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 -