1. Why Should We Care About the Stratosphere?
Mark P. Baldwin
Northwest Research Associates, Bellevue, WA USA
Department of Meteorology
University of Reading
16 July 2006 to 15 July 2007
WCRP Workshop on Seasonal Prediction
Barcelona, Spain, 4 June 2007

2. Overview
Variability in the stratosphere
The Arctic Oscillation and Annular Modes
Downward coupling to the troposphere
Effects of ozone loss on surface climate
Extended-range to seasonal prediction

6. Stratospheric Sudden Warming

8. Annular Modes

9. Northern Annular Mode (NAM).
The NAM can be defined at any pressure level.
Also called the Arctic Oscillation at 1000 hPa.

10. 60 days following sudden warmings
Baldwin and Dunkerton, 2001
60 days following sudden warmings

11. Composite surface maps for high and low AO index.
(From Thompson and Wallace, Science 2001)
>0.9°C

12. Leading EOFs of daily, zonally-averaged surface pressure.
Weak Winds
Leading EOFs of daily, zonally-averaged surface pressure.
From Baldwin (2001).

13. Effects of Ozone Loss on Southern Hemisphere Climate

14. Ozone Depletion

15. Southern Hemisphere surface climate response to ozone depletion
Observations and model
Springtime ozone loss appears to drive changes in surface climate from late spring to summer.

16. Simulated and observed geopotential height and temperature changes
Model results from Gillett & Thompson, Science 2003

17. Model results from Gillett & Thompson, 2003
Tropospheric changes
←Annular Mode pattern
Model results from Gillett & Thompson, 2003

18. Downward Coupling to the Troposphere

19. Leading EOFs of daily, zonally-averaged surface pressure.
Weak Winds
Leading EOFs of daily, zonally-averaged surface pressure.
From Baldwin (2001).

20. The lowest level is the AO index.
Weak Winds
Strong Winds
NAM index for 1998–1999.
The lowest level is the AO index.
From Baldwin and Dunkerton, 2001

21. Composites of strong and weak vortex events
Select strong events based only on the daily 10-hPa NAM index.
Form positive and negative composites.
Examine average behavior for the composites.

22. Time Delay
Long timescale
Baldwin and Dunkerton, 2001

23. 60 Days after Sudden Warmings
Surface pressure anomalies for 60 days after stratospheric sudden warmings look like the Arctic Oscillation.

24. 60 days following sudden warmings
Baldwin and Dunkerton, 2001
60 days following sudden warmings

25. Autocorrelation of daily surface AO index
(Norton, GRL, 2003)
Control Run
Normal Model
Model with damped stratosphere
Without stratospheric variability, the timescale of the surface AO is shorter.

26. In the troposphere the longest timescale
occurs during winter.
A diagnostic for seasonal prediction models?

28. Predict the Monthly-Mean AO after a 10-day lead
Persistence of the AO
Another seasonal forecast model diagnostic?

29. Possible Dynamical Mechanisms
Direct effect of stratospheric wave driving and stratospheric temperature anomalies
Indirect effects involving waves:
Effect on baroclinic waves/life cycles
Effect on planetary-scale waves
Wave reflection in the stratosphere

30. mark@nwra.com www.nwra.com/baldwin
Summary
Changes to the circulation of the lower stratosphere affect surface weather and climate.
Without well-represented stratospheres, seasonal prediction models will not capture stratospheric effects.
The definition of a “well-represented stratosphere” is not yet clear. But the model stratosphere has to have realistic variability.
A key diagnostic of seasonal prediction models could be the timescale of the annular modes.
We do not fully understand the dynamics of stratosphere- troposphere coupling.