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The Source of Europe's Mild Climate


Richard Seager


Figure 1. Members of the Serpentine Swimming Club...Click to Enlarge Image

This is not to say that there is no climatological mystery to be explained. The countries of northern Europe do indeed have curiously mild climates, a phenomenon I didn't really appreciate until I moved from Liverpool to New York. I arrived in the Big Apple just before a late-summer heat wave, at a time when the temperature soared to around 35 degrees Celsius. I had never endured such blistering temperatures. And just a few months later I was awestruck by the sensation of my nostrils freezing when I went outside. Nothing like that happens in England, where the average January is 15 to 20 degrees warmer than what prevails at the same latitude in eastern North America. So what keeps my former home so balmy in the winter? And why do so many people credit the Gulf Stream?

Like many other myths, this one rests on a strand of truth. The Gulf Stream carries with it considerable heat when it flows out from the Gulf of Mexico and then north along the East Coast before departing U.S. waters at Cape Hatteras and heading northeast toward Europe. All along the way, it warms the overlying atmosphere. In the seas between Norway and Newfoundland, the current has lost so much of its heat, and the water has become so salty (through evaporation), that it is dense enough to sink. The return flow occurs at the bottom of the North Atlantic, also along the eastern flank of North America. This overturning is frequently referred to as the North Atlantic thermohaline circulation, or simply the "Atlantic conveyor." It is part of the global pattern of ocean circulation, which is driven by winds and the exchange of heat and water vapor at the sea surface.

The Gulf Stream indeed contributes to Europe's warmth, but it is wrong to conflate the climate difference across the North Atlantic with the northward flow of warm water in the Gulf Stream. This erroneous logic leads to such statements as (from The Times of London): "The British Isles lie on the same latitude as Labrador on the East Coast of Canada, and are protected from a similarly icy climate by the Atlantic conveyor belt." Such claims are absolutely wrong.

The statements scientists make about Atlantic thermohaline circulation typically read more like this one from my Columbia University colleague, Wallace S. Broecker:

One of the major elements of today's ocean system is a conveyor-like circulation that delivers an enormous amount of tropical heat to the northern Atlantic. During winter, this heat is released to the overlying eastward air masses, thereby greatly ameliorating winter temperatures in northern Europe.

This assertion has the benefit of being both correct and misleading. Because it does not specify what European climate is ameliorated relative to (the climate of eastern North America?), it leaves unchallenged the incorrect version expounded in the popular media—thus contributing to the erroneous beliefs of millions.

The idea that the Gulf Stream is responsible for Europe's mild winters seems to have originated with Matthew Fontaine Maury, an American naval officer who in 1855 published The Physical Geography of the Sea, which is often considered the first textbook of physical oceanography. The book was a huge success, went through many printings and was translated into three languages. The role of the Gulf Stream in shaping climate is a recurring theme in Maury's book. For example, he stated:

One of the benign offices of the Gulf Stream is to convey heat from the Gulf of Mexico, where otherwise it would be excessive, and to disperse it in regions beyond the Atlantic for the amelioration of the climates of the British Isles and of all Western Europe.

According to Maury, if this transport of heat did not take place, "… the soft climates of both France and England would be as that of Labrador, severe in the extreme, and ice bound." Despite the differences in language and style, the modern statements clearly owe their provenance to this 1855 treatise.

Maury thought that God set the ocean up to work this way apparently as part of His design to keep Europe warm (for unspecified reasons). But holding such religious beliefs did not stop Maury from also providing a scientific explanation for the Gulf Stream. His idea was that it was the oceanic equivalent of what in the atmosphere is known as a Hadley cell, a convection cell wherein warm air flows upward and poleward, and cold material flows downward and equatorward. In the ocean, heated surface waters take a northeastward route, in Maury's view, because of the need to conserve angular momentum as they move north and, hence, closer to the axis of the Earth's rotation. Maury did not recognize that winds drive ocean currents. And it was not until a century later that a valid explanation of the Gulf Stream emerged: In the jargon of oceanographers, it is a westward-intensified boundary current within a subtropical gyre (a large circular current system) driven by the trade winds, which blow from east to west in the tropics, and mid-latitude westerlies, which move in the opposite direction.

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