Comment on cp-2021-178

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 (−1day/decade) and Maitland (−1.1days/decade) during the period 1950–2018. A significant decline in wet days also occurred at the SAAO between 1880 and 1940 (−3.3 days/decade, p = 0.005). Dry spells longer than 5 days 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-day running sums and pentad means; these were applied to each year containing adequate daily data, so as to track changes during the wet season. Mean season length over recent decades (1950–2018) is 186 days, but this has declined over this period of time such that it averages 183 days for the most recent c. 4 decades (1979–2018). 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.