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

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

Stratospheric Sudden Warming

Annular Modes

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

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

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

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

Effects of Ozone Loss on Southern Hemisphere Climate

Ozone Depletion

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.

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

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

Downward Coupling to the Troposphere

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

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

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

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

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

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.

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

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

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

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