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

ODSs Ozone

Northern Annular Mode (NAM)

Annular Modes – Latitudinal Profile

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) 1000 hPa (Arctic Oscillation) 10 hPa 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

Annular Mode Terminology Arctic Oscillation (AO): At or near Earth’s surface. North Atlantic Oscillation (NAO): Very similar to AO, but more oriented to the Atlantic sector. Northern Annular Mode (NAM): Same as the AO, but can describe higher levels in the atmosphere. The surface NAM is the Arctic Oscillation.

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

Ozone Depletion

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

Model results from Gillett & Thompson, 2003 Tropospheric changes ←SAM pattern Model results from Gillett & Thompson, 2003

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, 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

Storm tracks during weak and strong regimes

Baldwin and Dunkerton, 2001

The Quasi-Biennial Oscillation (QBO)

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.

IPCC: Altitude of the Model Top Stratopause Tropopause Number of levels Finally in relation to CO2 or greenhouse warming there is the vexatious question of the role of cloud. GO THROUGH To some extent the role of cloud depends on its height in the atmosphere. Low level cloud tends toward warming through long wave trapping, while high level cloud leads to cooling through solar radiation reflection In our final slide today, lets briefly look at surface characteristics. (IPCC slides from Eugene Cordero)

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 How will stratospheric change affect surface climate? The answer depends on 1) trends in the strength of the polar vortex 2) 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.

mark@nwra.com www.nwra.com/baldwin 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. mark@nwra.com www.nwra.com/baldwin