Assessing the influence of the El Niño-Southern Oscillation phase on rainfall variability in the Gauteng province of South Africa

South Africa is subject to large rainfall variability, with El Nino-Southern Oscillation (ENSO) being the most important driver across the summer rainfall region. The aim of this study was to relate ENSO phase to historical rainfall variability over the densely populated and economically important Gauteng province of South Africa. Monthly rainfall totals for the period 1960-2021 were used to calculate Standardised Precipitation Index (SPI) values at different timescales for five stations. The Oceanic Nino Index (ONI) was used to classify seasons according to ENSO phase. For each station, the number of wet, near-normal or dry seasons falling in a particular ENSO phase was tallied and used to calculate probabilities of occurrence. Results revealed spatial inhomogeneities despite the study area being relatively small. Analysis of 3- and 6-month periods revealed that dry conditions occur more frequently during spring and autumn El Nino events, but are very unlikely during summer and autumn La Nina events. Analysis of 12-month periods revealed that near-normal conditions occur most frequently regardless of ENSO phase, with an increased chance of experiencing wet as opposed to dry conditions under La Nina. This information can aid various decision-makers to make better use of seasonal climate predictions.