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The Source of Europe's Mild Climate

The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth

Richard Seager

Questioning the Myth

After completing my Ph.D. at Columbia University in New York City, I took a temporary postdoctoral position at the University of Washington in Seattle, where I should have immediately realized that something was wrong with the Gulf Stream-European climate story. Seattle and British Columbia, just to the north, I discovered, have a winter climate with which I was very familiar—mild and damp, quite unlike the very cold conditions that prevail on the Asian side of the Pacific Ocean. This contrast exists despite the fact that the circulation of currents in the Pacific Ocean is very different from the situation in the Atlantic.

Figure 2. Average January air temperatures...Click to Enlarge Image

The analogue of the Gulf Stream in the Pacific Ocean is the Kuroshio Current, which flows north along the coast of Asia until it shoots off into the interior of the Pacific Ocean east of Japan. From there, it heads due east (unlike the Gulf Stream, which heads northeast) toward Oregon and California. As such, there is almost no heat carried northward into the Pacific Ocean at the latitudes of Washington and British Columbia. Hence oceanic heat transport cannot be creating the vast difference in winter climate between the Pacific Northwest and similar latitudes in eastern Asia—say, chilly Vladivostok.

Strangely, experiencing a Seattle winter firsthand was not enough to make me question the myth. However, in Seattle I did become good friends with David S. Battisti, a professor of atmospheric sciences at the University of Washington. Battisti is one of those great scientists who, with relish and an air of mischief, loves to question conventional wisdom. Over the years he and I have enjoyed many a long evening indulging our shared passions for Italian cooking and wine while talking about climate research. During one of those conversations, sometime in 2000 as I recall, he brought up that he wanted to test the Gulf Stream-European climate idea. It was perfect timing, because just then I had been conducting a series of experiments with a numerical climate model, ones designed to examine the role the ocean plays in determining the global and regional features of the Earth's climate. So Battisti and I went to work.

First we had to consider the range of possibilities. If oceanic heat transport does not create the differences in regional climate across the North Atlantic (or North Pacific), what does? An obvious alternative explanation is that standard of high school geography education: Because the heat capacity of water is so much greater than that of rock or soil, the ocean warms more slowly in summer than does land. For the same reason, it cools more slowly in winter. That effect alone means that the seasonal cycle of sea-surface temperature is considerably less than that of land surfaces at the same latitude, which is why summers near the sea are cooler and winters are warmer than at equivalent sites located inland.

Figure 3. Thermohaline circulation...Click to Enlarge Image

The effect of differing heat capacities is augmented by the fact that the Sun's heat is stored within a larger mass in the ocean than on land. The heat reservoir is bigger because, as the Sun's rays are absorbed in the upper several meters of the ocean, the wind mixes that water downward so that, in the end, solar energy heats several tens of meters of water. On land, the absorbed heat of the Sun can only diffuse downward and does not reach deeper than a meter or two during a season. The greater density of soil and rock (which ranges up to three times that of water) cannot make up for this difference in volume of material that the Sun heats and for the difference in heat capacity of water compared with soil or rock.

Because sea-surface temperatures vary less through the seasonal cycle than do land-surface temperatures, any place where the wind blows from off the ocean will have relatively mild winters and cool summers. Both the British Isles and the Pacific Northwest enjoy such "maritime" climates. Central Asia, the northern Great Plains and Canadian Prairies are classic examples of "continental" climates, which do not benefit from this moderating effect and thus experience bitterly cold winters and blazingly hot summers. The northeastern United States and eastern Canada fall somewhere in between. But because they are under the influence of prevailing winds that blow from west to east, their climate is considerably more continental than maritime.

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