atlantic meridional overturning circulation” — which, in turn, is just one part of the larger global “thermohaline” circulation (“thermohaline” conjoins terms meaning “temperature” and “salty”).

For the whole system, a key driver occurs in the North Atlantic ocean. Here, the warm Gulf Stream flows northward into cooler waters and splits into what is called the North Atlantic Current. This stream flows still further toward northern latitudes — until it reaches points where colder, salty water sinks due to its greater density, and then travels back southward at depth.

This “overturning circulation” plays a major role in the climate because it brings warm water northward, thereby helping to warm Europe’s climate, and also sends cold water back towards the tropics. Here’s a helpful visualization, from Rahmstorf and the Potsdam Institute, of how it works: ……… (see top pic)

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The system above has a key vulnerability. What keeps everything churning in the North Atlantic is the fact that cold salt water is more dense than warm water — so it sinks. However, if too much ice melts in the region — from, say Greenland — a freshening of the cold salt water could occur. If the water is less salty it will also be less dense, reducing its tendency to sink below the surface.

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Rahmstorf points to a recent release by the National Climatic Data Center of the National Oceanic and Atmospheric Administration, finding that the winter of December 2014 through February 2015 was the warmest on record for the globe as a whole. However, there were several anomalies — not just a cold winter for the eastern U.S., but also record cold temperatures in the middle of the North Atlantic: ……… (see bottom pic)

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However, there are many other effects, ranging from dramatic impacts on fisheries to, perhaps most troubling of all, the potential for extra sea level rise in the North Atlantic region.

That may sound surprising, but here’s how it works. We’re starting out from a situation in which sea level is “anomalously low” off the U.S. east coast due to the motion of the Gulf Stream. This is for at least two reasons. First, explains Rahmstorf’s co-author Michael Mann of Penn State University, there’s the matter of temperature contrast: Waters to the right or east of the Gulf Stream, in the direction of Europe, are warmer than those on its left or west. Warm water expands and takes up more area than denser cold water, so sea level is also higher to the right side of the current, and lower off our coast.

“So if you weaken the ‘Gulf Stream’ and weaken that temperature contrast…sea level off the U.S. east coast will actually rise!” explains Mann by e-mail.

This is going to get amazingly ugly.