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HOME > PAST ISSUE > July-August 2006 > Article Detail

FEATURE ARTICLE

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

A Model of Contrasts

Battisti and I naturally wondered whether we could explain the difference in winter conditions between Europe and eastern North America as simply the difference between a maritime climate and a more continental one. To find the answer, he and I used two climate models, ones that normally serve for studies of natural climate variability or for assessments of future climate change. As in all such models, Earth's atmosphere is represented on a three-dimensional grid (latitude, longitude and pressure level in the vertical). For each grid point, the computer solves the relevant equations for the winds, temperature, specific humidity, fluxes of solar and terrestrial radiation and so forth while keeping track of the precipitation and energy fluxes at Earth's surface. The packing of the grid points was sufficiently dense so that we could accurately capture the endless progression of storm systems, which transport vast quantities of heat and moisture poleward. As with the computer models used to forecast the weather (which are basically the same as climate models), the computer code we used calculated conditions forward in time until, for these experiments, a statistical steady state was achieved. To get a representative picture of overall climate, we averaged together many years of simulated weather.

Figure 4. Gulf Stream currents..Click to Enlarge Image

The joy of such numerical models is that you can make radical changes to a virtual Earth's climate system with nothing more than a click of the mouse. To assess the importance of the heat transported by ocean currents such as the Gulf Stream, we compared the results of two versions of these climate models. The first versions were the standard ones, which compute sea-surface temperature after accounting for the heat moved by ocean currents, the absorption of the Sun's rays, and the exchange of heat between the ocean and the atmosphere. In the second versions, the computer code accounted for solar warming and the relevant surface heat exchanges but did not allow the model ocean to transport heat horizontally.

What we found in these tests was that, south of northern Norway, the difference in winter temperature across the North Atlantic was always the same, whether or not we let the ocean move heat around. This result would suggest that oceanic heat transport does not matter at all to the difference between the winter climates of western Europe and eastern North America! We concluded that the temperature difference must, as we had speculated before, be caused by other processes, most likely the seasonal absorption and release of heat by the ocean and the moderating effect this process has on maritime climates downwind.

Our revised view of things did not, however, mean that heat transport in the ocean does not influence climate. The ocean indeed absorbs more heat from the Sun near the equator than it loses back to the atmosphere (primarily by evaporation). And oceanic currents indeed move the excess heat poleward before releasing it to the atmosphere in the middle latitudes. Consequently, removal of the oceanic heat transport globally in our modeling exercise warmed the equator and cooled everywhere else. The climates produced by the models deprived of oceanic heat transport were colder in the subpolar North Atlantic by as much as 8 degrees in some places. The cooling over land areas was more modest, typically less than 3 degrees. These temperature changes, large as they are, are not terribly dramatic compared with the much larger temperature contrast across the North Atlantic Ocean.

Figure 5. Results of two weather-prediction models...Click to Enlarge Image

Why doesn't the ocean exert a greater influence on North Atlantic climate? According to scientists' best estimates, the ocean and atmosphere move about an equal amount of heat in the deep tropics. But at mid-latitudes, the atmosphere carries several times more heat. Thus, if one considers the region north of, say, 35 degrees North, the atmosphere is much more effective than the ocean in warming winter climates. Also, the winter release of the heat absorbed during the summer is several times greater than the amount of heat that the ocean transports from low to high latitudes in a year. Hence it is the combined effect of atmospheric heat transport and seasonal heat storage and release that keep the winters outside the tropics warmer than they otherwise would be—by several tens of degrees.

Although these numbers are instructive, they are not directly relevant to understanding the warming of Europe. For that, one needs to consider some details of geography. The Gulf Stream and associated current systems in the North Atlantic focus heat (and lose it to the atmosphere) in two clearly defined areas. One is immediately to the east of the United States, where the warm Gulf Stream flows north after leaving the Gulf of Mexico and rounding the tip of Florida. During winter, the prevailing winds blow frigid, dry air off the North American continent and across the Gulf Stream. Because of the large difference in moisture and temperature content between air and sea, the heat lost from the ocean through evaporation and direct heat transfer is immense—a few hundred watts per square meter. Much of this heat is picked up by storms in the atmosphere and carried over the eastern United States and Canada, effectively mitigating what would otherwise be a cold continental climate.

Figure 6. Experiments with numerical climate models...Click to Enlarge Image

Where else does the Gulf Stream deposit its heat? After departing the American coast, the Gulf Stream heads northeast and turns into what is called the North Atlantic Drift and, farther downstream, the Norwegian Current. After spawning many Atlantic storms, it loses most of the remainder of its heat in the Nordic seas. There the heat can effectively be moved eastward by the prevailing winds to warm northwest Europe. Thus the transport of heat taking place in the North Atlantic warms both sides of the ocean and by roughly the same amount, a few degrees. This leaves the much larger, 15-to-20-degree difference in winter temperatures to be explained by other processes.

One subtle but important effect stems from a fundamental principle in physics: the conservation of angular momentum. In meteorology, this principle translates to a rule that atmospheric flow must closely conserve the total angular momentum of a column of air. The angular momentum of the air contains two components: one arising from the rotation of the Earth (which meteorologists call the "planetary component") and another from the curvature of the fluid flow itself. The planetary component, which in the Northern Hemisphere is directed counterclockwise, is at a maximum at the pole and zero at the equator.

Figure 7. Average winds...Click to Enlarge Image

The conservation of angular momentum, it turns out, causes the mountains of North America to contribute substantially to the dramatic difference in temperatures across the Atlantic. To fathom why, you must first understand that the troposphere (the lower part of the atmosphere, where weather takes place) is bounded at the top by the tropopause, a region of stability where temperature increases with height and which acts somewhat like a lid. Thus when air flows over a mountain range—say, the Rockies—it gets compressed vertically and, as a consequence, tends to spread out horizontally. When a spinning ice skater does as much, by spreading his arms, the conservation of angular momentum slows his spin. An atmospheric column going up a mountain behaves  in a similar way and swerves to the south to gain some clockwise spin, which offsets part of the counterclockwise planetary component of its spin.

Figure 8. The waviness in the flow of the mid-latitude westerlies...Click to Enlarge Image

On the far side of the Rockies, the reverse happens: The air begins to stretch vertically and contract horizontally, becoming most contracted in the horizontal when it reaches the Atlantic. And as with an ice skater pulling in his arms, conservation of angular momentum demands that the air gain counterclockwise spin. It does so by swerving to its left. But having moved to the south after crossing the mountains, it is now at a latitude where the planetary component of its angular moment is less than it was originally. To balance this reduction in angular momentum, the air acquires more counterclockwise spin by curving back around to the north. This first southward and then northward deflection creates a waviness in the generally west-to-east flow of air across North America and far downwind to the east.

Such waves are of massive scale. The southward flow takes place over all of central and eastern North America, bringing Arctic air south and dramatically cooling winters on the East Coast. The return northward flow occurs over the eastern Atlantic Ocean and western Europe, bringing mild subtropical air north and pleasantly warming winters on the far side of ocean.

Topographically forced atmospheric waves contribute significantly to the large difference in winter temperature across the Atlantic. When Battisti and I removed mountains from our climate models, the temperature difference was cut in half. Our conclusion was that the large difference in winter temperature between western Europe and eastern North America was caused about equally by the contrast between the maritime climate on one side and the continental climate on the other, and by the large-scale waviness set up by air flow over the Rocky Mountains.








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