Numerical forecasting of monthly climate in southern Africa

Abstract

The South African Weather Bureau began issuing monthly forecasts of rainfall and temperature for seven regions in South Africa in 1995. With the availability of reanalysis datasets, historical forecasts may be done to assess the utility of numerical monthly forecasts. Monthly hindcasts with observed sea surface temperature data were run every four weeks from 1979 to 1995 using the T30 18-layer Center for Ocean-Land-Atmosphere Studies (COLA) general circulation model. Verification results show that bias-correction of model output and ensemble averaging improves forecast skill substantially. Monthly average sea level pressure and 500 hPa height forecasts are better than using persistence for all areas globally. Model surface temperature and outgoing long-wave radiation anomalies have a negligible bias but model rainfall anomalies suffer from geographically dependent biases. Variations in monthly forecast skill was found not to correspond to ENSO events and a priori prediction of skill using ensemble spread was not successful. In southern Africa, monthly forecasts that complement seasonal forecasts hold great potential to benefit economic activities in the region.