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Atmospheric Teleconnections and Climate Change
Mike BlackburnNational Centre for Atmospheric Science,
University of Reading
Talk for Lighthill Research Network 2 May 2008
© University of Reading 2008
Teleconnections - Outline
Well known examples: El Niño, North Atlantic Oscillation
Processes – atmospheric response to heating – wave propagation
Spatial correlations: linked regions of variability; patterns of variability
Downstream developments on jet
Tropical convection – MJO
Climate change: potential methodology; ENSO, NAO examples
El Niño and La Niña
NOAA, Climate Prediction Center
(global teleconnections)
El Niño impacts (northern winter)
NOAA, Climate Prediction Center
La Niña impacts (northern winter)
El Niño / La Niña
Changes in the location of tropical clouds over the Pacific
David Neelin, UCLA
• Atmospheric response during El Niño (schematic)
• Pressure and flow in the upper troposphere
• La Niña - opposite sign to a first approximation, but weaker over N. America
Propagation of Rossby wavesfrom a region of tropical convection (schematic)
Trenberth et al. (1998)Horel and Wallace (1981)
Propagation of Rossby waves
Model response to steady heating DJF Meridional wind ~250hPa
Ambrizzi and Hoskins (1997)
Climatological winds (DJF)
• Stationary phase (trough/ridge)
• fast eastward group propagation
After 10 days
32ºN
Propagation of Rossby waves
Model response to steady heating DJF Meridional wind ~250hPa
Ambrizzi and Hoskins (1997)
• Model gives good agreement with theory
• Great circle paths + refraction by the climatological winds
• Jet streams are preferential paths for propagation (waveguides)
After 15 daysAfter 9 days
Taxonomy of Teleconnections
50 years of daily global atmospheric analyses
One-point correlation maps → “centres of action”, associated patterns of variability
Originally by Wallace & Gutzler (1981): correlations of monthly mean 500hPa height
Figure 3.26
IPCC: from Hurrell et al (2003)
PNA
Pacific North-American pattern
NAO
North Atlantic Oscillation
*
*
From http://www.ldeo.columbia.edu/NAO by Martin Visbeck
NAO+ NAO-
The North Atlantic Oscillation(an example of regional teleconnections)
Sea level pressure pattern Dec-March
Hurrell (1995), Science
Winter NAO index based on Portugal – Iceland pressure difference
Now associated with breaking Rossby waves
The North Atlantic Oscillation
Summer 2007 UK floods - jet stream
- strength of wind at 250hPa -
Average from 12 June to 25 July
Climatology 2007
• Persistent pattern of waves on the jet stream – trough (low pressure) over UK
• Jet “joins up” over Europe – possible continuous waveguide & stationary free wave
(ms-1)
Summer 2007 UK floods
(potential temperature on the tropopause)
• Air moving equatorwards from the cold polar reservoir becomes cyclonic
• Repeated pattern of waves associated with each flooding event
• Slow-moving cyclonic anomalies over UK, forcing air to ascend and rain
15 June 25 June 20 July
THORPEX International Science Plan
Impacts of severe weather associated with four Rossby wave-trains that encircled the globe during November 2002
Global tropopause trough-ridge pattern (Rossby Waves)
Alan Thorpe
Time/longitude diagram 250hPa meridional wind (ms-1); 35-60ºN 6-28 November 2002
Hovmöller diagram of 250-mb meridional wind component (ms-1) 28 July -14 August 2002 (40-60ºN)
On 1 August 2002, a Rossby wave train was excited by cyclogenesis east of Japan, followed by rapid downstream development of high-amplitude Rossby waves, culminating in severe flooding in Central Europe on 11 August 2002.
Mel Shapiro, NOAA
Prague
Madden-Julian Oscillation (MJO)
Main mode of intraseasonal variability in tropical convection and rainfall
Rainfall
Composite life-cycle
(DJF)
mm/day
Adrian Matthews, University of East Anglia
MJO teleconnections (1)
Acts as a moving source of extra-tropical wave-trains
cloudy clear
Source: U.S. CLIVAR
MJO teleconnections (2)
Modulation of Atlantic hurricane activity in Summer, especially strongest (cat. 3-5)
Source: U.S. CLIVAR
Madden-Julian Oscillation (MJO)
Main mode of intraseasonal variability in tropical convection and rainfall
Propagates eastwards from Indian Ocean to west Pacific
● time lagged correlations along the equator
● a moving source of waves propagating into mid-latitudes
Modulation of the Asian Summer monsoon (active/break cycle)
Poorly simulated in models (latest ECMWF model much improved)
Potential for extended predictability in both tropics and extra-tropics (few weeks)
Teleconnections and climate change
Increasing analysis of variability in models (IPCC, AR4)
El Niño Southern Oscillation (ENSO)
NAO and Northern Annular Mode (NAM)
Other possible changes:
• jet stream waveguide characteristics; triggering
More understanding of present day variability is needed to underpin this
• e.g. apparent phase locking; persistence of anomalies
Assess simulations of present day variability and teleconnections
Figure 10.16
Oldenborgh et al (2005)
in IPCC, AR4 (2007)
Climate Change - El Niño
• Weak tendency for change in mean Pacific state to be “El Niño like”
• No agreement on El Niño variability
• Weaker teleconnections over N. America
Climate Change - NAO
Figure 10.17
Average change projects onto Northern Annular Mode (NAM)
Recent variability not captured (NAM)
Caveat on methodology
Climate Change – Annular Indices
Miller et al (2006)
in IPCC, AR4 (2007)
Teleconnections - Conclusions
Remote effects of modes of variability, e.g. El Niño, NAO
Wave propagation – tropics; great circles; jet streams as waveguides
Downstream developments on the jet stream:
• examples of linked weather, e.g. November 2002, August 2002
• triggering: storm growth; tropical cyclone transitions
• persistent anomalies, e.g. Summer 2007 (Autumn 2000)
Tropical convection – MJO:
• eastward propagation - time lagged correlations in tropics
• moving source of waves for extra-tropics (predictability)
Climate change:
• uncertainty, even for El Niño and NAO
• potential methodology
- End -