Proposal Summary

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.