02 Feb 2022
02 Feb 2022
Status: this preprint is currently under review for the journal CP.

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

Nothabo Elizabeth Ndebele1, Stefan Grab2, and Herbert Hove1 Nothabo Elizabeth Ndebele et al.
  • 1School of Statistics and Actuarial Science, University of the Witwatersrand, South Africa
  • 2School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, South Africa

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.

Nothabo Elizabeth Ndebele et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2021-178', Neil Macdonald, 08 Apr 2022
    • AC1: 'Reply on RC1', Nothabo Ndebele, 16 Apr 2022
  • RC2: 'Comment on cp-2021-178', Linden Ashcroft, 11 May 2022
    • AC2: 'Reply on RC2', Nothabo Ndebele, 07 Jun 2022

Nothabo Elizabeth Ndebele et al.

Nothabo Elizabeth Ndebele et al.


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Short summary
An investigation of the wet season characteristics including wet day frequencies, wet and dry spells, season onset, end and length is done for Cape Town South Africa. The temporal changes since 1841 in these characteristics indicate an increased incidence of shorter wet seasons and long dry spells in the most recent three decades compared to previous years. There is evidence of correlations between solar cycles and the Southern Oscillation index cycles with the wet season characteristics.