FIGURE 1. The North Atlantic heat conveyor.

Most warm waters in the upper ocean circulate clockwise in a giant horizontal swirl in the subtropics, but some flow farther north and cross the Greenland–Scotland Ridge (GSR). This branch warms the northern North Atlantic and Europe, and keeps most of the Nordic Seas free of ice. Here the water sinks (indicated by the star) and flows back southwards at depth, mostly down the western edge of the Atlantic basin. The Scandinavian monitoring array tracks only the northern limb of the overturning circulation, but more deep water is added south of the GSR and in the Labrador Sea (stars). The 257 N section covers all of the overturning circulation, and also includes the horizontal recirculation in the subtropics. According to Bryden and colleagues' results¹, the former is weakening and the latter strengthening.