Section 2.2 Interannual Variability

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Section 2.2 Interannual Variability  2.2.1 El
Nino Southern Oscillation (ENSO)   (i)
Observations (ii) Theory for ENSO (iii)
Impacts 2.2.2 Interannual variability in
Atlantic SSTs 2.2.3 The North Atlantic
Oscillation (NAO)  
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2.3.2 Interannual variability in Atlantic
SSTs So far we have emphasized interannual
variability in the Pacific in association with
ENSO It should be noted that other ocean basins
exhibit interannual variability (e.g. Atlantic,
Indian Ocean etc). Here we briefly highlight
some key modes of variability in the Atlantic.
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2.3.2 Interannual variability in Atlantic
SSTs Unlike the Pacific the Atlantic is not
dominated by any single mode of climate
variability such as ENSO. This region is subject
to multiple competing influences. See Sutton
et al 2000 The Elements of Climate Variability
in the Tropical Atlantic Region, Journal of
Climate
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2.3.2 Interannual variability in Atlantic
SSTs Three different forced SST patterns have
been identified (a) Remote response to
ENSO (b) An Atlantic Dipole SST pattern (c) A
response to Equatorial Atlantic SST anomalies
(La hermanita!) There relative importance
varies according to season.
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2.3.3 The North Atlantic Oscillation
The North Atlantic Oscillation most slides
courtesy Martin Visbeck
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Useful website for NAO provided by David
Stephenson (University of Exeter) http//www1.se
cam.ex.ac.uk/cat/NAO And the NOAA
website http//www.cpc.ncep.noaa.gov/products/pre
cip/CWlink/pna/nao.shtml
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General CirculationThe surface pressure

• NorthernWinter (January)
• High over Land, Low over Ocean

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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.
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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 (Iceland) 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 is then the winter NAO index.

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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".

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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
  deeper 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.

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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.

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Impacts of the NAO
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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.
• 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.

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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.

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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.

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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.

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Atlantic Ocean SSTs and the NAO

• Some scientists 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.

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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.

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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?

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Summary

• 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 scientists argue that the NAO is strongly
  coupled to the stratosphere and will be
  significantly influenced by "global warming".
• Other scientists 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.