1. . Is the Gulf Stream responsible for Europe's mild winters? By R. Seager et Al Q.J.R.Meteorol.Soc. (2002), 128 Presenter: Rabah Aider, November, 19 th 2006

2. Summary. Ocean heat transport ( O.H.T) has a minor effect on the difference between the mild winters of Western Europe and the harsh ones of Eastern North America.. It is the interaction between atmospheric circulation and Atlantic ocean which is the principal cause of the contrast in the east-west asymmetry across the North Atlantic Ocean.

3. The Gulf Stream The Gulf Stream with its northern extension, the North Atlantic Drift, is one of the strong ocean currents that carries vast quantities of warm water from tropics to higher latitudes. The Gulf Stream originates from the Gulf of Mexico and flows North along the the Eastern coast before departing U.S. Waters at about 30°W, 40°N (Cape Hatteras) and heading northeast toward Europe.

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5. All along the way, it warms the overlaying atmosphere. In the seas between Norway and Newfoundland the current has lost so much of its heat and the water has become so salty, that it is dense enough to sink. The return flow occurs at the bottom of the North Atlantic and becomes the North Atlantic Deep Water (NADW)

6. The coldest waters are shown as blue, with green, yellow, representing progressively warmer water. Temperatures range from about 7 to 22 degrees Celsius.

7. The Gulf Stream and the Europe's winters For a long time scientists believed that the Gulf Stream is the responsible for Europe's mild winters.

8. This idea seems to be originated with M. Fontaine Maury who published “The Physical Geography of the Sea” in 1855. Maury:“One benign offices of the Gulf Stream is to convey heat from, and to disperse it in regions beyond the Atlantic for the amelioration of the climates of the British Isles and of Western Europe”

9. During 150 years a lot of scientific publications supported this idea with some reservations sometimes. Recently new results seem to challenge this “myth”.

10. Observational Analysis Trenbeth et al (2001) using reanalysis products (NCEP,ECMWF) and satellite estimates, computed oceanic and atmospheric annually averaged northward heat transport.  15 Deg S-15 Deg N: (O.H.T.) & Atmospheric heat transport are equal.  Midlatitudes: Atmospheric heat transport =5 times OHT  Large dominance of the atmosphere in the poleward heat transport.

11. Atmosphere ocean

12. How can we explain the zonal asymmetry of the winter temperatures between Western Europe and North America?  Seager & al used the NCEP reanalyses from 1949 to 2000 to compute the different terms of the temperature equation:  This equation is integrated vertically between 700mb and 1000 mb  The terms are averaged from December to February.

13. Results 1 st Term: Stationary horizontal advection of heat.  It creates the Western Europe-North America contrast  However the cooling of North Eastern America is stronger than the warming of W. Europe

14. . Stationary horizontal advection of temperature averaged from December to February (Wm-2) Unit= W/m2

15. 3 rd Term: Horizontal transient heat flux Cools Western Europe and warms North America Unit= W/m2

16. 2 nd Term: Stationary vertical advection Cooling of the Pacific coast of North America opposing the warming by horizontal advection Unit= W/m2

17. 4 th Term: Transient vertical heat flux Cooling of both North Atlantic and North Pacific Oceans Unit=w/m 2

18. Diabatic Heating Q: *Positive over Atlantic and Pacific Ocean *Strong in the region of Gulf Stream *East of North America the heating is strong (120W/m2) but it is balanced by the cooling horizontal advection (1 st term)

19. Diabatic heating Unit= W/m2

20. Conclusion The horizontal stationary advection of temperature creates the East-West asymmetry in winter climate while the transient heat flux attempts to damp this contrast.

21. Models results Seager & al used performed 02 experiments using a model based on a combination of Atmospheric GCM and mixed-layer ocean (CCM3-AGCM-ML). Simulation 1:The ocean heat transport O.H.T. is specified Simulation 2: Without OHT.  In both experiments the sea ice cover is fixed to its annual mean value  15 years integration and results for averaged 08 last years

22. Main results 1. The ocean and atmosphere contributions to the poleward heat transport Ocean atmosphere total Without OHT

23. 2. Contribution of the OHT to winter temperature in Northern Hemisphere The difference in January surface temperature are simulated for : the case without OHT – the case with OHT The results are shown in the figure below:

24. The difference in January Temperature in Deg C for the case with OHT minus with no OHT

25. Comments The OHT contributes to the winter temperatures by about 3deg C for Western Europe. Eastern North America is warmed by the the same amount. Removing the OHT results in a cooling of the zonal mean temperature by 4.5 deg C North of 35deg The OHT warms uniformly Northern Atlantic winters and by a few degC only. so it can not be the responsible for the West-East temperature contrast across the Atlantic Ocean.

26. The GISS model The change in surface air temperature is simulated by the GISS model (Goddard Institute for Space Studies). In this model the sea ice cover is allowed to vary. Globally similar results are found for the case with OHT minus with no OHT With the GISS model the removing of the OHT in winter causes an expansion of the sea ice cover in some regions North of 60 deg: Norwegian and Barents seas which causes the temperatures to drop by many degrees (about 20DegC),but the cooling of land areas is more modest (typically 3DegC)

27. The impact of the OHT is to prevent the expansion of the sea ice cover North of Norway, but does not significantly affect the temperatures in Western Europe The difference in January Temperature in Deg C for the case with OHT minus with no OHT simulated by GISS model. Negative values less than 6DegC are shaded Negative values<6 DegC are shaded.

28. 3. Impact of OHT on the winter temperature contrast across the North Atlantic The difference in January surface temperature are computed with the AGCM-ML model for the case with OHT and for the case without OHT. The results (figures below) show clearly that the removing of the ocean heat transport has a modest impact on the difference in winter temperatures across the North Atlantic. The departure from the zonal mean temperature between 45DegN and 60Deg N are: With OHT: 12-21 DegC Without OHT: 9-18 DegC

29. Departure of January surface temperature from the zonal mean With OHT With no OHT

30. Contribution of the ocean heat convergence to the release of heat over the Atlantic The total heat released by the ocean to the atmosphere in winter is equal to the sum of the heat converged by the ocean and the latent heat stored locally. These 03 quantities are computed and the results show that the most part of the heat released is due to the latent heat stored locally.

31. Total heat releasedHOC Total heat-HOC

32. Influence of mountains on the difference in winter temperature across the North Atlantic Experiment is performed with OHT and without mountains. Figures below show the sea level pressure and the difference of surface temperature from the zonal mean

33. No OHT,With mountains With OHT,no mountains With OHT and mountains

34. The effect of mountains on the surface temperature is shown in the figure below In absence of mountains the trough over North America is weakened  The northerlies and southerlies are weakened  This leads to a large warming of North America and a cooling of Northern Europe.  The temperature contrast across the Atlantic is reduced

35. The difference in January temperature (DegC)_surface for the case with mountains minus the case without mountains

36.  The mountains exert a strong effect on the temperature contrast across the North Atlantic. It results by a warming of the British Island by about 3DegC an by a cooling of Eastern North America by as much as 6 DegC. Other model results show that when the mountains are removed the temperature difference cut in half

37. Conclusion The transport of heat taking place in the North Atlantic warms both sides of the ocean by roughly the same amount, a few degrees. Thus it has a minor effect on the temperature contrast, 15°-20° C in winter, between Eastern North America and Western Europe. This strong asymmetry does not require the dynamical ocean and must be explained by other processes.

38. The most important are: Advection by stationary waves: the south- westward flow takes place over Eastern North America bringing Arctic air south and much colder winters for the East coast, the northeastward flow occurs over the eastern Atlantic Ocean and Western Europe bringing mild subtropical air north and warming winters of this side of Atlantic. Local heat storage: Latent heat due to solar radiation is stored locally and released to the Atmosphere in winter

39. Mountains effect: The Rokies mountains causes the Icelandic Low to strengthen and consequently to intensify the north-westerlies and the south- easterlies.