Using composite analysis, the timing and extent of El Niño-Southern Oscillation (ENSO) impacts on maximum (T max ) and minimum temperatures (T min ) during austral summer are investigated for South Africa over the period 1940-2016. Pearson correlation coefficients determined between temperature data at stations within regions indicate that temperature records are coherent. During austral summer, composite analysis exhibits positive T max /T min anomalies for El Niño years while negative anomalies are recorded during La Niña years across all regions of South Africa. Statistical significance of composite average temperature anomalies was determined using the Student's t-test. T max for El Niño years are significantly different from the neutral years over the central interior of South Africa for the period 1940-2016. The most notable finding of this study is that El Niño events have had a stronger warming effect during austral summer over many regions in South Africa after the late 1970s, than before. Such an impact has been most prominent over the northern and central interior regions, where, respectively, associated T max record an average of 1.1 C and 0.73 C higher values for the period 1979-2016 compared to the earlier period . Chi-squared statistics indicate that ENSO phases exert a stronger influence on temperatures over the interior of South Africa than along the coast.

To establish precise climate trend analyses, highly reliable and accurate homogenous historical climate data are required. To this end, we undertake a robust quality control and homogenization process of daily Tmax and Tmin data (1916–2013) for the Western Cape Province, South Africa, using RClimDex, ProClimDB and Anclim software. Inhomogeneities were detected using the pairwise method in AnClim, suggesting possible artificial shifts in the time series. The adjustment of time series utilized ProClimDB software to create reference series using highly correlated or nearest neighbour stations. Given few available long‐term data sets, the study is limited to eight suitable stations. The modified Mann Kendall test in XLSTAT 2015 software identified annual and seasonal trends in the newly homogenized monthly Tmax and Tmin data. Annual Tmax and Tmin adjusted data over the Western Cape region indicate statistically significant increasing temperature trends over the period 1916–2013, with the exception of an insignificant decreasing Tmax trend at Cape St Blaize. A statistically significant increasing trend (0.13 °C/decade) for all stations used in this study is recorded for the common period 1937–2001. The seasonal trends also support significant increasing trends, with the exception of Tmax trends for summer (−0.03 °C/decade) and autumn (0 °C/decade) at Kirstenbosch.

Climate change has the potential to alter the spatio-temporal distribution of rainfall, subsequently affecting the supply and demand of water resources. In a water-stressed country such as South Africa, this effect has significant consequences. To this end, we investigated annual and winter rainfall and river flow trends for the Western Cape Province over two periods: 1987–2017 and 1960–2017. Annual rainfall for the most recent 30-year period shows decreasing trends, with the largest magnitude of decrease at the SA Astronomical Observatory rainfall station (-54.38 mm/decade). With the exception of the significant decreasing winter rainfall trend at Langewens (-34.88 mm/decade), the trends vary between stations for the period 1960–2017. For the period 1987–2017, statistically significant decreasing winter trends were found at four of the seven stations, and range from -6.8 mm/decade at Cape Columbine to -34.88 mm/decade at Langewens. Similarly, the magnitudes of decreasing winter river flow at Bree@Ceres and Berg@Franschoek are greater for the more recent 30-year period than for 1960–2017. Correlation coefficients for Vilij@Voeliv rainfall and four river flow stations Berg@Franschoek, Bree@Ceres, Wit River@Drosterkloof and Little Berg@Nieuwkloof) are stronger for shorter periods (i.e. 1987–2017 and 2007–2017) than that for the longer period, 1960–2017. The Intergovernmental Panel on Climate Change emphasises the importance of studies to assist with model prediction uncertainties. To this end, our study expands the understanding of regional hydrological responses to rainfall change in the water stressed region of the Western Cape Province.