New research shows that the Gulf Stream has little effect on the contrast in winter temperatures between Europe and eastern North America, dispelling a long-held assumption. Instead, atmospheric circulation, augmented by the Rocky Mountains, plays a larger role, say Richard Seager of Columbia's Lamont-Doherty Earth Observatory, David Battisti of the University of Washington and their colleagues.
Published in the Quarterly Journal of the Royal Meteorological Society, this new data suggests that atmospheric circulation is more important to understanding climate variability than is the ocean circulation.
What Seager, Battisti, and their co-workers found was that much of the difference in temperature between eastern North America and western Europe can be explained by the simple and well-known fact that the ocean stores heat in the summer and gradually releases it in winter. Where winds blow from west to east, as across the North Atlantic, the heat released in winter preferentially warms the land areas to the east of the ocean. That this is a big effect is well known, but the new research shows that the winter temperature contrast is much bigger than can be accounted for by this simple difference between a warm 'maritime' climate in Europe and a frigid 'continental' climate in North America.
The Rocky Mountains play a major role. Analogous to an island in a stream, the Rockies set up a persistent wave in the winds downstream that brings cold winds from the north into eastern North America and warm winds from the south into western Europe. This pattern of movement of heat by the winds accounts for half of the total difference in winter temperatures between the two regions, with much of the other half attributable to the release of heat stored in the ocean.
"That the Gulf Stream heat transport has a minor effect while the Rocky Mountains loom large in causing the differing winter conditions of western Europe and eastern North America will 'certainly require some rewriting of textbooks as well as tourist guides' says Seager. "But now we must also look differently at theories of climate change, which in the past have revolved around water circulation in the Atlantic Ocean."
The research team analyzed observational data to first make their argument and then performed a set of experiments with computer models of the atmosphere and ocean to prove it. In some model experiments they accounted for the movement of heat by ocean currents and in others they stopped the ocean from moving. In other experiments they removed mountains and made the Earth flat. When the Rocky Mountains were removed from the model, temperatures in eastern North America warmed, and they dropped in western Europe.
Richard Seager is a Senior Research Scientist with the Lamont-Doherty Earth Observatory, a research unit of the Earth Institute of Columbia University. The research reported in the Quarterly Journal of the Royal Meteorological Society was supported by funding from NOAA.