Is the Gulf Stream responsible for Europe's mild winters By R' Seager et Al Q'J'R'Meteorol'Soc' 2002

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

• .

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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.
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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 30W,
  40N (Cape Hatteras) and heading northeast
  toward Europe.

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t
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• 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)

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

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• This idea seems to be originated with M. Fontaine
  Maury who published The Physical Geography of
  the Sea in 1855.
• MauryOne 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

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

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Atmosphere
ocean
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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.
  

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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

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

• .

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3rd Term Horizontal transient heat fluxCools
Western Europe and warms North America
Unit W/m2
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2nd Term Stationary vertical advection Cooling
of the Pacific coast of North America opposing
the warming by horizontal advection
Unit W/m2
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4th Term Transient vertical heat fluxCooling
of both North Atlantic and North Pacific Oceans
Unitw/m2
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Diabatic Heating QPositive over Atlantic and
Pacific OceanStrong in the region of Gulf
StreamEast of North America the heating is
strong (120W/m2) but it is balanced by the
cooling horizontal advection (1st term)
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Diabatic heating
Unit W/m2
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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.

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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 1The 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

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Main results

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

total
Without OHT
atmosphere
Ocean
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• 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

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

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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)

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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 valueslt6 DegC are shaded.
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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

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

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Total heat released
HOC
Total heat-HOC
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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

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No OHT,With mountains
With OHT and mountains
With OHT,no mountains
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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

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

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• 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

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