North Atlantic Oscillation Lecture Outline

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North Atlantic OscillationLecture Outline

• Development of ideas
• Westerlies and waves in the westerlies
• North Atlantic Oscillation basic
  pattern impact on Northern Hemisphere
• Forcing of NAO
• Economical impacts

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Development of Ideas

• Hans Egede Saabye made the following observation
  in a diary which he kept in Greenland during the
  years 1770-78

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• 1920s Walker NAO
• 1950s Jerome Namias Index Cycle
• 1950s Ed Lorenz Lorenz Boxes
• 1980s Lamb and Peppler
• 1995 onwards Work begins!

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Westerlies and waves in the westerlies
Poles
Subtropics
Pressure Gradient Force
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Vertical motion in Rossby Wave
LOW
Vorticity maximum
LOW
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North Atlantic OscillationBasic patterns and
impact on Northern Hemisphere
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North Atlantic Setting

• High Pressure over the Azores
• Low Pressure over Iceland
• Westerly wind
• low pressure systems (rain) travel in the
  westerlies

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North Atlantic Oscillation

• Azores high and Icelandic Low are the centres of
  action
• Azores high can weaken or strengthen
• Icelandic low can weaken or strengthen
• Tendency for Azores high and Icelandic low to be
  negatively correlated

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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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NAO 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.

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• The NAO index interannual signal
• decadal variability (quite strong at the
  beginning and end of the 20th century)
• the recent 30 years trend
• possibly linked to "global warming".

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The negative NAO index phase

• Azores high weak
• Icelandic Low shallow
• westerlies weak
• NW Europe cold and dry
• wet conditions in Med and N Africa
• US east coast cold outbreaks and snow
• Greenland mild conditions

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The positive NAO index phase

• Azores High strong
• Icelandic Low deep
• westerlies strong
• wind and rain
• mild conditions in NW Europe
• dry conditions in Med and N Africa
• Dry and cold in N Canada and Greenland
• Eastern USA wet and mild

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NCAR CCM3 simulations. Difference in sea level
pressure between winters (December-March) with an
NAO index value gt 1.0 and those with an index
value lt 1.0 (high minus low index winters) since
1899. The contour increment is 2 hPa and
negative values are dashed. The index (right) is
based on the difference of normalized sea level
pressure between Lisbon, Portugal and
Stykkisholmur/Reykjavik, Iceland from 1864
through 2000. The heavy solid line represents the
index smoothed to remove fluctuations with
periods less than 4 years.
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Impacts of the North Atlantic Oscillation
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NAO -
NAO
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NAO -
NAO
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Temperature
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NAO -
NAO
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Rainfall
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- NAO
Negative NAOI
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NAO
Positive NAOI
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U Component (East-West) NAO
- NAO
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• Forcing of NAO

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Forcing of NAO

• Internal variability in the atmosphere
• Ocean forcing
• Ocean-Atmosphere coupling
• Atmosphere Ocean
• Stratospheric forcing

Observational studies
Modelling Studies
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Internal Variability

• Models forced with non-varying SST still produce
  a NAO response (Barnett, 1985 Marshall and
  Molteni, 1993, e.g., Kitoh et al. 1996 Saravanan
  1998 Osborn et al. 1999 Shindell et al. 1999).
• Fundamental mechanism of NAO may be internal
  atmospheric variability
• Phase and amplitude of NAO can be forced
• Ocean models forced with noise can generate
  coherent decadal SST patterns

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SST Forcing

• Modelling
• Davies et al 1997 forced HADAM1 with observed SST
  1949-1993.
• NAO pattern similar to observed
• Rodwell et al 1999 Nature
• Observational
• Sutton and Allen 1997 Nature
• Hurrell

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Coupled Mechanisms

• NAO may be determined by an inherently coupled
  interaction between ocean and atmosphere
• Low frequency response of the ocean to
  atmospheric forcing and its feedback on the
  atmosphere result in decadal oscillations
• 2 possible mechanisms exist

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• Mechanical and thermal interaction between the
  wind-driven ocean gyres and overlying atmospheric
  circulation - gyre dynamics then set decadal time
  scales (e.g. Deser and Blackmon, 1993)
• variability is governed by processes that
  modulate the strength of the meridional or
  thermohaline circulation (and heat transport) -
  hence SSTs (e.g. Latif, 1996)

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• CMIP, the Coupled Model Intercomparison Project,
  is the analog of AMIP for global coupled
  ocean-atmosphere general circulation models.
• CMIP began in 1995 under the auspices of the
  Working Group on Coupled Models (WGCM) of CLIVAR.

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• NAOMIP is a multi-national CMIP sub-project to
  compare the coupled ocean-atmosphere model
  simulations of the annual, interannual, and
  interdecadal variability in the North Atlantic
  Oscillation.

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OCEAN - ATMOSPHERE INTERACTION
Sensible and latent heat flux wind stress
Sensible and latent heat flux
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OCEAN - ATMOSPHERE INTERACTION AOGCM Experiments
Sensible and latent heat flux wind stress
Sensible and latent heat flux
Model calculates SSTs Wind stress heat fluxes
O-A and A-O
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OCEAN - ATMOSPHERE INTERACTION AOGCM Experiments
Sensible and latent heat flux wind stress
Sensible and latent heat flux
Hypotheses dependent on model behaviour
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ATMOSPHERE - OCEAN INTERACTION OGCM Experiments
wind stress
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• 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.

SLP and Windstress As observed
Observed land temps Modelled SSTs
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Stratospheric forcing

• NAO might be more appropriately thought of as an
  annular (zonally symmetric) hemispheric mode of
  variability characterized by a seesaw of
  atmospheric mass between the polar cap and the
  middle latitudes in both the Atlantic and Pacific
  Ocean basins (Thompson and Wallace, 1998 1999),
  called the Arctic Oscillation. During winter, its
  vertical structure extends deep into the
  stratosphere.
• Similar structure is evident in the Southern
  Hemisphere.

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.....stratospheric forcing

• During winters when the stratospheric vortex is
  strong, the AO (and NAO) tends to be in a
  positive phase.
• Baldwin and Dunkerton (1999) suggest that the
  signal propagates from the stratosphere downward
  to the surface.
• Recent trends in the tropospheric circulation
  over the North Atlantic could be related to
  processes which affect the strength of the
  stratospheric polar vortex, e.g. tropical
  volcanic eruptions (Robock and Mao 1992 Kodera
  1994 Kelly et al. 1996), ozone depletion
  (Volodin and Galin 1999), and changes in
  greenhouse gas concentrations

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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.
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• Economic impacts

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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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• Can current generation climate model's capture
  the observed features of the North Atlantic
  Oscillation ?
• If so, which features depend on coupled
  ocean-atmosphere processes and which are purely
  atmospheric ?
• How much role do land-atmosphere interactions
  play in the NAO ?
• Useful website http//www.ldeo.columbia.edu/NAO/l
  inks.html