Can we trust the simulated gravity-wave response to climate change?

1. NCAR TIIMES Gravity-Wave Retreat, 2006 Can we trust the simulated gravity-wave response to climate change? Ted Shepherd Department of Physics University of Toronto

2. GW parameterizations are highly tuned to reproduce current climate • So why should we trust their response to climate change? • There are two issues: • Changes in source characteristics • Changes in propagation and dissipation • This talk will only address the latter • Will focus primarily on polar vortex

3. Impact of parameterized mesospheric GW drag on downwelling and temperature over the winter pole in a zonal mean model Dashed line is without GW drag, solid line is with GW drag From Garcia & Boville (1994 JAS)

4. Cumulative contribution of resolved and parameterized wave drag at various altitudes on polar downwelling at 10 hPa in CMAM with only orographic GW drag Parameterized Resolved Total From Beagley et al. (1997 Atmos.-Ocean)

5. Wave driving vs polar temperature in the Antarctic Heat flux at 100 hPa estimates the (resolved) wave activity entering the stratosphere More wave forcing implies more polar downwelling and a warmer pole Differences reflect GW drag From Austin et al. (2003 ACP)

6. Holton (1983 JAS) explained the mesospheric cooling observed above stratospheric sudden warmings as due to a GW feedback • Filtering of GW momentum fluxes leads to a positive wave drag anomaly • Same reasoning applies to climate perturbations, e.g. to the ozone hole • How robust is this effect? Shaw & Shepherd (JAS, in press)

7. Response of downwelling over SH polar cap to combined effects of climate change and ozone depletion • Solid line shows October, dashed shows November • Left is total downwelling, right only from resolved EPFD From Manzini et al. (2003 JGR)

8. There is a strong constraint from (angular) momentum conservation • In the steady limit, downwelling is constrained by “downward control” (Haynes et al. 1991 JAS) [F is force/unit mass] • For GWs, this simplifies to (assuming no flux of momentum to space)

9. Thus the downwelling at a given height is independent of exactly where the waves break above that height • What goes up must come down • But what happens at the model lid? • If any momentum flux remaining at the model lid is thrown away, then which now depends on model lid height

10. To conserve momentum, any remaining momentum flux at the model lid must be deposited as a drag, e.g. in the top few levels of the model • This maintains the integrity of the downward control relation • Throwing away momentum flux is equivalent to imposing an opposite drag above the model lid • Also, there must be no Rayleigh drag or zonal mean sponge layer (Shepherd et al. 1996 JGR)

11. GW feedbacks to a radiative perturbation Rayleigh drag (violates momentum constraint) Physically consistent From Shepherd & Shaw (2004 JAS)

12. Difference between 80-km and 96-km lids with Hines GWD in Met Office UM (letting momentum flux at model lid escape to space) • Influence extends to low altitudes From Lawrence (1997 JGR)

13. Top: ratio of downwelling in 96-km model from below 80 km to below 96 km • Bottom: ratio of downwelling in 80-km model to that in 96-km model From Lawrence (1997 JGR)

14. The effect of a background jet on an anti-symmetric source spectrum is to create a dipole of negative drag above positive drag, hence polar downwelling

15. Imposing a polar cooling shifts each part of the drag dipole, so the difference drag is composed of two dipoles, driving two circulation cells (left) Rayleigh drag gives a single-signed response (unphysical)

16. Circulation response to polar cooling at ~15 km AD99 Non-MC AD99 • Enforcing momentum conservation can improve the robustness of GWD feedback to polar cooling MC AD99 Low lid MC AD99 + RD

17. Vertical profile of downwelling in response to polar cooling around 15 km, with AD99 GWD scheme • Dashed is 80°N, solid is 85°N • a=control • cf. Garcia & Boville (JAS, 1994) • c=MC AD99 • Physical response is significant • e=non-MC AD99 (50 km lid) • Spurious response is also significant

18. Tropospheric circulation Downwelling at 25 km, 85°N • Sensitivity of AD99 and H97 induced downwelling to model lid height MC Actual (solid) Inferred from downward control (dashed) non-MC (solid)

19. Sensitivity to the source spectrum Anti-symmetric Asymmetric Resting state With polar cooling Difference

20. Conclusion: GW induced warming above an imposed polar cooling is robust to • Model lid height • Source spectrum • Breaking criterion • Background flow if any only if momentum is conserved

21. Zonal mean wind at SH midlatitudes in CMAM and in observations • GW drag doesn’t just slow the mesospheric jet, it reverses it above about 90 km altitude (so isn’t really a “drag”) • Requires non-zero GW phase speeds From Beagley et al. (2000 GRL)

22. Doubled CO2 simulations with the CMAM (note no heterogeneous chemistry in these runs) • We separate the effect of doubled CO2 from that of the associated change in SSTs (taking SSTs from CCCma coupled atmosphere-ocean run) • The combined response is surprisingly linear • Figure shows temperature change in January (blue is 99% significant, purple 90%) Total response From 2xCO2 From SSTs Fomichev et al. (JC, 2006)

23. There is a robust dynamical temperature response at the summer mesopause • Tropospherically induced dynamical changes negate the CO2-induced cooling • From gravity-wave drag • Consistent with the lack of a cooling trend in observations Fomichev et al. (JC, 2006)

24. Summary • There are some robust aspects to the GW response to climate change (assuming fixed source spectra) • Based on filtering of GW fluxes • Robustness depends on enforcing momentum conservation • Without momentum conservation, model intercomparisons will be ill-posed

25. However there is a robust response in the lower tropical stratosphere • Tropospherically induced changes now augment the CO2-induced cooling • Increased upwelling from stratospheric wave drag (in both NH and SH) Fomichev et al. (JC, in revision)

26. The annual cycle of tropical and extratropical 50 hPa temperature (global mean is subtracted) points to a strengthened diabatic circulation 2xCO2+SST Extratropics Control Tropics 2xCO2+SST Fomichev et al. (JC, in revision)

27. Changes to tropical upwelling at 70 hPa • Black from resolved EPFD, gray total • Half these models use Rayleigh drag From Butchart et al. (CD, in press)