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Is the Gulf Stream responsible for Europe's mild winters?

By

R. Seager et AlQ.J.R.Meteorol.Soc. (2002), 128

Presenter: Rabah Aider , November, 19th 2006

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.

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.

t

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)

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

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.

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”

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

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.

Atmosphere

ocean

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.

Results

1st 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

.

Stationary horizontal advection of temperature averaged from December to February (Wm-2)

Unit= W/m2

3rd Term: Horizontal transient heat fluxCools Western Europe and warms North America

Unit= W/m2

2nd Term: Stationary vertical advection Cooling of the Pacific coast of North America opposing the warming by horizontal advection

Unit= W/m2

4th Term: Transient vertical heat flux

Cooling of both North Atlantic and North Pacific Oceans

Unit=w/m2

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 (1st term)

Diabatic heating

Unit= W/m2

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.

Models resultsSeager & 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

Main results

1. The ocean and atmosphere contributions to the poleward heat transport

Ocean

atmosphere

total

Without OHT

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:

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

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

35degThe 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.

The GISS modelThe 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)

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.

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

Departure of January surface temperature from the zonal mean

With OHT

With no OHT

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.

Total heat released

HOC

Total heat-HOC

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

No OHT,With mountains

With OHT,no mountains

With OHT and mountains

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 weakenedThis leads to a large warming of North America and a cooling of Northern Europe.The temperature contrast across the Atlantic is reduced

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

mountains

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

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.

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

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