NAO and its Predictability

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NAO and its Predictability

• Åke Johansson
• SMHI
• Visiting Scientist EMC

Acknowledgements Suranjana Saha, EMC Huug van den
Dool, CPC
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What is the NAO ?
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What is the NAO ?

• Meridional seesaw in atmospheric mass between 40
  N and 60 N in the Atlantic sector which
• Modifies the westerlies across the Atlantic
• 35 N and 55 N Anticorrelated
• A variation of the zonal index in the Atlantic
  Blocked flow vs zonal flow
• Quadrapole pattern of Temp and Prec over
  surrounding land masses
• Influence from Rockies to Ural
• Large influence on regional weather and climate

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NAO and Global Warming
Source Hurrell (1997)
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NAO and Global Warming
Source Hurrell (1997)
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How is it defined ?

• No strict definition
• Teleconnection pattern with centers of action
  near Iceland and the Azores
• Alternatively defined as leading mode in
  analyis like rotated EOF, Regional EOF (Atlantic
  sector) or EOT
• Spectral component that explains a large portion
  of the variance

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Source Saha
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Even in extreme NAO cases, the pattern is not
very modal. East and west Atlantic are often
acting independenly
Hard to see in Instantenous flows
Source CPC
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Only mode present year round
Source Peng
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Present on all Timescales Seasonal Monthly
Daily 21.1 16.6
8.1
Z500
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From Sea Level up into the Stratosphere
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From Sea Level up into the Stratosphere
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From Sea Level up into the Stratosphere
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NAO-Index

• Traditional station-based indices
• Stykkisholmúr, Iceland vs Ponta
  Delgada, Azores
• SW Iceland vs Lisbon, Portugal
• PC time series - LEV-VAR e g Z500
• EOT base point time series - LEV-VAR e g
  Z500

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NAO Index based on Z1000 EOT
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Modulates the Jets and concomitant storm tracks
Source Ambaum et al. (2001)
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• N-S shift of polar jet
• Simultaneous strengthening and weakening of the
  polar and subtropical jets
• Modulation of the stratospheric jet

NAO
NAO-
Source Ambaum et al. (2001)
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NAO Definition

• Named by Walker (1910-20), but its frequent use
  started in 1980s and exploded in 1990s
• No precise definition
• Alternative definition and name AO,NAM

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AO or NAM

• Produced by a zonally symmetric mechanism
• Modified by zonally asymmetric forcing
• Defined as EOF-1 of SLP in NH
• Extends thru the Troposphere and Stratosphere

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Source Thompson and Wallace (2000)
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Source Thompson and Wallace (2000)
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• AO vs NAO/PNA
• The climatological flow is not symmetric ?
  asymmetric response,
• e g Azore High and Aleutian Low

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Source Thompson and Wallace (2000)
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Troposphere StratosphereConnections
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Source Ambaum and Hoskins (2002)
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Troposphere StratosphereConnections

• Increase in NAOI ? Stronger stratospheric vortex
• A delay of 4 Days
• Stronger stratospheric vortex ? Lower surface
  pressure over the North Pole ? Direct
  reinforcement of the NAO and/or dynamical
  interaction with the NAO

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Memory through interaction with the stratosphere
Source Ambaum and Hoskins (2002)
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Temporal behavior
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Temporal behavior of NAO

• Mostly a winter phenomenon
• Tendency that same sign repeats itself from
  winter to winter, even though different in the
  intervening seasons

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Runoff to Swedish Hydro-Power Magasin
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Source Hurrell et al. (2003)
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Spectra

• Very similar to a first order Auto Regressive
  AR(1) spectra (Red noise)
• Spectral peaks come and go with the time period
  of study, e.g. 8-10 yrs recently vs 2 yrs around
  1900
• Apparent trends similar to those observed during
  recent decades is consistent with AR(1) Wunsch
  (1999)

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The Ocean and NAO
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SST Pattern due to NAO-forcing
Source Rajagopalan (1998)
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SVD-MODES Z500 - SST
LAG 0 Weeks
LAG -2 Weeks
LAG 2 Weeks
Source Deser and Timlin 1997
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SST caused by sensible and latent heat fluxes
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SST and Z500Atlantic sector - Winter

• The Atmosphere leads the ocean
• ?t 2-3 Weeks
• Atmospheric pattern resembles the NAO
• SST pattern resembles the pattern obtained when
  regressing SST on NAO Index
• Anomalous Sensible and Latent heat fluxes related
  to NAO fluctuations cause a tripole
• SST-pattern

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Plausible Mechanism

• High-frequency stochastic forcing by the
  Atmosphere on the ocean mixed layer
• AR(1)-process ? Red noise spectrum
• Frankignoul and Hasselmann 1977

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Stochastic atmospheric forcing of the ocean mixed
layer
Atmospheric time scale
4-10 days
Relaxation time scale of ocean mixed layer
4- 8 months
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Source Deser and Timlin 1997
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Ensemble AMIP Integrations
r 21 / 41
DJF
Low-pass filtered T gt 6.5 Years
r 43 / 74
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Source Rodwell et al. 1999
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A Linear coupled ocean-Atmosphere model
All of the variability in the mid-latitude ocean
is explicitly driven by unpredictable atmospheric
variability
Source Bretherton and Battisti 2000
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Danger with AMIP type integrations

• Heat fluxes in the wrong direction
• Cause and effect can be misleading

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Physical Processes
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Physical Processes

• NAO exist with climatological SST
• NAO forces SST anomalies in the
  Atlantic

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Physical Processes

• NAO exist with climatological SST
• NAO forces SST anomalies in the
  Atlantic
• NAO exist without a stratosphere

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Physical Processes

• NAO exist with climatological SST
• NAO forces SST anomalies in the
  Atlantic
• NAO exist without a stratosphere

Basic mechanism is internal dynamics in the
extratropical troposphere
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Transient eddy fluxes alone drive variability in
the jets
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Forecast skill
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NAO vs other modes
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From Jae Schemms 5 year calibration data set
(NCEP-GFS model)
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Sometimes forecasts for NAO are
remarkablyaccurate!
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CFS
r43
r27
ECMWF
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The prediction of NAO in the GFS
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THE END THANK YOU TACK SÅ MYCKET !!