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Interannual variability of the stratospheric wave driving during northern winter

Interannual variability of the stratospheric wave driving during northern winter. Alwin J. Haklander 1,2 Supervisor: dr. P.C. Siegmund 2 Promotor: prof. dr. H.M. Kelder 1,2 1 Eindhoven University of Technology (TU/e) 2 Royal Netherlands Meteorological Office (KNMI)

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Interannual variability of the stratospheric wave driving during northern winter

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  1. Interannual variability of the stratospheric wave driving during northern winter Alwin J. Haklander 1,2 Supervisor: dr. P.C. Siegmund 2 Promotor: prof. dr. H.M. Kelder 1,2 1 Eindhoven University of Technology (TU/e) 2 Royal Netherlands Meteorological Office (KNMI) Chapman Conference on RSCCC Santorini, Greece, Thu 27 Sep 2007

  2. Intro • Why is the NH midwinter wave driving relevant? • How is it quantified? • Which factors determine the observed interannual variability?“Fundamental causes for interannual variability poorly understood” (2006 WMO/UNEP Ozone assessment, SPARC Newsletter 29, July 2007)

  3. Why is the NH midwinter wave driving relevant? • Planetary waves mainly drive Brewer-Dobson circulation (BDC) • BDC draws ozone-rich air poleward from the Tropics • Ozone is long lived in polar night, so little loss there • Meanwhile, new ozone production in the Tropics by the adjustment towards photochemical equilibrium • Therefore, BDC causes net ozone production in NH winter (Fusco&Salby,1999) • Adiabatic compression of air in downward branch at the pole yields higher early-spring temperatures → less PSC’s

  4. Why is the NH midwinter wave driving relevant? • Strong correlation January wave driving at 100 hPa with extratropical ozone column increase (Fusco & Salby, 1999) Wave driving ~ Extratropical ozone increase (Ozone increase) (Wave driving)

  5. Why is the NH midwinter wave driving relevant? • Strong correlation Jan-Feb wave driving at 100 hPa with Feb-Mar mean T poleward of 60°N at 50 hPa (Newman, 2005) Wave driving ~ Adiabatic heating at high latitudes (Wave driving)

  6. How is the wave driving quantified? • Fz = net zonal-mean upward flux of wave activity: Fz(upward component Eliassen-Palm flux F) • Fz is proportional to zonal-mean poleward eddy heat flux [v*T*], with v southerly wind component T temperature [] zonal mean* deviation from zonal mean • January-February average of [v*T*] over 40-80°N at 100 hPa (~16 km, lower stratosphere) good measure of total midwinter wave driving in NH (Austin et al. 2003)

  7. How is the wave driving quantified? • We define H100 as the January-February average of [v*T*] over 40-80°N at 100 hPa(following, e.g., Austin et al., 2003; Newman, 2005) • ERA-40 reanalysis data for 1979-2002 2.5° × 2.5° lat-lon grid 6-hourly wind and temperature fields 23 levels between 1000 and 1 hPa

  8. How is the wave driving quantified? • We define H100 as the January-February average of [v*T*] over 40-80°N at 100 hPa

  9. How is the wave driving quantified? • H100 ranges between 11.2 and 19.2 K m/s • Zonal waves 1-3 account for >90% of H100 • Zonal wave 1 dominates the interannual variability

  10. Which factors determinethe observed interannual variability in H100? • Variable planetary-wave spectrum at 100 hPaPerform linear regression analyses of H100 with different wave components constituting H100 Regression coefficients The sum of regression coefficients for all wave components is 1.

  11. Which factors determinethe observed interannual variability in H100? → Zonal-wave 1 dominates the interannual variability of the total wave driving, primarily its stationary component → Waves 1+2 account for about 85% of the interannual variability (r=0.88) s = zonal wavenumber

  12. Which factors determinethe observed interannual variability in H100? • Variable tropospheric wave sourceCorrelation coefficients of H100 with [v*T*] averaged over 20-90°N not significant in the troposphere.→Total tropospheric wave source probably not a very important factor 95% significance level [v*T*] poorly defined at lowermost levels due to extrapolation below the Earth’s surface

  13. Which factors determinethe observed interannual variability in H100? → Fig. a) Only wave 1,2 components of H100 significantly correlated with the same component of the upward wave activity flux in the troposphere → Fig. b) Wave 4 preferred mode for breaking of very long waves in upper stratosphere s = zonal wavenumber

  14. Which factors determinethe observed interannual variability in H100? So far… → Most of the interannual variability of the total wave driving at 100 hPa can be attributed to stationary wave 1, about 85% to wave 1+2 (r=0.88) →Total heat flux at 100 hPa not significantly correlated with the total heat flux in the troposphere → However, a significant coupling between 100 hPa and the troposphere does exist for the separate wave-1 and wave-2 components → The variability of the total wave driving at 100 hPa cannot be significantly attributed to the separate wave-1 and wave-2 components

  15. Which factors determinethe observed interannual variability in H100? Further focus… → Q: What does this coupling look like in the meridional plane? Wave 1+2 dominate wave driving at 100 hPa, both have separate coupling with troposphere

  16. Wave activity is refracted towards higher index of refraction • Stationary wave-1 correlation maximum ~ coincides with climatological wave guide

  17. Conclusions → Most of the interannual variability of the total wave driving at 100 hPa can be attributed to stationary wave 1, about 85% to wave 1+2 (r=0.88) →Total heat flux at 100 hPa not significantly correlated with the total heat flux in the troposphere → A significant coupling exists between 100 hPa and the troposphere for the separate stationary wave-1 and stationary wave-2 components → Location of tropospheric stationary-wave 1 source significantly affects its contribution to wave driving.

  18. Way forward… → Analyze the effect of CO2 doubling on the wave driving with the MA-ECHAM4 GCM, by comparing a 30-yr doubled CO2 run with a 30-yr control run → Preliminary results: Highly significant increase of the wave driving (>12%) in a doubled CO2 climate: ~25% increase in stationary wave-1 component,~100% increase in transient wave-5 component. Stronger heat flux associated with larger zonal temperature perturbations in the lower stratosphere. Presented work recently published in ACP: Haklander, A.J., P.C. Siegmund and H.M. Kelder,2007: Interannual variability of the stratospheric wave driving during northern winter. Atmos. Chem. Phys., 7, 2575-2584.

  19. Questions?

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