Wet season rainfall characteristics and temporal changes for Cape Town, South Africa, 1841–2018

Abstract

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.