North Atlantic Oscillation Lecture Outline Development of Ideas Westerlies and waves in the westerlies North Atlantic Oscillation basic pattern impact on Northern Hemisphere Forcing of NAO

Hans Egede Saabye made the following observation in a diary which he kept in Greenland during the years :

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

Subtropics Poles Pressure Gradient Force

Vertical motion in Rossby Wave LOW Con Div LOW Vorticity maximum

Lecture Outline Westerlies and waves in the westerlies North Atlantic Oscillation basic pattern impact on Northern Hemisphere Forcing of NAO

North Atlantic Oscillation Pattern of Climate Variability that determines weather and climate in N.Atlantic Controls much of the month to month variability of temperature and rainfall in N.W. Europe

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

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

North Atlantic Oscillation High 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 Low Phase Azores high weak Icelandic Low shallow westerlies weak clear and still NW Europe cold and dry wet conditions in Med and N Africa US east coast cold outbreaks and snow Greenland mild conditions

Temperature

Rainfall

- NAO Negative NAOI

+ NAO Positive NAOI

U Component (East-West) + NAO - NAO

Lecture Outline Westerlies and waves in the westerlies North Atlantic Oscillation basic pattern impact on Northern Hemisphere Forcing of NAO

Internal variability in the atmosphere Ocean forcing Ocean-Atmosphere coupling Atmosphere – Ocean Stratospheric forcing Observational studies Modelling Studies

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 ). 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

SST Forcing Modelling Davies et al 1997 forced HADAM1 with observed SST NAO pattern similar to observed Rodwell et al 1999 Nature Observational Sutton and Allen 1997 Nature Hurrell

OCEAN - ATMOSPHERE INTERACTION Sensible and latent heat flux Sensible and latent heat flux wind stress

OCEAN - ATMOSPHERE INTERACTION Sensible and latent heat flux Sensible and latent heat flux wind stress

OCEAN - ATMOSPHERE INTERACTION Sensible and latent heat flux Sensible and latent heat flux wind stress

OCEAN - ATMOSPHERE INTERACTION: AGCM experiments Sensible and latent heat flux SSTs (and therefore heat fluxes to atmosphere) prescribed by observed SSTs BUT Heat fluxes and wind stress from atmosphere are ignored SST hypotheses

OCEAN - ATMOSPHERE INTERACTION: AGCM experiments Sensible and latent heat flux SSTs (and therefore heat fluxes to atmosphere) fixed by constant SSTs Any variability must be internal to atmosphere (i.e. chaos) Internal variability hypotheses

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:

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)

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.

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.

OCEAN - ATMOSPHERE INTERACTION Sensible and latent heat flux Sensible and latent heat flux wind stress

OCEAN - ATMOSPHERE INTERACTION: AOGCM Experiments Sensible and latent heat flux Sensible and latent heat flux wind stress Model calculates SSTs Wind stress heat fluxes O-A and A-O

OCEAN - ATMOSPHERE INTERACTION: AOGCM Experiments Sensible and latent heat flux Sensible and latent heat flux wind stress Hypotheses dependent on model behaviour

ATMOSPHERE - OCEAN INTERACTION: OGCM Experiments wind stress

SLP and Windstress As observed Observed land temps Modelled SSTs

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.

Stratospheric forcing contin 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

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: