1. Extra-tropical climate and the modelling of the stratosphere in coupled atmosphere ocean models. E Manzini Istituto Nazionale di Geofisica e Vulcanologia Centro Euro-Mediteraneo per i Cambiamenti Climatici, Bologna, Italy (email: manzini@bo.ingv.it)manzini@bo.ingv.it MA Giorgetta, M Esch and E Roeckner Max Planck Institute for Meteorology, Hamburg, Germany Thanks: C Cagnazzo, PG Fogli, E Scoccimarro (CMCC) MP Baldwin, PJ Kushner, and J Perlwitz

2. Motivation: To what extent we need to model the stratosphere for climate prediction on decadal time scales. Representation of the troposphere - stratosphere system in Climate Models. Representation of stratosphere - troposphere connections in Climate Models. “Climate Model”: Atmosphere - Ocean - Sea Ice Coupled Model.

3. Methodology: Inter-comparison of High Top and Low Top Models. Low Top Model: Standard Climate Model, such as those used in AR4 IPCC simulations High Top Model: Extended version of the Low Top, constructed to be as consistent as possible with the Low Top Model to (i)Minimize spin up shocks with the coupling to the ocean model. (ii)Reduce spurious causes of difference (parameter settings).

4. Low Top Model (surface to 10 hPa): ECHAM5/MPIOM (T63L31): 5 levels from 100 hPa to 10 hPa High Top Model (surface to 0.01 hPa): MAECHAM5/MPIOM (T63L47): 21 levels from 100 hPa to 0.01 hPa (9 levels from 100 hPa to 10 hPa). 1000 hPa to 100 hPa: 26 levels in each model (identical grid). Differences in the vertical grids occur only in the atmosphere above 100 hPa: The vertical resolution is almost doubled in MAECHAM5 with respect to ECHAM5 between 100 and 10 hPa. Identical horizontal resolution; Identical coupling to ocean and ice models; Identical setting for clouds, convection, and orographic gravity wave drag parameterization.

5. However, some differences in the design of High and Top Atmospheric Models remains: Local dissipation within the 10-100 hPa layer, generally larger in the low top models (“surrogate” stratosphere). Inclusion of processes relevant to the new part of the atmosphere considered. => Momentum flux deposition from mesospheric gravity wave breaking. (but there can be more)

6. Simulations: ECHAM5/MPIOM: 100 years from a control simulation for the IPCC AR4 set (total length of the simulation: few 100 years) MAECHAM5/MPIOM: 100 years from a simulation started with the ocean state from the above, spin up excluded (60 years) Pre-Industrial Conditions (GHGs, Aerosols, etc)

7. High Top - Low Top Models DJF Zonal Mean TemperatureDJF Zonal Mean Zonal Wind Difference in mean climate: Difference between 100 year means Boville (1984) U changes: 50 m/s at 10 hPa and 20 m/s at 100 hPa => Here we are looking at effects of substantially smaller changes in the lower and middle stratosphere

8. Low Top Model - ERA40 High Top Model - ERA40 Zonal Mean Temperature

9. High Top - Low Top Models DJF Sea Level Pressure Difference in mean climate: Difference between 100 year means DJF Sea Level Pressure for the Low Top Model

10. High Top - Low Top Models DJF Surface Air TemperatureDJF Sea Ice Concentration 20-90 N50-90 N Local Difference: Temperature - Sea Ice Coupling Difference in mean climate: Difference between 100 year means

11. Tropospheric Extra Tropical Variability and its Connection to the Stratosphere: Following the Baldwin and Dunkerton (2001) Approach: Weak Vortex and Strong Vortex Composites for SLP (Sea Level Pressure) and SAT (Surface Air Temperature) Selection criteria: Daily NAM index less than -2 Std (Weak Vortex Composite) Daily NAM index more than 1.5 Std (Strong Vortex Composite) (average over days 10 to 60 after the threshold is passed) Daily NAM index (10 hPa): projection of the daily gopotential anomalies onto the leading low frequency EOF pattern for NDJFMA (November to April). Applied to the Low-Top and High-Top Coupled Models

12. High TopLow Top High Top (left) and Low Top (right) Models Leading mode of variability from low pass filtered Nov to Apr (daily) Geopotential at 10 hPa from ~40-year time series Period 1 Period 2 49%43% 46% 47%

13. Warm colors: negative High Top (top) and Low Top (low) Models Days from Nov 1 High Top Low Top Daily NAM index For selected years of the Period 1 time series (large neg NAM Index at 90)

14. High Top (top) and Low Top (low) Models Days from Nov 1 High Top Low Top Zonal Mean Zonal Wind For selected years of the Period 1 time series (large neg NAM Index at 90)

15. 40 years (Period 1) Sea Level Pressure Composites for the Weak Vortex Case High Top Model Baldwin and Dunkerton 2001

16. Sea Level Pressure Composites for the Strong Vortex Case High Top Model 40 years (Period 1)

17. 40 years (Period 2) High Top Climate Model Sea Level Pressure Composites: Weak VortexStrong Vortex

18. 80 years (Period 1 and 2) Weak VortexStrong Vortex High Top Climate Model Sea Level Pressure Composites:

19. 80 years (left) 70 years (right) Weak Vortex Sea Level Pressure Composites High Top ModelLow Top Model

20. 80 years (left) 70 years (right) Strong Vortex Sea Level Pressure Composites High Top ModelLow Top Model

21. 80 years (left) 70 years (right) Weak Vortex Surface Air Temperature Composites High Top ModelLow Top Model

22. 80 years (left) 70 years (right) Strong Vortex Surface Air Temperature Composites High Top ModelLow Top Model

23. Autocorrelation function for DJF daily NAM index High Top Model Low Top Model 10 hPa: Longer decorrelation time for the High Top Model Time [day]

24. Conclusions and Outlook Mean Climate: Differences between the High and Low Top Climate Models (atmosphere ocean coupled models) less severe than in “idealized” comparisons with degraded models of the stratosphere. => Model sensitivity per a unit change (in stratospheric polar temperature, for instance) Tropospheric Extra Tropical Variability and its Connection to the Stratosphere: Issues related to the selection criteria (normalized indexes versus background zonal winds): The range of the internal variability of the models is different. Issues related to the time period considered: Multi-decadal variations in the processes behind the stratosphere-troposphere connection? Low Top Model: Connection to North Pacific SLP. Realistic? High Top Model: More promising, although a weak connection. Missing sources of Variability: QBO, no external forcing.