The coupled stratosphere-troposphere response to impulsive forcing from the troposphere

1. The coupled stratosphere-troposphere response to impulsive forcing from the troposphere Thomas J. Reichler Geophysical Fluid Dynamics Laboratory / Princeton University, Princeton NJ Paul J. Kushner Department of Physics / University of Toronto, Toronto Lorenzo M. Polvani Dept. of Applied Physics and Applied Mathematics / Columbia University, New York NY

2. 3 z t 2 4 stratosphere 1 5 troposphere t0 t0+ t Stratosphere-troposphere coupling • Physical mechanisms for ST-coupling are largely unknown • Goal: Explore ST-coupling with a simple GCM • Approach: Stimulate interaction from the lower troposphere 1. Generate pulse of planetary waves 2. Waves propagate upward 3. Waves break in the stratosphere 4. Anomalies propagate downward 5. Tropospheric response • This setup is motivated by the observation that stratospheric anomalies are usually caused by planetary wave activity from the troposphere

3. Simple GCM after Polvani & Kushner 2002 • GFDL spectral dynamical core, T42 • 40 vertical levels, from troposphere to mesosphere • Simple dry “physics”: • Newtonian cooling to prescribed reference profile Teq Teq: troposphere: Held & Suarez 1994 stratosphere: cool polar cap for polar vortex > perpetual JAN • Rayleigh drag in PBL (p>700 hPa) and in sponge (p<0.5 hPa) • Zonally symmetric forcing, no ocean or continents • Simple problem, few tunable parameters

4. Shape : Temporal evolution T: 5000 m 10 days Perturbation experiments Each experiment is 100 days long Perturb lower boundary over a period of 10 days to create pulse of planetary waves Run out to day 100 and observe response Repeat many times (403) from different ICs from control run

5. Evolution of the response REM ensemble mean geopotential, polar cap averaged and normalized ? 0 • Ensemble mean shows no downward signal • Response to forcing is very variable • Need to classify responses

6. day 25-50: “Early” (38%) day 50-75: “Intermediate” (30%) day 75-100: “Late” (32%) Classification R-REM R • Based on time of maximum tropospheric R • Exclude case with: abs(AM)t=0>2/3 • Remain with 201 out of 401 • tropospheric response: ~0.8 SDEV, ~40 m, ~4 hPa

7. Weak and poleward shifted polar vortex Decreased lower stratospheric wave drag Composite initial conditions: “Late” meridional structure at t=0 u’ F’ thick contours indicate 10% of climatological standard deviation

8. Predicting “Late” cases R-REM • Based on t=0: I. Lower stratospheric wave drag: II. Upper stratospheric geopotential: I. + II.

9. c2 c1 = c2 c1 Dynamical Interpretation “Late” cases are favored by: Weak and poleward shifted polar vortex • more wave activity is absorbed at higher levels (“preconditioning”) • delayed tropospheric response Anomalously positive F in lower stratosphere • strengthening of westerlies and less wave activity absorption in the lower stratosphere

10. Rate of downward descent Rate of descent increases as thermal damping rate increase • from linear theory: c~ks (Dickinson 1968) • eddy driving is stronger if damping rate is stronger

11. Summary • We have used an externally imposed lower-tropospheric wave pulse in a simple GCM to stimulate stratosphere-troposphere interaction in a controlled and initial condition independent way. • This basic experimental setup can be modified in many ways for the investigation of stratosphere-troposphere coupling. • The response to the forcing is highly non-linear. • Downward propagating signals appear when cases are separated by the time of the tropospheric return signal. • The evolution of the response depends on the state of the stratosphere-troposphere system at the initial time. • The rate of downward descent is controlled by the thermal relaxation parameter.