1 / 27

Can the Stratosphere Control the Extratropical Circulation Response to Surface Forcing?

Can the Stratosphere Control the Extratropical Circulation Response to Surface Forcing?. Chris Fletcher and Paul Kushner Atmospheric Physics Group University of Toronto Canada Judah Cohen AER Inc., Lexington, MA, USA.

sela
Télécharger la présentation

Can the Stratosphere Control the Extratropical Circulation Response to Surface Forcing?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Can the Stratosphere Control the Extratropical Circulation Response to Surface Forcing? Chris Fletcher and Paul Kushner Atmospheric Physics Group University of Toronto Canada Judah Cohen AER Inc., Lexington, MA, USA Thanks: Steven Hardiman (U Toronto), Michael Sigmond (UVic) and CFCAS funding AGU Chapman Conference, Santorini. September 27, 2007.

  2. In this talk I will: • Explain why we are still interested in Siberian snow cover • Show results from a large ensemble of transient simulations using a low top model forced with anomalous Siberian snow extent • Demonstrate the large variability in tropospheric responses to snow forcing • Try to convince you that, to better predict how the troposphere will respond to snow forcing, you should look at the initial state of the stratosphere rather than the troposphere • Conclude that it’s probably not just about snow

  3. Why are we interested in Siberian snow? • Snow forcing in autumn/fall is proposed as a source of seasonal predictability in the NH extratropics during winter • Dynamical mechanism involves wave-mean flow interaction in the stratosphere Cohen et al. [2007]

  4. Results from NCEP Reanalysis • Correlations ~0.5  suggests it’s unreliable • Stratospheric circulation anomalies can arise without a clear tropospheric precursor Cohen et al. [2007] Cohen et al. [2007]

  5. Results from NCEP Reanalysis • Correlations ~0.5  suggests it’s unreliable • Stratospheric circulation anomalies can arise without a clear tropospheric precursor Cohen et al. [2007] Cohen et al. [2007]

  6. Results from ECHAM3 low top AGCM Gong et al [2003] Cohen et al. [2007]

  7. Motivation • Reanalysis data are suggestive but not conclusive; r ~ 0.5 • Previous modeling results are from a small ensemble, an old AGCM and with no information about response variability Research Questions: Can snow really act as a precursor to strat-trop interaction? If so, are there conditions where interaction is more likely to occur? Cohen et al. [2007]

  8. GFDL AM2 low-top AGCM Full atmospheric GCM run in standard configuration for tropospheric seasonal/climate prediction: • Finite-volume dynamical core: 2o x 2.5o (lat x lon) • 24 vertical levels with lid at 1hPa; 4 above 100hPa • Rayleigh drag in top level sponge layer • Decorrelation timescales of NAM and geopotential height compare well with reanalyses

  9. Experimental Design • Set of 100 independent Oct 1st initial conditions from long pre-industrial control run: • Atmospheric composition = 1870 levels • Climatological SST / sea ice

  10. Experimental Design HIGH SNOW k Snow Mass for Ensemble Member k Initial Conditionk 40cm Siberia LOW SNOW k Time: Oct 1st . . . . . . . . . . . . . . . . . . . . . . .Dec 31st • Set of 100 independent Oct 1st initial conditions from long pre-industrial control run: • Atmospheric composition = 1870 levels • Climatological SST / sea ice • From each initial condition we fix snow mass at Oct 1st levels then run two new simulations Oct 1st - Dec 31st: (1) HIGH SNOW = Fixed Oct 1st snow + 40cm snow over Siberia (2) LOW SNOW = Fixed Oct 1st snow

  11. Experimental Design HIGH SNOW k Snow Mass for Ensemble Member k Initial Conditionk 40cm Siberia LOW SNOW k Time: Oct 1st . . . . . . . . . . . . . . . . . . . . . . .Dec 31st • Set of 100 independent Oct 1st initial conditions from long pre-industrial control run: • Atmospheric composition = 1870 levels • Climatological SST / sea ice • From each initial condition we fix snow mass at Oct 1st levels then run two new simulations Oct 1st - Dec 31st: (1) HIGH SNOW = Fixed Oct 1st snow + 40cm snow over Siberia (2) LOW SNOW = Fixed Oct 1st snow • Response is defined as <HIGH> minus <LOW>

  12. Surface Albedo Response Snow Perturbation Region (typical January extent) Albedo response ~0.4-0.5

  13. Early Surface Response to Snow Forcing Peak Cooling ~ 12 K in 2 weeks SLP maximum ~ 6 hPa

  14. Polar Cap Height Response 70 -25 15

  15. Polar Cap Height Response Response highly variable in the troposphere: 70 -25 15

  16. Can we Better Predict the Response? [Z] Initial Condition: 5 days before perturbation 48 -12 Strongest and most significant ‘predictor’ is located in the lower stratosphere Following Reichler et al. [2005]

  17. Can we Better Predict the Response? [Z] Initial Condition: 5 days before perturbation Initially WARM 115 Initially COLD 50 25 Strongest and most significant ‘predictor’ is located in the lower stratosphere Following Reichler et al. [2005]

  18. Strat-Trop Interaction Diagnostic d1-50: Find day of max upward ∆WAF at 50 hPa Wait 10 days, then record 30-day mean ∆SLP • Snow forcing begins Oct 1st, but strat-trop interaction is associated with WAF pulses whose timing is difficult to predict: • - Find strongest WAF pulses then look at lagged SLP response • - Does strat. initial condition influence interaction? (e.g. Polvani and Waugh [2004])

  19. 30-day Mean ∆SLP Following WAF Pulses Init. WARM: 18/51 Init. COLD: 20/49 (e.g. Polvani and Waugh [2004])

  20. Annular Mode Response in SLP Init WARM Init COLD (e.g. Polvani and Waugh [2004])

  21. Annular Mode Response in SLP Init WARM Init COLD p(NAM < -1.0): WARM = 0.18 COLD = 0.10 Subtle difference in NAM probabilities, but focused in areas of low seasonal predictability (e.g. East Coast US, Northern Europe).

  22. Conclusions • Can snow really act as a precursor to strat-trop interaction? • Response is highly variable but snow forcing does induce WAF pulses, causing strat. and trop. warming responses • Are there conditions where interaction is more likely to occur? • Initial condition in polar strat. provides useful predictor of trop. response to snow forcing (better than trop. predictor) • Similar freq of interaction in “Init WARM” and “Init COLD” cases • “Init WARM”: interaction events produce more systematic downward prop and negative NAM response • Is this really about the snow? • No. Results should also apply to SST/sea ice forcing

  23. Conclusions • Can snow really act as a precursor to strat-trop interaction? • Response is highly variable but snow forcing does induce WAF pulses, causing strat. and trop. warming responses • Are there conditions where interaction is more likely to occur? • Initial condition in polar strat. provides useful predictor of trop. response to snow forcing (better than trop. predictor) • Similar freq of interaction in “Init WARM” and “Init COLD” cases • “Init WARM”: interaction events produce more systematic downward prop and negative NAM response • Is this really about the snow? • No. Results should also apply to SST/sea ice forcing • Results suggest a dynamical mechanism • Limitations: strat. resolution/drag scheme, persistence? • Ongoing: do we observe the same behavior in a strat-resolving model? Initial findings: confusing… Fletcher, C. G. et al. (2007) Geophys. Res. Let., In Press. Preprint: http://www.atmosp.physics.utoronto.ca/people/cgf/

  24. The end.

  25. Northern Annular Mode in SLP AM2 Reanalysis Source: NCEP/CPC

  26. Zonal mean climatologies AM2-strat. ERA-40 AM2-trop.

More Related