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Abrupt Climate Change: Inevitable Surprises
marily an Atlantic rather than Pacific phenomenon, the heat transport is focused in the Atlantic. The somewhat greater warmth of European coastal regions than of similar latitudes on the Alaskan coast might well be linked to this THC heat transport (Plate 5), although debate continues about where and by how much ocean heat transport warms the atmosphere, and the extent to which changes in oceanic heat transport would be balanced by changes in atmospheric heat transport.
Popular treatments sometimes imply that the THC is responsible not only for the temperature difference between Europe and Alaska, but also for the larger difference between Europe and the east coast of North America. This is unlikely because the US Pacific Northwest coast is warmer than the east coast of Asia without an active THC in the North Pacific. Two factors appear to provide the large differences between east and west coasts. First, the prevailing westerly winds pick up some heat during winter while blowing over the ocean, spreading the moderating influence of the ocean downwind. Second, and perhaps more important, the planetary wave pattern—the global-scale sinuous bending of the jet stream—brings Arctic air down over the US New England region and the Asian coast of the Pacific and warmer breezes to the US Pacific Northwest and Europe. The pattern is known to be driven primarily by mountains in the winter (Nigam et al., 1988), and is influenced relatively little by land-sea temperature contrasts. An improved quantitative understanding of the east-west (North America to Greenland to Europe) gradient of temperature fluctuations in past abrupt climate changes is crucial to furthering the understanding of mechanisms. On the other hand, the relative warmth of Norway, which is just to the east of the second major center of heat loss in the North Atlantic (the first being in the Gulf Stream off the coast of the United States), is probably due to the THC; the Pacific lacks similar warmth to that off Norway.
A major problem with THC-based theories of the Younger Dryas and Dansgaard/Oeschger events is that atmospheric models yield a predominantly local North Atlantic cooling in response to THC shutdown, with few of the global repercussions that seem to be demanded by the data (see Chapter 2). In fact, the atmosphere alone does not seem able to extend the influence of extratropical influences to the rest of the globe efficiently. Even such a major forcing as the introduction of Northern Hemisphere ice sheets of the last glacial maximum into models has little influence south of the equator (Manabe and Broccoli, 1985; Broccoli and Manabe, 1987). In models generally, North Atlantic cooling affects the strength of the tropical