Q U A R T E R L Y J O U R N A LO F T H ER O Y A L M E T E O R O L O G I C A L S O C I E T YVol. 128 OCTOBER 2002 Part B No. 586Q. J. R. Meteorol. Soc. (2002), 128, pp. 2563–2586 doi: 10.1256/qj.01.128Is the Gulf Stream responsible for Europe’s mild winters?By R. SEAGER1¤, D. S. BATTISTI2, J. YIN2, N. GOR DON1, N. NAIK1, A. C. CLEMENT3and M. A. CANE11Lamont Doherty Earth Observatory of Columbia University, USA2University of Washington, USA3Rosenstiel School of Marine and Atmospheric Sciences, USA(Received 23 July 2001; revised 19 April 2002)SUMMARYIs the transport of heat northward by the Gulf Stream and North Atlantic Drift, and its subsequent releaseinto the midlatitude westerlies, the reason why Europe’s winters are so much milder than those of eastern NorthAmerica and other places at the same latitude? Here, it is shown that the principal cause of this temperaturedifference is advection by the mean winds. S outh-westerlies bring warm maritime air into Europe and north-westerlies bring frigid continental air into north-eastern North America. Further, analysis of the ocean surface heatbudget shows that the majority of the heat released during winter from the ocean to the atmosphere is accountedfor by the seasonal release of heat previously absorbed and not by ocean heat- ux convergence. Therefore, theexistence of the winter temperature contrast between western Europe and eastern North America does not requirea dynamical ocean. Two experiments with an atmospheric general-circulation model coupled to an ocean mixedlayer con rm this conclusion. The difference in winter temperatures across the North Atlantic, and the differencebetween western Europe and western North America, is essentially the same in these models whether or not themovement of heat by the ocean is accounted for. In an additional experiment with no mountains, the  ow acrossthe ocean is more zonal, w estern Europe is cooled, the trough east of the R ockies is weakened and the cold ofnorth-eastern North America is ameliorated. In all experiments the west coast of Europe is warmer than the westcoast of North America at the same latitude whether or not ocean heat transport is accounted for. In summary thedeviations from zonal symmetry of winter temperatures in the northern hemisphere are fundamentally caused bythe atmospheric circulation interacting with the oceanic mixed layer.KEY WORDS: European winters1. INTRODUCTIONIt is widely believed by scientists and lay peo ple alike that the transpo rt of warmwater north in the Gulf Stream and North Atlantic Drift, and its release to the atmos-phere, is a majo r reason why western Europe’s winters are so much milder (as muchas 15–2 0 degC) than those of eastern North A merica (Fig. 1). The idea appears to havebeen popu larized by M. F. Maury in his book The ph ysical geography of the sea andits meteorology (1855) which went through many printings in the United States and theBritish Isles and was translated into three languages. In the boo k Maury says:One of the benign of ces of the Gulf Stream is to convey heat from the Gulf of Mexico, whereotherwise it would become excessive, and to disperse it in regions beyond the Atlantic for theamelioration of the climates of the British Isles and of all Western Europe. Maury, 1855.¤Corresponding author: Lamont Doherty Earth Observatory of Columbia University, Palisades, New York,NY 10964, USA. e-mail: [email protected]° Royal Meteorological Society, 2002.25632564 R. SEAGER et al.90°E 120°E 150°E 180°150°W 120°W 90°W 60°W 30°W 0°30°E 60°E0°30°N 60°N 90°N-21-21-18-18-18-15-15-15-12-12-12-12-9-9-9-9-9-9-6-6-6-6-6-6-6-3-3-3-3-3-3-3-3000000033366699121518212427Figure 1. Departure from the zonal mean of January surface air temperature (degC) from NCEP/NCARReanalysis. Parts of Western Europe (taken to mean the part of Europe west of the longitude that joins the Adriaticand the Baltic), are 15–20 degC warmer than parts of eastern North America and are also warmer than westernNorth America at the same latitudes. The contrast is greatest in the maritime regions of north-west Europe (theBritish Isles and Scandinavia). The contour interval is 3 degC with negative values dashed.Maury says that were this not to occur:the soft climates of both France and England would be as that of Labrador, severe in the extreme,and ice-bound. Maury, 1855.He continues:Every west wind that blows crosses the stream on its way to Europe, and carries w ith it a portion ofthis heat to temper there the northern winds of winter. It is the in uence of this stream upon climatethat makes Erin the ‘Emerald Isle of the Sea’, and that clothes the shores of Albion in evergreenrobes; while in the same latitude, on this side, the coasts of Labrador are fast bound in fetters ofice. Maury, 1855.The idea that the poleward ocean heat transport (OHT) helps make westernEurope’s winters th e mildest of their latitude has gained wid e currency, with the subtledifference th at the poleward  ow of warm water is now more likely to be ascribed to thethermohaline circulation (THC) than the Gulf Stream per se. For example, in a recentpaper about the possible impact of rising greenhouse gases on the THC, Latif et al. state:In the North Atlantic the Gulf Stream transports enormous amounts of heat poleward (1 PW) aspart of the THC, thereby warming western Europe. Latif et al., 2000.In a prior paper on th e same subject Broecker states:One of the major elements of today’s ocean system is a conveyor-like circulation that delivers anenormous amount of tropical heat to the northern A tlantic. During winter, this heat is releasedto the overlying eastward moving air masses, thereby greatly ameliorating winter temperatures innorthern Europe. Broecker, 1997.These statements are somewhat ambiguous (wa rming and ameliorating relative towhat?—the ambiguity is consciously echoed in the title of this paper), but both implythat the OHT preferentially affects temperatures in parts of Europe, increasing the zona ltemperature asymmetry. Hartmann (1 994) explicitly appeals to the OHT as one factorneeded to explain why winters in western Europe are milder than those in eastern NorthAmerica:It appears that some of the heat carried northward by the Gulf Stream is picked up by theNorwegian Current and carried into polar latitudes. As a result, at middle and high latitudes theGULF STREAM AND EUROPEAN WINTERS 2565eastern Atlantic is much warmer at