The WaPo article to which you refer is a gross over-simplification of a scientific paper, which itself is an over-simplification. Stefan Rahmsdorf and his gullible pupils at the Potsdam Institute for Climate (PIK) have been pushing this silly line of "research" for decades. And every few years they publish the pretty-much same paper.
You may recall a rather alarmist movie came out in about 2004 - called "The Day After Tomorrow". At the time, most physical scientists who reviewed it panned it completely, or commented that it was great fiction but “lousy science”. Carl Wunsch, of MIT (Earth, Atmospheric and Planetary Sciences) - a proper oceanographer - even wrote to Nature journal, as follows,
"Sir —Your News story "Gulf Stream probed for early warnings of system failure" (Nature 427, 769; 2004) discusses what the climate in the south of England would be like "without the Gulf Stream". Sadly, this phrase has been seen far too often, usually in newspapers concerned with the unlikely possibility of a new ice age in Britain triggered by the loss of the Gulf Stream.
European readers should be reassured that the Gulf Stream's existence is a consequence of the large-scale wind system over the North Atlantic Ocean, and of the nature of fluid motion on a rotating planet. The only way to produce an ocean circulation without a Gulf Stream is either to turn off the wind system, or to stop the Earth's rotation, or both.
Real questions exist about conceivable changes in the ocean circulation and its climate consequences. However, such discussions are not helped by hyperbole and alarmism. The occurrence of a climate state without the Gulf Stream any time soon — within tens of millions of years has a probability of little more than zero.."
One of the founding fathers of oceanography was Harald Sverdrup — who was made director of the Scripps Institution of Oceanography in California in 1936. He developed a theory of the general ocean circulation, the ‘Sverdrup balance’. In 1948, he was joined by Walter Munk and they collaborated on the subject of wind-driven ocean circulation. The resulting research showed that Gulf Stream transport can be estimated by integrating the wind stress across the Atlantic.
In 1948, Henry Stommel (of Scripps and later of Woods Hole) proposed a circulation for the entire ocean depth by starting with the same equations as Sverdrup but adding bottom friction. This showed a process known as western intensification — e.g. the Gulf Stream and the Kuroshio Current. These western boundary currents play a major role in heat transport from the tropics to high latitudes. Then in 1950 Walter Munk was able to combine the results of Rossby (eddy viscosity), Sverdrup (upper ocean wind driven flow) and Stommel (western boundary current flow) for a complete solution for the ocean circulation. This proposed the following wind-driven gyres: North Pacific Ocean, South Pacific Ocean, Indian Ocean, South Atlantic and North Atlantic. In addition there was the Antarctic Circumpolar Current.
I’m not sure why the Antarctic Circumpolar Current wasn’t included among the other wind-driven gyres, but it is a) wind-driven and it is b) the most powerful current of them all — the key to the whole of the Earth’s system of ocean currents.
Now let’s talk about the Deepwater circulation.
Since the mid-18th century sea captains knew that very cold water underlay even the hottest waters of the tropical Atlantic and there had been speculation that this cold water could only be flowing to the tropics from the polar regions. By the early 20th century it was clear that this deep cold water was flowing the full-length of the Atlantic from the Arctic to the Southern Ocean.
But it was Stommel (and Arons) who solved the puzzle mathematically using two theoretical cold-water sources — one in the North Atlantic near the south tip of Greenland, and the other in the Antarctic’s Weddell Sea. 3403_111_76.jpg (546×331) (climate-policy-watcher.org)
Look at the diagram in the above link and note the dominant role of the Southern Ocean circulation — i.e. the deepwater equivalent of the Antarctic Circumpolar surface current (with its chicane at the Drake Passage). Oceanographers have called this “the great Mixmaster” (i.e. referring to the Sunbeam Mixmaster — a popular household appliance in the 1960s). When the Atlantic deepwater enters the Southern Ocean it is blended so efficiently that it ‘loses its identity’ before it is even halfway round the Antarctic loop. The resulting blend consists of one part north Atlantic deep water to two parts of Antarctic deep water — which proceeds to flood into the deep Pacific and Indian Oceans — where numerous zones of upwelling counterbalance the two near-polar downwelling zones, thus maintaining mass-equilibrium. Once it has upwelled, the (former) deepwater rejoins the system of surface currents to be randomly whirled-around in the various wind-driven gyres. It does not behave anything like a conveyor belt!
What drives the Deepwater circulation?
The Deepwater circulation is relatively sluggish — probably because the forces driving it (the Thermohaline effect — modified by Coriolis (a.k.a. ‘Geostrophic’) Forces — and maybe some other effects as well) are inherently weak.
To move the winds and the ocean currents requires a vast amount of ‘work’ (in the physical sciences sense of the word — i.e. force x distance). To obtain work from a heat source (ultimately the Sun) requires a natural heat engine — of which there are two types: efficient gas heat engines and inefficient liquid heat engines.
The gas heat engine(s) which drive the winds are the Hadley Cell, Ferrell Cell and the Polar Cell. The heat which drives the Hadley Cell comes from the sun beating down on the tropical oceans and land. This causes a powerful gas expansion and updraft — the notorious Intertropical Convergence Zone (ITCZ) — up to the top of the troposphere. Air then travels along the tropopause to about 30oN or 30oS before descending as a hot dry downdraft, and then returning as the Tradewinds back to the ITCZ.
By contrast, in the Arctic, the Thermohaline effect is more like a “cold engine” than a heat engine. Moreover it is an inefficient liquid type engine — unlike the gas engines that drive the winds.
When polar seawater (some of which may have originated from the Gulf Stream) is exposed to the freezing arctic night-sky, the surface layer freezes, while the remainder is cooled to near 0oC.
The seawater that freezes gives-up its salt — thus increasing the salinity of the remaining unfrozen water. This remaining water, being both very cold and more saline becomes denser, and tends to sink to the bottom. This must happen right across the polar oceans wherever water freezes. The unique bathymetry of the various straits that divide the Arctic Ocean and the North Atlantic means that this cold dense water tends to enter the North Atlantic via the Faroe Bank channel, the Iceland-Faroe channel, and the Denmark Strait — i.e. not too far away from the location in Strommel’s and Arons’ model. (I understand that no such localised sources of thermohaline water have yet been found in the Antarctic.)
To recap, the Thermohaline effect is just one of the drivers of the sluggish Deepwater Circulation. It does not drive the Gulf Stream.
What is the AMOC?
AMOC stands for Atlantic Meridional Overturn Circulation. The likes of PIK often refer to it only by its initials without defining exactly what they mean by it. I think their lack of a clear definition tells you a lot about the quality of PIK’s research. AMOC seems to mean whatever people want it to mean (per Humpty Dumpty in “Alice though the Looking Glass”). I eventually found an authoritative source (NOAA) which gave a clear definition. (To preserve the source’s authenticity, I have not edited out any of the technical jargon.)
“The terms Atlantic Meridional Overturning Circulation (AMOC) and Thermohaline Circulation (THC) are often used interchangeably but have distinctly different meanings. The AMOC is defined as the total (basin-wide) circulation in the latitude depth plane, as typically quantified by a meridional transport stream-function. Thus, at any given latitude, the maximum value of this stream-function, and the depth at which this occurs, specifies the total amount of water moving meridionally above this depth (and below it, in the reverse direction). The AMOC, by itself, does not include any information on what drives the circulation.”
Finally two relevant quotes (which I hope will set your mind more at ease):
“Here, it is also crucial to make the distinction between the Gulf Stream and the AMOC. The Gulf Stream is driven by the atmospheric winds which do not show strong trends. The AMOC, however, is associated with convection and the formation of cold dense water around Greenland. Although the two circulations muddle up along the coast of the US, a healthy Gulf Stream can remain in place even with a weak AMOC.” (Dr David Ferreira, Associate Professor in Meteorology, University of Reading)
“Linking AMOC variability to the Gulf Stream, as stated in the press release, is somewhat misleading. The Gulf Stream is primarily a wind-driven ocean circulation, and there is no evidence that I am aware of that the Gulf Stream is itself weakening. [ . . .] Overall, this is an interesting study and one that needs continued investigation. However, it shouldn’t be over-interpreted.” (Prof Tim Palmer, Royal Society Research Professor in Climate Physics, University of Oxford)
