A decade ago, however, Richard Seager of Columbia University's Lamont-Doherty Earth Observatory and his colleagues produced an explanation for Europe's warmer winter that had nothing to do with the Gulf Stream. Seager's modeling study indicated that when the atmospheric jet stream, which flows around the earth from west to east, hits the Rocky Mountains, it begins to oscillate north and south. The oscillation produces winds that flow from the northwest over the western side of the Atlantic basin and from the southwest over the Atlantic's eastern side. The northwesterly winds bring cold continental air to the northeastern U.S., whereas the southwesterly ones bring warm maritime air to northwestern Europe.
In this view, it is not heat carried by the Gulf Stream that moderates the European climate. Instead heat that is stored off the shores of Europe, in the upper 100 meters of the ocean during the summer, is released to the atmosphere in winter when the southwesterly winds mix the surface ocean waters. In this scenario, the classic conjecture of Maury is incorrect: large-scale wind patterns directed by mountain ranges, plus local storage of heat by the ocean near Europe, set the temperature differences between the western and eastern sides of the Atlantic [see box on next two pages].
It is important to keep in mind that Seager's model simulations did not explicitly take into consideration the transport of heat by the ocean, a point addressed in a study released soon after Seager's by Peter Rhines of the University of Washington and Sirpa Häkkinen of the NASA Goddard Space Flight Center. They put forth a counterargument that offered some modern support for Maury's historical ideas. After examining archived sea-surface temperature data, the two oceanographers concluded that the amount of heat stored in the upper layer of the eastern Atlantic Ocean at the latitudes of northern Europe is enough to maintain mild air temperatures only through December of an average year. The additional heat required to moderate the climate over the remainder of the winter had to be imported from elsewhere. The most likely source: the northeastward-flowing Gulf Stream.
Measurements showed that at 35 degrees north latitude—roughly the latitude of North Carolina—the North Atlantic transports about 0.8 petawatt of heat northward, mostly by the Gulf Stream. Yet at 55 degrees north latitude—the latitude of Labrador in Canada—this poleward heat transport is negligibly small. Where does all the heat go? Rhines and Häkkinen suggested that it is released by the ocean into the atmosphere along the path of the Gulf Stream. The prevailing winds then carry the heat eastward, where it moderates the European climate. Rhines and Häkkinen essentially argued for Maury's Gulf Stream conjecture, and Seager argued against it, focusing on the role of the atmospheric jet stream.
In 2011 Yohai Kaspi, now at the Weizmann Institute of Science in Rehovot, Israel, and Tapio Schneider of the California Institute of Technology unveiled a third idea, based on novel numerical experiments of the atmosphere and the ocean. They suggested a degree of truth in both the Seager and Rhines scenarios but concentrated mostly on patterns of atmospheric pressure. Kaspi and Schneider's model indicated that the loss of heat from the ocean to the atmosphere along the path of the Gulf Stream where it leaves the U.S. East Coast generates a stationary, atmospheric low-pressure system to the east—on the European side of the Atlantic. It also creates a stationary high-pressure system to the west—over the eastern edge of the North American continent. For complex reasons, the net result of this pattern is that the stationary low-pressure system delivers warm air to western Europe via the jet stream's southwesterly winds, which pick up heat released all winter long by the Gulf Stream. The stationary high pulls in cold air from the Arctic, cooling eastern North America and increasing the contrast in temperature between North America and Europe.