AGU Fire Poster

1
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)t0 gt 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 t0
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
  precondition the 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.