The North Atlantic Oscillation

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The North Atlantic Oscillation
Martin Visbeck Department of Earth and
Environmental Sciences Lamont-Doherty Earth
Observatory of Columbia University with
contributions from Gerd Krahmann, Heidi Cullen
and Jessica Cherry many thanks to my
colleagues Yochanan Kushnir and Richard Seager
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Outline of the presentation

• What is the North Atlantic Oscillation ?
• Impacts of the NAO- Temperature-
  Precipitation- Storminess- Economics
• NAO and the North Atlantic Ocean.
• NAO, the stratosphere and global warming.
• Conclusions

Martin Visbeck
29 August, 2014
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What is the North Atlantic Oscillation ?

• A sea saw of atmospheric mass which alternates
  between the polar and subtropical regions.
• Changes in the mass and pressure fields lead to
  variability in the strength and pathway of storm
  systems crossing the Atlantic from the US East
  coast to Europe.
• The NAO is most noticeable during the winter
  season (November - April) with maximum amplitude
  and persistence in the Atlantic sector.

Martin Visbeck
29 August, 2014
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The North Atlantic Oscillation Index

• An Index can be constructed that represents the
  phase of the NAO. Most commonly the NAO index is
  based on the surface pressure (SLP) difference
  between the Subtropical (Azores) high and the
  Subpolar (Island) low.
• Very often the pressure readings from two
  stations one on Iceland and the other either the
  Azores, Lisbon or Gibraltar are used to construct
  the NAO index. The twice daily reading are
  averaged from November through March and the
  difference in then the winter NAO index.

Martin Visbeck
29 August, 2014
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The North Atlantic Oscillation Index

• The NAO index shows large variations from year to
  year. This interannual signal was especially
  strong during the end of the 19th century.
• Sometimes the NAO index stays in one phase phase
  for several years in a row. This decadal
  variability was quite strong at the beginning and
  end of the 20th century.
• One might also interpret the recent 30 years as a
  trend in the NAO index possibly linked to "global
  warming".

Martin Visbeck
29 August, 2014
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The positive NAO index phase

• The positive NAO index phase shows a stronger
  than usual subtropical high pressure center and a
  deep than normal Icelandic low.
• The increased pressure difference results in more
  and stronger winter storms crossing the Atlantic
  Ocean on a more northerly track.
• This results in warm and wet winters in Europe
  and in cold and dry winters in northern Canada
  and Greenland.
• The eastern US experiences mild and wet winter
  conditions.

Martin Visbeck
29 August, 2014
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The negative NAO index phase

• The negative NAO index phase shows a weak
  subtropical high and weak Icelandic low.
• The reduced pressure gradient results in fewer
  and weaker winter storms crossing on a more
  west-east pathway.
• They bring moist air into the Mediterranean and
  cold weather to northern Europe.
• The US east cost experiences more cold air
  outbreaks and hence snowy winter conditions.
• Greenland, however, will have milder winter
  temperatures.

Martin Visbeck
29 August, 2014
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Impacts of the NAO
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Martin Visbeck
29 August, 2014
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Us East Coast Impacts of the NAO

• The US East coast experiences milder winter
  conditions during a positive NAO index phase.
• The amount of snow cover is reduced.
• Warmer than usual ocean temperatures cause more
  frequent occurrence of "red tides" in the summer.
• Colder than usual tropical ocean temperatures
  reduce the number of hurricanes in the following
  summer.
• Cold ocean temperatures in the spawning grounds
  over the Grand Banks cause less cod reproduction.

Martin Visbeck
29 August, 2014
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Impacts of the NAO in Europe

• Northern Europe experiences mild and wet winter
  during the positive NAO index phase.
• This has dramatic consequences for hydro-electric
  power generation and heating oil consumption.
• South-Eastern Europe receives less rain and hence
  causes significant problems with drinking water
  supply and reduced stream flow volume in the
  Middle East.
• Harvest yield of grapes and olives have been
  shown to depend significantly on the NAO.

Martin Visbeck
29 August, 2014
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NAO and Energy in Norway

• Norway experience cold winters during a negative
  NAO phase.
• Heating Oil consumption in Norway varies by 30
  in good (anti) correlation with the NAO.
• Correlation with precipitation results in
  variability in hydropower generation.

Martin Visbeck
29 August, 2014
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NAO and Water Resources in Turkey and the Middle
East
Precipitation in Turkey is well correlated with
the NAO. As a result spring stream flow in the
Euphrates River varies by about 50 with the
NAO. An upward trend in the NAO will lead to
drought conditions in the Middle East.
Martin Visbeck
29 August, 2014
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Temperatures in the Atlantic Ocean
Winter temperatures become colder along the
pathway of the Gulf Stream / North Atlantic
Current System because the ocean warms the
overlying atmosphere.
Martin Visbeck
29 August, 2014
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NAO impact on Atlantic Ocean SSTs

• Ocean surface temperatures (SST) changes with the
  phase of the NAO.
• During a positive year the ocean warms just east
  of the US east coast and cools in the subpolar
  gyre between England, Newfoundland and Iceland.
• The Gulf stream transports those temperature
  anomalies downstream towards Europe.

Martin Visbeck
29 August, 2014
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Atlantic Ocean SSTs and the NAO

• Some scientist have suggested that the storage
  and propagation of temperature anomalies by the
  ocean gives an important feed back to the
  atmosphere and is responsible for the decadal
  signal.
• If correct one could make use of the "slow ocean
  dynamics" to predict aspects of the NAO.

Martin Visbeck
29 August, 2014
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NAO movie

• Animation of sea level pressure and surface winds
  during an idealized NAO cycle of 12 year
  duration.
• The lower panel shows the land temperature
  response and the propagation of SST anomalies in
  the ocean.
• The ocean is simulated by the Lamont Ocean model
  (LOAM)
• All other data are regressions from the NCEP/NCAR
  reanalysis.

Martin Visbeck
29 August, 2014
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NAO and global warming

• Some scientist argue that changes in the
  stratospheric circulation can influence the phase
  of the NAO.
• Ozone depletion and increase of CO2 both result
  in a strong polar night vortex which might cause
  the NAO to prefer a positive state.
• Will "global warming" cause a persistent positive
  NAO phaes?

Martin Visbeck
29 August, 2014
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Conclusions

• The North Atlantic Oscillation is the largest
  mode of climate variability in the Atlantic
  Sector and possibly in the whole northern
  hemisphere.
• Its impacts reach from the upper atmosphere to
  the bottom of the ocean and reach from America
  over to Europe and far into Asia.
• The dynamics of the NAO are not fully understood
  and in partiuclar its sensitivity to ocean, land
  or changes in the sea-ice conditions need more
  study.
• Some scientist argue that the NAO is strongly
  coupled to the stratosphere and will be
  significantly influenced by "global warming".
• Other scientist see evidence for coupling with
  the North Atlantic Ocean.
• It has also been suggested that tropical ocean
  temperatures can influence the phase of the NAO.
• It is unlikely that we will ever be able to
  predict the NAO with the same accuracy as we do
  for ENSO today.

Martin Visbeck
29 August, 2014