Behavior of tropopause height and atmospheric temperature in models, reanalyzes, and
observations: Decadal changes
B. D. Santer, R. Sausen, T. M. Wigley, J. S. Boyle, K. AchutaRao, C. Doutriaux, J. E.
Hansen, G. A. Meehl, E. Roeckner, R. Ruedy, G. Schmidt, and K. E. Taylor
We examine changes in tropopause height, a variable that has hitherto been neglected in
climate change detection and attribution studies. The pressure of the lapse rate
tropopause, p_LRT, is diagnosed from reanalyzes and from integrations performed with
coupled and uncoupled climate models. In the National Centers for Environmental
Prediction (NCEP) reanalysis, global-mean p_LRT decreases by 2.16 hPa/decade over
1979-2000, indicating an increase in the height of the tropopause. The shorter European
center for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA) has a
global-mean p_LRT trend of -1.13 hPa/decade over 1979-1993. Simulated p_LRT trends
over the past several decades are consistent with reanalysis results. Superimposed on the
overall increase in tropopause height in models and reanalyzes are pronounced height
decreases following the eruptions of El Chichon and Pinatubo. Interpreting these
p_LRT results requires knowledge of both T(z), the initial atmospheric temperature
profile, and delta T(z), the change in this profiled in response to external forcing. T(z) has a
strong latitudinal dependence, as does delta T(z) for forcing by well-mixed greenhouse
gases and stratospheric ozone depletion. These dependencies help explain why overall
tropopause height increases in reanalyzes and observations are amplified toward the
poles. The pronounced increases in tropopause height in the climate change integrations
considered here indicate that even AGCMs with coarse vertical resolution can resolve
relatively small externally forced changes in tropopause height. The simulated decadal-
scaled change in p_LRT are primarily thermally driven and are an integrated measure of
the anthropogenically forced warming of the troposphere and cooling of the stratosphere.
Our algorithm for estimating p_LRT (based on a thermal definition of tropopause height)
is sufficiently sensitive to resolve these large-scale changes in atmospheric thermal
structure. Our results indicate that the simulated increase in tropopause height over
1979-1997 is a robust, zero-order response of the climate system to forcing by well-
mixed greenhouse gases and stratospheric ozone depletion. At the global-mean level, we
find agreement between the simulated decadal-scaled p_LRT changes and those estimated
from reanalyzes. While the agreement between simulated p_LRT changes and those in
NCEP is partly fortuitous (due to excessive stratospheric cooling in NCEP), it is also
driven by real pattern similarities. Our work illustrates that changes in tropopause height
may be a useful "fingerprint" of human effects on climate and are deserving of further
attention.
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