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

 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 Stratosphere-troposphere coupling z t troposphere stratosphere t0t0 t 0 +  t

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 T eq T eq : 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

Perturbation experiments 1.Each experiment is 100 days long 2.Perturb lower boundary over a period of 10 days to create pulse of planetary waves 3.Run out to day 100 and observe response 4.Repeat many times (403) from different ICs from control run Shape  : Temporal evolution T: 10 days 5000 m

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

Classification day : “Late” (32%) R day 25-50: “Early” (38%) day 50-75: “Intermediate” (30%) R-R EM  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

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

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

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 c1c1 c2c2 c 1 = c 2

Rate of downward descent Rate of descent increases as thermal damping rate increase  from linear theory: c~k s (Dickinson 1968)  eddy driving is stronger if damping rate is stronger

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.