Impact of lower stratospheric dynamics on the predictability of summer rainfall over South Africa

Abstract

Seasonal climate prediction of rainfall and temperature in South Africa (SA) during summer is mainly related to the variability of El Niño Southern Oscillation (ENSO) besides other forcings from local surrounding oceans. However, the predictive skill is low whenever ENSO is dormant; therefore rainfall and temperature forecast become uncertain over the region. The study then aims to investigate the possible importance of proper representation of stratospheric processes in climate prediction models. These stratospheric processes include mainly ozone and green-house gases. A comparison of trends in lower stratospheric temperatures and summer zonal wind fields in 27 years of the National Centre for Environmental Prediction (NCEP) of the Department of Energy (DOE) Reanalysis II data and output from hindcast simulations using an ocean-atmosphere general circulation model (OAGCM) is conducted. Lower stratospheric ozone in the OAGCM is relaxed towards the observed climatology and increasing greenhouse gas concentrations are neglected. In reanalysis lower stratospheric ozone fields are better represented than in the OAGCM. The lower stratospheric/upper tropospheric cooling in the polar cap during spring (shown by previous studies as a result of lower ozone depletion) appears in reanalysis but not in the OAGCM. The corresponding summer tropospheric response also differs between datasets. Moreover, in reanalysis a statistically significant poleward trend of the summer jet position is found. However, contrary to this, no such trend is found in OAGCM. Furthermore, the dynamics of the stratosphere is analyzed using wave activity. The lack of wave activity in the model as opposed to observations further suggests that the dynamical coupling which is essential for proper simulation of the stratosphere is compromised. Lastly, the statistical forecasting system using winds and temperature during spring when the stratosphere is actively coupled also suggest potential predictability of summer rainfall over South Africa, particularly wet conditions when the tropospheric eddy-driven jet is shifted towards the poles.