Red-hot: Gulf Stream water (red) is the warmest in this infrared image of the Atlantic; yellow, green,blue and purple zones are progressively cooler. Image: COURTESY OF PETER MINNETT AND BOB EVANS University of Miami/MODIS/NASA
- Three new climate studies indicate that our long-held belief about the Gulf Stream's role in tempering Europe's winters may not be correct. Yet the studies themselves do not agree.
- Two of the three studies ascribe a surprisingly large role to the direction of the prevailing winds, and one focuses on the heat lost from the ocean.
- Many climate models indicate that extensive melting of Arctic ice would not actually shut down the Gulf Stream, as previously thought.
- The ocean's influence on climate in Europe and elsewhere should become clearer within a decade, now that a global array of more than 3,000 floating ocean sensors called Argo is making near-real-time maps of temperature and salinity down to 2,000 meters.
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For a century, schoolchildren have been taught that the massive ocean current known as the Gulf Stream carries warm water from the tropical Atlantic Ocean to northwestern Europe. As it arrives, the water heats the air above it. That air moves inland, making winter days in Europe milder than they are in the northeastern U.S.
It might be time to retire that tidy story. The explosion of interest in global climate has prompted scientists to closely study the climatic effects of the Gulf Stream only to discover that those effects are not as clear as conventional wisdom might suggest. Based on modeling work and ocean data, new explanations have emerged for why winter in northern Europe is generally less bitter than winter at the same latitudes in the northeastern U.S. and Canada—and the models differ on the Gulf Stream's role. One of the explanations also provides insight into why winter in the U.S. Northwest is warmer than it is across the Pacific in eastern Russia.
At the same time, recent studies have been casting doubt on the popular conjecture made a few years ago that melting of Arctic ice could “shut down” the Gulf Stream, thereby wreaking havoc with Europe's weather. Yet the studies do suggest that climate change could at least affect the strength of the Gulf Stream, which could lessen the impact of global warming on northern Europe.
Climate variations across the globe stem primarily from the earth's spherical shape. Because the sun's rays are more perpendicular to the earth's surface at lower latitudes, they impart more heat per unit area there than at higher latitudes. This differential heating leads to the prevailing atmospheric winds, whose instabilities redistribute that heat from the tropics to the poles. The oceans, covering 70 percent of the earth, also play a major role in this redistribution. The upper two meters of the oceans store more solar heat than the entire atmosphere above the seas because the specific heat (a property that determines the capacity to store heat) of a cubic meter of water is about 4,000 times greater than the same volume of air (and about four times larger than it is for soil). Water temperatures in the upper 100 to 200 meters of the oceans at midlatitudes might vary by 10 degrees Celsius over a year, storing and releasing an immense amount of heat compared with the atmosphere or the land. And because ocean currents, such as the Gulf Stream, move water around the globe, heat gained in the summer at one locale can later be released to the atmosphere thousands of kilometers away.
Given that movement and the oceans' ability to store heat, it is easy to hypothesize that ocean currents might be responsible for the fact that winter air temperatures in Ireland, at about 50 degrees north latitude, are nearly 20 degrees C warmer than they are at the same latitude across the Atlantic in Newfoundland. Similarly, air temperatures at 50 degrees north latitude in the eastern Pacific, near Vancouver, are about 20 degrees C warmer than they are at the same latitude at the southern tip of Russia's Kamchatka Peninsula.
In the 19th century geographer and oceanographer Matthew Fontaine Maury was the first to attribute the relatively mild climate of northwestern Europe to the Gulf Stream. This powerful ocean current flows northward along the southeastern U.S. coast, a product of warm waters from the subtropics and tropics. At about the latitude of Cape Hatteras, N.C., the Gulf Stream turns to the northeast and flows out into the Atlantic. Maury surmised that the Gulf Stream supplies heat to the overlying westerly winds that move across the Atlantic toward northwestern Europe. He also speculated that if the Gulf Stream were somehow diminished in strength, the winter winds would be much colder and that Europe would experience Arctic-style winters. Over the years Maury's idea became almost axiomatic—and until recently, it also remained largely untested.