Thus, the difference in the climate across the Atlantic arises not only because western Europe warms but also because eastern North America gets colder. Both regions have their characteristic temperatures because of the atmospheric circulation pattern established by heat loss from the ocean in the vicinity of the Gulf Stream.
The amount of heat loss from the Gulf Stream that is required to establish this circulation cannot be sustained only from heat that the mid-Atlantic gains during the summer, however. Heat transported by the Gulf Stream, from lower latitudes, is also needed. In this sense, Kaspi and Schneider lend some credence to Maury's earlier ideas. Although the atmospheric low- and high-pressure systems are created without any need to invoke the influence of the Rockies on the jet stream, this new work does highlight the importance of the southwesterly winds in bringing warmth to Europe.
Interestingly, the Kaspi-Schneider model can also explain why western Oregon, Washington State and British Columbia have much milder winters than what Kamchatka endures. This transpacific contrast has never been attributed to the presence of the Kuroshio, the counterpart of the Gulf Stream in the Pacific, primarily because the Pacific is a much larger ocean and the Kuroshio is a considerably weaker current than the Gulf Stream across much of it. Yet the Kaspi-Schneider result would suggest that heat lost over the Kuroshio could induce a stationary, atmospheric-pressure system similar to the one near the Gulf Stream in the Atlantic. The system would deliver cold polar air to northwestern Asia via northwesterly winds there, and southwesterlies would deliver warmer air to the northern U.S. Pacific Coast.
Shutting Down the Gulf Stream
The jury is still out on which model is correct, although the Kaspi-Schneider scenario seems plausible. The second part of Maury's conjecture—that a cessation of the Gulf Stream would lead to more intense winters over northwestern Europe—has also recently generated considerable interest. For many years the nature of the Gulf Stream's role in climate change has been framed as this question: If a warmer climate melts Arctic ice, will the excess freshwater that enters the ocean in the northern Atlantic decrease the overturning circulation there, shut down the Gulf Stream and rob northwestern Europe of an important source of heat?
The overturning circulation consists of warm upper waters in the North Atlantic that move northward toward the pole and of cold deep waters that move southward toward the equator. These shallow and deep currents link to form something of a conveyor belt by the sinking, or downwelling, of surface waters at high latitudes in the Labrador and Nordic seas and by deep water elsewhere in the global basin that rises, or upwells, to the surface. In essence, the cold waters that sink in the northern North Atlantic are replaced by relatively warm surface waters that upwell elsewhere in the global ocean.
In many climate-warming scenarios, the melting of Arctic ice would add a large quantity of freshwater to the ocean at high latitudes. Because freshwater is less salty (and thus less dense) than seawater, it might not sink—so the downwelling that feeds the deep currents of the overturning circulation would be inhibited. In this case, there would be no physical requirement for warm deep waters to rise up elsewhere because there would be no downwelling to compensate for; in consequence, with no new warm water rising to the surface, the northward flow of such water—the Gulf Stream—might be diminished. Alternative scenarios hold that freshwater additions at high latitudes would divert the Gulf Stream farther south or diminish its strength. In either case, a weakened or diverted Gulf Stream would provide less heat for European winters. Many models strongly predict that a decrease in the overturning circulation correlates with a subsequent cooling in the North Atlantic and northwestern Europe.