Abstract:
We conducted a comparison of trends in lower stratospheric temperatures and summer zonal wind fields
based on 27 years of reanalysis data and output from hindcast simulations using a coupled oceanatmospheric
general circulation model (OAGCM). Lower stratospheric ozone in the OAGCM was relaxed to
the observed climatology and increasing greenhouse gas concentrations were neglected. In the reanalysis,
lower stratospheric ozone fields were better represented than in the OAGCM. The spring lower stratospheric/
upper tropospheric cooling in the polar cap observed in the reanalysis, which is caused by a direct ozone
depletion in the past two decades and is in agreement with previous studies, did not appear in the OAGCM.
The corresponding summer tropospheric response also differed between data sets. In the reanalysis, a
statistically significant poleward trend of the summer jet position was found, whereas no such trend was
found in the OAGCM. Furthermore, the jet position in the reanalysis exhibited larger interannual variability
than that in the OAGCM. We conclude that these differences are caused by the absence of long-term lower
stratospheric ozone changes in the OAGCM. Improper representation or non-inclusion of such ozone
variability in a prediction model could adversely affect the accuracy of the predictability of summer rainfall
forecasts over South Africa.
