How do Long-Term Changes in the Stratosphere Affect the Troposphere? Mark P. Baldwin Northwest Research Associates, Seattle, WA, USA IAGA/ICMA/CAWSES Trends Workshop St. Petersburg, Russia 9-13 September 2008
Outline • Stratospheric vortex variability affects the troposphere and weather. • Ozone loss has affected Antarctic climate. • Will increasing GHGs produce a trend in polar vortex strength? • Will increasing GHGs affect the variability of the stratosphere? • Ozone will recover, reversing the trend from the past ~25 years.
From Bill Randel 44-58 km 34-51 km 25-44 km 18-37 km 13-22 km
What are “annular modes?” • They are patterns that maximize variance • Not exactly annular in the troposphere • Close to annular in the stratosphere • Similar to North Atlantic Oscillation • Arctic Oscillation (AO), Northern & Southern Annular Mode (NAM, SAM)
Northern Annular Mode (NAM) 10 hPa 1000 hPa (Arctic Oscillation) Annular mode patterns are similar from Earth’s surface to 50+km.
Composite surface maps for high and low AO index. (From Thompson and Wallace, Science 2001) >0.9°C
Southern Hemisphere surface climate response to ozone depletion • Springtime ozone loss appears to drive changes in surface climate from late spring to summer.
Simulated and observed geopotential height and temperature changes Model results from Gillett & Thompson, Science 2003
Tropospheric changes ←SAM pattern Model results from Gillett & Thompson, 2003
Weak Winds Strong Winds NAM index for 1998–1999. The lowest level is the AO index. From Baldwin and Dunkerton, Science 2001
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.
Time Delay Long timescale Baldwin and Dunkerton, 2001
Surface pressure anomalies after stratospheric events look like the Arctic Oscillation. Baldwin and Dunkerton, 2001
The Equatorial Quasi-Biennial Oscillation Baldwin et al., Reviews of Geophysics, 2001
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
The Future of the Stratosphere? • Increasing greenhouse gases cool the stratosphere. • Ozone recovery reverses cooling trends. • Stratospheric NAM index trend would depend on relative cooling of the polar cap. • Many (most?) models show a warmer, weaker NH vortex in winter.
How Will Climate Change Affect Ozone Recovery? • Colder stratosphere slows ozone-depleting reactions. • Colder stratosphere increases PSCs; more ozone loss in spring. • Ozone recovery will not be a simple reversal of ozone depletion.
How will stratospheric change affect surface climate? The answer depends on1) trends in the strength of the polar vortex2) future variability of the polar vortex
The Future of the Stratosphere? • Increasing greenhouse gases cool the stratosphere. • Ozone recovery reverses SH trends. • Stratospheric NAM index trend would depend on relative cooling of the polar cap. • Most models show a warmer, weaker NH vortex in winter and spring.
Summary • Changes to the circulation of the lower stratosphere affect surface climate. • The future evolution of the stratosphere will depend on increasing GHGs, ozone changes, and changes to the troposphere. • We do not know how the stratosphere will change in the coming decades and centuries, and we do not know how surface climate will be affected. • We do not fully understand the dynamics of stratosphere-troposphere coupling. email@example.com www.nwra.com/baldwin