My post about the possibility of the Gulf Stream switching off elicited two interesting emails.

Quentin pointed me to a letter in the Economist from Professor Carl Wunsch, an MIT oceanographer, which said, in part:

The Gulf Stream is a wind-driven phenomenon (as explained in a famous 1948 paper by Henry Stommel). It is part of a current system forced by the torque exerted on the ocean by the wind field. Heating and cooling affect its temperature and other properties, but not its basic existence or structure. As long as the sun heats the Earth and the Earth spins, so that we have winds, there will be a Gulf Stream (and a Kuroshio in the Pacific, an Agulhas in the Indian Ocean, etc).

Shut-off would imply repeal of the law of conservation of angular momentum. The primary mechanism of heat transport in the ocean is the wind-forcing of currents that tend to push warm water toward the poles, cold water toward the equator. Widely disseminated and grossly oversimplified pictures showing the ocean as a “conveyor belt” have misled people into thinking ocean circulation is driven by a sinking motion at high latitudes. A comprehensive literature shows that with no wind, heating and cooling could produce a weak flow, but one not at all resembling the observed circulation.

If the sinking motion at high latitudes were completely stopped, by covering that part of the ocean by sea ice for instance, there would still be a Gulf Stream to the south, and maybe an even more powerful one as the wind field would probably then become stronger. If the sinking were stopped by adding fresh water (a deus ex machina often invoked to change the climate), the Gulf Stream would hardly care except in so far as the wind system changed too. The amount of heat transported by the system would shift, but could not become zero.

Many writers, including scientists, toss around the words “Thermohaline Circulation” as though they constituted an explanation. In the ocean, most of the movement of heat and salt, the real Thermohaline Circulation, is driven directly and indirectly by the wind field. Thus the Gulf Stream, and hence the wind, rather than being minor features of oceanic climate are best regarded as the primary elements. Many real climate change effects exist and require urgent attention; focusing on near-impossible Gulf Stream failure is an unproductive distraction.

My son Brian wrote to point out the irony that GulfStream is the brand name of a very successful executive jet aircraft!