6. Independent test

1. R-REM R EARLY INTERM. LATE The coupled stratosphere-troposphere response to impulsive forcing from the troposphere Thomas Reichler1, Paul J. Kushner2, Lorenzo M. Polvani3 1GFDL/Princeton University (NJ), 2University of Toronto (CA), 3Columbia University (NY) SPARC 2004, Victoria, British Columbia, Canada Princeton University 6. Independent test From the control run we select 6 independent additional ICs, which most strongly satisfy the LATE criteria, and make perturbation experiments. 4. Classification First, we exclude cases with abs(AM)t=0 > 2/3 and remain with 201 out of 403 cases. Based on the timing of the tropospheric return signal, we then classify the experiments into EARLY, INTER-MEDIATE and LATE cases. 1. Objectives Investigate the life cycle of stratosphere-troposphere interaction in a simple GCM by stimulating a wave activity pulse from the lower troposphere. Z’ u’ R experiments R control 2. Experimental design We use a relatively simple model of the stratosphere-troposphere system (Polvani and Kushner 2002) and perturb the lower boundary for 10 days. Then, we run out to day 100 and observe the response. This experiment is repeated 403 times using independent ICs i 0 50 100 0 50 100 5. Composite ICs meridional structure at t=0 3. Mean response EARLY LATE Initial states that strongly satisfy the LATE criteria respond in the expected way. Rindividual Z’ 7. Dynamical interpretation • The initial weak upper stratospheric vortex causes more wave activity to be absorbed at higher levels, which delays the tropospheric response. • The initial EP-flux signature indicates that wave activity just propagated out of the lower into the upper stratosphere, which helps to preconditionthe polar vortex. REM u’ F’ The mean response REM does not show a distinct downward propagating signal. This is because the individual outcomes are highly variable. Therefore, we have to objectively classify the response types to understand their variability. LATE cases are favored by a weak and poleward shifted polar vortex and by decreased lower stratospheric wave drag.