A few times a year the British media of all stripes goes into a tizzy of panic when one climate scientist or another states that there is a possibility that the North Atlantic ocean circulation, of which the Gulf Stream is a major part, will slow down in coming years or even stop. Whether the scientists statements are measured or inflammatory the media invariably warns that this will plunge Britain and Europe into a new ice age, pictures of the icy shores of Labrador are shown, created film of English Channel ferries making their way through sea ice are broadcast... And so the circus continues year after year. Here is one example.
The Gulf Stream-European climate myth The panic is based on a long held belief of the British, other Europeans, Americans and, indeed, much of the world's population that the northward heat transport by the Gulf Stream is the reason why western Europe enjoys a mild climate, much milder than, say, that of eastern North America. This idea was actually originated by an American military man, Matthew Fontaine Maury, in the mid nineteenth century and has stuck since despite the absence of proof.
We now know this is a myth, the climatological equivalent of an urban legend. In a detailed study published in the Quarterly Journal of the Royal Meteorological Society in 2002, we demonstrated the limited role that ocean heat transport plays in determining regional climates around the Atlantic Ocean. Popular versions of this story can be found here, here and, in French, here.
The determinants of North Atlantic regional climates We showed that there are three processes that need to be evaluated:
1. The ocean absorbs heat in summer and releases it in winter. Regions that are downwind of oceans in winter will have mild climates. This process does not require ocean currents or ocean heat transport. 2. The atmosphere moves heat poleward and warm climates where the heat converges. In additions, the waviness in the atmospheric flow creates warm climates where the air flows poleward and cold climates where it flows equatorward. 3. The ocean moves heat poleward and will warm climates where it releases heat and the atmosphere picks it up and moves it onto land.
Using observations and climate models we found that, at the latitudes of Europe, the atmospheric heat transport exceeds that of the ocean by several fold. In winter it may even by an order of magnitude greater. Thus it is the atmosphere, not the ocean, that does the lion's share of the work ameliorating winter climates in the extratropics. We also found that the seasonal absorption and release of heat by the ocean has a much larger impact on regional climates than does the movement of heat by ocean currents.
Seasonal storage and release accounts for half the winter temperature difference across the North Atlantic Ocean. But the 500 pound gorilla in how regional climates are determined around the Atlantic turned out to be the Rocky Mountains. Because of the need to conserve angular momentum, as air flows from the west across the mountains it is forced to first turn south and then to turn north further downstream. As such the mountains force cold air south into eastern North America and warm air north into western Europe. This waviness in the flow is responsible for the other half of the temperature difference across the North Atlantic Ocean.
Hence:
1. Fifty percent of the winter temperature difference across the North Atlantic is caused by the eastward atmospheric transport of heat released by the ocean that was absorbed and stored in the summer. 2. Fifty percent is caused by the stationary waves of the atmospheric flow. 3. The ocean heat transport contributes a small warming across the basin.
The seasonal ocean heat storage and pattern of atmospheric heat transport add up to make winters in western Europe 15 to 20 degrees C warmer than those in eastern North America. A very similar process occurs across the Pacific Ocean. The ocean heat transport warms the North Atlantic Ocean and the land on both sides by a modest few degrees C. The only place where the ocean heat transport fundamentally alters climate is along the coast of northern Norway which would be sea ice-covered were it not for the warm northward flowing Norwegian Current.
The Gulf Stream and future climate change A slowdown of the Gulf Stream and ocean circulation in the future, induced by freshening of the waters caused by anthropogenic climate change (via melting glaciers and increased water vapor transport into high latitudes) or simply by warming, would thus introduce a modest cooling tendency. This would leave the temperature contrast across the Atlantic unchanged and not plunge Europe back into the ice age or anything like it. In fact the cooling tendency would probably be overwhelmed by the direct radiatively-driven warming by rising greenhouse gases.
North Atlantic Ocean circulation and abrupt climate change The conflation of the Gulf Stream, ocean heat transport and Europe's climate has led to changes in ocean circulation being the reigning theory of the cause of glacial era abrupt climate change. These abrupt changes - the Dansgaard-Oeschger events of the last ice age and the Younger Dryas cold reversal of the last deglaciation - are well recorded in the Greenland ice core and Europe and involved changes in winter temperature of as much as thirty degrees C! For the Younger Dryas it has been proposed that the sudden release of glacial meltwater from ice dammed Lake Agassiz freshened the North Atlantic and shut down the overturning circulation causing dramatic regional coooling.
Only through an inflated view of the impact of ocean circulation could it be thought that the enormous glacial era abrupt changes were caused by changes in ocean circulation. Instead, as we have argued, changes in atmospheric circulation regimes had to be the driver, see (Seager and Battisti,2007). Determining how this could happen has become more of a priority now that the geological evidence for the Lake Agassiz flood has not been found, see (Broecker,2006).
Moving beyond the myth It is long time that the Gulf Stream-European climate myth was resigned to the graveyard of defunct misconceptions along with the Earth being flat and the sun going around the Earth. In its place we need serious assessments of how changes in ocean circulation will impact climate change and a new look at the problem of abrupt climate change that gives the tropical climate system and the atmosphere their due as the primary drivers of regional climates around the world.
Publications
Seager, R., D. S. Battisti, J. Yin, N. Gordon, N. H. Naik, A. C. Clement and M. A. Cane, 2002: Is the Gulf Stream responsible for Europe's mild winters? Quarterly Journal of the Royal Meteorological Society, 128(586): 2563-2586. PDF Seager, R. and D. S. Battisti, 2007: Challenges to our understanding of the general circulation: abrupt climate change. In: T. Schneider and A.S. Sobel (Editors), The Global Circulation of the Atmosphere: Phenomena, Theory, Challenges. Princeton University Press, pp. 331-371. PDF. Seager, R., 2006: The source of Europe's mild climate. American Scientist, 94(4): 334-341. PDF. Seager, R., 2008: Setting the record straight on Europe's mild winters. The Plantsman, Royal Horticultural Society,7, Part 1 March, p.22-27. PDF. Seager, R., 2003: Gulf Stream la fin d'un mythe. La Recherche(361): 40-46.PDF
No comments:
Post a Comment