The rainfall series from the South African Astronomical Observatory in Cape Town, South Africa, is one of the longest known single site instrumental records in the southern hemisphere, spanning over 176 years. Rainfall data are analysed to determine trends and periodicity in the series for annual, seasonal and monthly time scales. Using the Mann Kendall test and Sen's slope, significant negative rainfall trends are recorded for the months of March and October, and for the spring season (from September to November). Using the Mann Kendall and its modified versions to account for serial correlation, as well as a multi‐temporal trend analysis, we demonstrate a positive rainfall trend during the first 60 years (i.e., 1841–1900), which thereafter changes to a long‐term (1900–2016) negative trend, but incorporating a shorter 40 years significant positive trend between 1930 and 1970. We identify cyclic patterns with recorded periods of 9–12 years, 16–30 years and 30–42 years for rainfall, the Southern annular mode (SAM) and Southern oscillation index (SOI). In addition to the notable 9–12 years rainfall cycle that is evidently associated with sunspot cycles, 20–30 years and longer 32–40 years rainfall, solar, SAM and SOI cycles are also identified.

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.

Abstract. Wet seasons may be characterized by the frequency of wet–dry days, duration of wet–dry spells and season length. These properties are investigated for Cape Town using rainfall data from four weather stations in the Cape Town metropolitan area located at the South African Astronomical Observatory (SAAO), Maitland, Kirstenbosch and Cape Town International airport. The primary focus is on the long SAAO daily rainfall record dating back to 1841, with the specific aim to statistically assess attributes of the wet season (April to October) and its temporal variability over the period 1841–2018. The decade 1950–1959 had significantly high frequencies of wet days, but there was a subsequent significant decline in wet days at the SAAO (−1 d per decade) and Maitland (−1.1 d per decade) during the period 1950–2018. A significant decline in wet days also occurred at the SAAO between 1880 and 1940 (−3.3 d per decade, p=0.005). Dry spells longer than 5 d have become more prevalent since the beginning of the 20th century. A rain-based definition for the onset and termination of the wet season is presented using 5 d running sums and pentad means; these were applied to each year containing adequate daily data, so as to track changes during the wet season. We identify a recent decadal decline in mean wet season length (−1.4 d per decade) since ca. 1940, particularly between 1941–1994 (−4.2 d per decade), such that wet seasons since 2000 have only averaged 182 d in length while the long-term (1841–2018) average is 188 d. This decline is attributed to an increased incidence of late onsets (after 15 April) and early terminations (earlier than 18 October) of the wet season, or a combination of both, particularly since the year 2000. Interannual variability in wet season characteristics is associated with solar (sunspot) cycles and fluctuations in the Southern Oscillation Index and Southern Annular Mode.

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