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Overview

Overview. Aim of the Study: To establish those aspects of predicted stratospheric climate and circulation changes that are robust i.e. model independent Basic climate parameters: temperature, wind, water vapour Brewer Dobson circulation (age of air), wave driving Model Simulations

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Overview

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  1. Overview • Aim of the Study: • To establish those aspects of predicted stratospheric climate and circulation changes that are robust i.e. model independent • Basic climate parameters: temperature, wind, water vapour • Brewer Dobson circulation (age of air), wave driving • Model Simulations • Transient CCM simulations of the past and future were performed to support the WMO/UNEP Ozone assessment 2006 • The simulations used nearly identical forcings and experimental set up to eliminate many of the uncertainties in the conclusions of previous assessments • Eyring et al. 2006 “Assessment of temperature, trace species and ozone in chemistry-climate model simulations of the recent past“ • Eyring et al. 2007 “Multi-model projections of ozone in the 21st century” • http://www.pa.op.dlr.de/CCMVal/

  2. Neal Butchart (Met Office) Veronika Eyring (DLR), Eugene Cordero (SJSU), Darryn Waugh (JHU) and the SPARC CCMVal Team AGU Chapman Conference “The role of the stratosphere in climate and Climate Change” 24-28 September 2007, Santorini, Greece Stratospheric climate and circulation changes in the CCM simulations used for the 2006 WMO/UNEP ozone assessment

  3. Participating Models (13 of 19 CCMs)

  4. CCMVal Reference Simulations supporting WMO (2007)

  5. Prescribed forcings in CCMVal Simulations WMO/UNEP 2002 Ab IPCC A1B(medium)

  6. SSTs and REF2 simulations

  7. Ozone Forcing Global Total Ozone Anomalies Pressure (hPa) future past 0.1 1.0 10.0 100. 1000.0 Global Vertical Ozone Trends in ppmv/decade Pressure (hPa) -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.6 -0.4 -0.2 0.0 0.2 0.4 Eyring et al., JGR, 2007

  8. Temperature trends in the 21st century All the model predict a cooling of the global mean and equatorial stratosphere with good agreement between the models Also a cooling of the polar middle & upper stratosphere in both solstice seasons Arctic trends Antarctic trends Recovery of the ozone hole causes a warming of the Antarctic lower stratosphere in DJF 5 of the 11 models predict a near-zero trend in the Arctic lower stratosphere cooling in DJF

  9. Winds Most models indicate a westerly shift in the winds in the sub-tropical lower stratosphere in both hemispheres and in all seasons In nearly all the models recovery of Antarctica ozone produces an easterly acceleration in spring and (early) summer, most likely due to an earlier breakdown of the polar vortex

  10. Water Vapour Increases in water vapour over the polar regions in early winter are generally not large enough to lead to any enhancement of PSCs All models indicate an increase in the global and tropical annual mean water vapour – probably due to a warming of the tropical tropopause From Eyring et al., J. G R.. 2006

  11. Conclusions: Basic climate parameters • All models predict a cooling of the annual mean global and equatorial stratosphere with good agreement between models. • In the NH polar winter the temperature trend is near zero in the lower stratosphere in most models. • Recovery of the Antarctic ozone leads to a warming of the lower stratosphere in DJF in all but two of the runs though there is a wide spread between models. • Climate change produces a westerly shift in the sub-tropical lower stratospheric winds throughout the year. • Ozone recovery produces an easterly acceleration over Antarctica in SON and DJF. • Models predict a small increase in stratospheric water vapour.

  12. Brewer Dobson circulation and wave driving

  13. Brewer-Dobson circulation after J.R.Holton et al., Rev. Geophys. 1995 All the models have upwelling in the tropical stratosphere and downwelling in the extratropics. The upwelling extends 30° either side of the equator In response to climate change there is no change in the width of the upwelling region

  14. Projectedtropical upwelling mass fluxes @70 hPa • Most models predict an increase in the mass flux in the 21st century • Reasonable agreement between models

  15. Seasonality Annual mean Dec-Jan-Feb mean Jun-Jul-Aug mean Upwelling mass fluxes are up to 50% larger in DJF than JJA, apart from two models The increase occurs in all seasons Interannual variability is generally much smaller than the annual cycle

  16. Vertical profile of trend Trend in annual mean mass flux in per cent per decade • The trend in the mass flux occurs throughout the depth of the stratosphere • The largest percentage trends are in the lower and upper stratosphere

  17. Age of Air Corresponding to the increase in tropical upwelling there is a decrease in the age of air in the tropics Models with the weaker upwelling generally had older air WACCM From WMO (2007) Chapter 5

  18. Wave driving & downward control For steady conditions “downward control” gives the mass streamfunction in terms of the vertical integral of the zonal forces • EP-flux divergence • Gravity wave drag • Rayleigh friction where is the zonal forcing from: Vertical velocity w*=0 at Ψmax andΨmin 2πaΨ(Φ,z) is the net downward mass between the pole and latitude Φ TOTAL UPWARD MASS FLUX = 2πa (Ψmax – Ψmin)

  19. Mass fluxes derived from EP-flux divergence Mass fluxes derived from the EP-flux divergence using downward control can account for most of the actual upwellling mass flux The trend in the downward control mass fluxes is stronger during DJF than JJA

  20. Mass fluxes derived from Gravity Wave Drag The contribution from GWD to the upwelling mass flux varies considerably between models. There is no obvious relationship to model horizontal resolution The amplitude of the annual cycle is rather weak though there is much greater inter-annual variability in JJA. There is a positive trend in all the models in all seasons

  21. Contributions from wave driving • Mass flux • Downward control (EP-flux) mass flux • Downward control (GWD) mass flux 1 1 12 1 12 12 123 123 12 123 123 123 12 1

  22. Trend in upward EP-Flux Most models show an increase wave flux in the middle latitudes and a decrease in high latitudes Small trend in the latitudinal averaged flux • Mid-latitude trend larger in the NH than SH

  23. Wave Driving From REF1 1980-1999 Jan-Feb 100 hPa Heat Flux vs Feb-Mar 50 hPa Temperatures Northern Hemisphere Jul-Aug 100 hPa Heat Flux vs Aug-Sep 50 hPa Temperatures Southern Hemisphere From Eyring et al. 2006

  24. Heat Flux trend Southern Hemisphere Jul-Aug mean Northern Hemisphere Jan-Feb mean Trend in averaged heat flux between 40°-80° latitude From Chapter 5 of WMO (2007)

  25. Conclusions: Brewer Dobson circulation • A strengthening of the Brewer-Dobson circulation is a robust feature of model climate predictions. • The mass flux across the tropopause will increase by about 2% per decade due to climate change. • The tropical upwelling increases thoughout the depth of the stratosphere. • Corresponding to the increased upwelling there is reduction in the mean age of air in the tropical stratosphere. • Some of the inter-model differences in age-of-air can therefore be attributed to differences in circulation not “tracer advection.” • The increased upwelling results from increased wave driving mainly through the EP-divergence. • Causes of the increased wave driving are uncertain.

  26. Trend Upward EP-Flux

  27. Trends in mass flux at 70 hPa Downward control estimate Modeled • Trend is much larger during the Dec-Jan-Feb than Jun-Jul-Aug • The mass flux increases in all seasons • There is considerable spread between models with the smallest trends for models simulating past changes • The multi-model average trend in the annual mean mass flux is 11.0 ± 2.0 kt s-1 year-1 Mean Trend ~2% per decade

  28. Modelled streamfunction at 70 hPa From vertical velocity From downward control Shading denotes inter-annual variability

  29. Trend derived from resolved waveforcing Actual Trend Contribution to trend from wave driving On average increased wave driving accounts for 58% of the increase in the annual mean mass flux (74% in DJF and 53% in JJA) Downward control estimate Modeled ~40% of the increase in mass flux in the models is due to changes in gravity wave (unresolved wave) forcing (assuming downward control)

  30. Winds All but one model indicate a westerly shift in the winds in the sub-tropical lower stratosphere due to climate change Wave flux from troposphere Change in wave flux x10 In the Unified Model a westerly shift in the subtropical winds (green shading) enhanced the wave flux from the troposphere Increased wave driving and stronger Brewer-Dobson circulation From Butchart and Scaife, Nature 2001

  31. From Butchart and Scaife, Nature 2001 Age-of-air and lifetimes Predictions from WACCM (From WMO 2007, Chapter 5) Without climate change With climate change Effect of a 3% increase in mass flux on the lifetimes of CFC11 and CFC12 With 3% increase in mass flux

  32. Low frequency variability Smoothed (11-year running mean) Dec-Jan-Feb polar temperature anomalies UM49L Dec-Jan-Feb @ 10 hPa temperature Three runs of CMAM with identical climate forcings but different realisations of SSTs Two runs (a & b) of UM49L with identical climate forcings and SSTs From Butchart et al., J. Climate. 2000

  33. Impact of circulation and climate change on ozone recovery In the AMTRAC model ozone recovery occurs earlier in the NH than SH because of the greater increase in the Brewer-Dobson circulation. Adapted from Austin and Wilson, JGR 2006. Percentage change from 1980 amounts in near global total ozone from simulations with (solid lines) and without (dotted lines) climate change for 3 different models. (From WMO 2007, Chapter 5)

  34. Projectedtropical upwelling mass fluxes @70 hPa • Inter-annual variability much less than amplitude of the annual cycle • Reasonable agreement between models • All models predict an increase in the mass flux in the 21st century

  35. Wave driving & downward control Momentum equation: Continuity equation: • EP-flux divergence • Gravity wave drag • Rayleigh friction Where F is the zonal forcing from: For steady seasonal mean conditions ∂u/∂t=0 “DOWNWARD CONTROL”

  36. Eyring et al., BAMS, 2005 Mesosphere Photolysis and radiative heating 50 Tropics Meridional Circulation Upper Stratosphere Photochemically controlled Ozone Production Stratospheric response to wave driving Large-scale ascent 40 Vortex barrier Subtropical barrier Large-scale descent Height (km) 30 Lower Stratosphere Ozone controlled by both chemistry and transport Surf zone QBO Source gases enter the stratosphere Polar Vortex Cold trap Ozone Depletion via polar chemical processes 20 TTL ozone depleting substances UTLS Lowermost Stratosphere Complex chemistry and transport greenhouse gases 10 Troposphere Large scale descent and two-way stratosphere - troposphere exchange aerosols Forcing and propagation of planetary waves 0 90° 60° 30° 0° Polar Region Mid-Latitudes Tropics

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