Chemistry-climate models nearly universally predict a
strengthening of the Brewer-Dobson circulation of the stratosphere with
increasing greenhouse gases. Detection of the impact of such a circulation
change has proven difficult, but the reported negative trend in stratospheric
ozone in a narrow band above the tropical tropopause deduced from SAGE data is
qualitatively consistent with the predicted impact of a circulation change.
Unfortunately, SAGE data has limited sampling, the sampling decreases starting
in 2001, and the record ends in 2005. This proposal revisits the issue of the
profile of tropical ozone trends and their potential relationship to changes in
the Brewer-Dobson circulation. Specifically, we will:
1) Examine time-series analysis of the SAGE data record with an emphasis on
uncertainties and interpretation. Preliminary results indicate that the time
series of ozone measurements at 75 hPa, averaged over 15oS-15oN is consistent
with a step-function drop during 2000. This is exactly the time when SAGE went
from sampling both sunrise and sunset to sampling either sunrise or sunset. It
is also the time when a rapid change in tropical lower stratospheric water vapor
was recorded by HALOE measurements on UARS.
2) Analyze the spatial and temporal structures in MLS data from Aura and
ozonesondes in the SHADOZ network in order to extend the SAGE data record for
tropical ozone. MLS data begin in late 2004 and extend through the present time
with good spatial coverage. SHADOZ data have high vertical resolution and
extend from 1998 to the present in a few locations. We will apply time series
analysis to the extended data set to test for the importance of various
influences on ozone variability in the lower tropical stratosphere. We will
investigate the possibility of using other data sets, such as ACE-FTS, to
evaluate the validity of our combined SAGE/MLS/SHADOZ data set.
3) Use the GEOS CCM to compare trends and variability of tropical lower
stratospheric ozone derived from data to expectations from theory. This task
will also examine the apparent inconsistency between altitude profile of trends
and total ozone trends. We will use the combination of profile-integrated
trends from SBUV measurements with the derived profile trend itself.
Results from the proposed studies should enable a clearer understanding of
tropical ozone trends and their relationship to changes in the Brewer-Dobson
circulation of the stratosphere. Predictions concerning the state of ozone in
the post-chlorofuorocarbon world are dependent upon understanding of how
circulation changes will affect the ozone layer.
Figure: Magnitude of the annual cycle (%) in ozone deduced from measurements
make by the SAGE II instrument using data from 1984 to 2005. Note the maximum
in magnitude just above the tropical tropopause that is offset to the northern
hemisphere.