Abstract
Mounting evidence across South Africa’s southwestern winter rainfall zone (WRZ) reflects consistent drying since ~ 1980, and projected trends suggest this will continue. However, limited evidence exists for changes in the region’s rainfall seasonality. To improve our understanding of these WRZ drying trends, especially within the context of Cape Town’s 2015–2017 “Day Zero” drought, it is necessary to explore long-term rainfall seasonality trends. Thus, we use the longest WRZ meteorological record from the South African Astronomical Observatory (SAAO) in Cape Town to investigate rainfall seasonality shifts during 1841–2020. Consistent with recorded poleward migrations of the subtropical high-pressure belt and mid-latitude westerlies, known drivers behind the drought and drying trends, calculated trends demonstrate strengthening of WRZ conditions, primarily from a later start-date trend leading to a shorter wet-season. Long-term drying trends are quantified for the wet and dry seasons; however, analysis of trend evolution reveals much variability, reflecting that drying has only persisted since ~ 1892. Comparative analyses of the first and last 59 years of 1841–2020 reveal a rainfall decline of almost 10% across both seasons—highlighting that the extreme “Day Zero” drought was not only driven by wet-season rainfall declines. Results demonstrate that these drying trends were consistently driven by a long-term decline in rain day counts and a more recent decline in average rainfall per rain day. Correspondence between our results and projected rainfall seasonality trends suggests the trends we quantified will likely continue; thus, improvements and continuation of existing water conservation and management strategies are imperative for Cape Town.
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Data for 1841–1899 can be obtained by direct request to the authors. Data for 1933–2020 can be obtained through a direct request to the South African Weather Service (SAWS).
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References
Aguilar E, Auer I, Brunet M, Peterson TC, Wieringa J (2003) Guidance on metadata and homogenization. World Meteorological Organization Technical Document 1186:53
Beaulieu C, Chen J, Sarmiento JL (2012) Change-point analysis as a tool to detect abrupt climate variations. Philos T R Soc A 370(1962):1228–1249. https://doi.org/10.1098/rsta.2011.0383
Blamey RC, Ramos AM, Trigo RM, Tomé R, Reason CJC (2018) The influence of atmospheric rivers over the South Atlantic on winter rainfall in South Africa. J Hydrometeorol 19(1):127–142. https://doi.org/10.1175/JHM-D-17-0111.1
Blamey R, Reason CJC (2007) Relationships between Antarctic sea-ice and South African winter rainfall. Clim Res 33:183–193. https://doi.org/10.3354/cr033183
Booysen MJ, Rpunda C, Visser M (2019) Results from a water-saving maintenance campaign at Cape Town schools in the run-up to Day Zero. Sustain Cities Soc 50:101639. https://doi.org/10.1016/j.scs.2019.101639
Botai CM, Botai JO, de Wit JP, Ncongwane KP, Adeola AM (2017) Drought characteristics over the Western Cape province. South Africa Water 9(11):876. https://doi.org/10.3390/w9110876
Burls NJ, Blamey RC, Cash BA, Swenson ET, Fahad M, Bopape MJM, Straus DM, Reason CJ (2019) The Cape Town “Day Zero” drought and Hadley cell expansion. Npj Clim Atmos Sci 2:27. https://doi.org/10.1038/s41612-019-0084-6
Chavaillaz Y, Codron F, Kageyama M (2013) Southern westerlies in LGM and future (RCP 4.5) climates. Clim Past 9(2):517–524. https://doi.org/10.5194/cp-9-517-2013
De Kock WM, Blamey RC, Reason CJC (2021) Large summer rainfall events and their importance in mitigating droughts over the South Western Cape. South Africa J Hydrometeorol 22(3):587–599. https://doi.org/10.1175/JHM-D-20-0123.1
Dieppois B, Pohl B, Rouault M, New M, Lawler D, Keenlyside N (2016) Interannual to interdecadal variability of winter and summer southern African rainfall, and their teleconnections. J Geophys Res-Atmos 121(11):405–442. https://doi.org/10.1002/2015JD024576
du Plessis JA, Schloms B (2017) An investigation into the evidence of seasonal rainfall pattern shifts in the Western Cape, South Africa. J S Afr Inst Civ Eng 59(4):47–55. https://doi.org/10.17159/2309-8775/2017/v59n4a5
Dunning CM, Black EC, Allan RP (2018) Later wet seasons with more intense rainfall over Africa under future climate change. J Clim 31(23):9719–9738. https://doi.org/10.1175/JCLI-D-18-0102.1
Durre I, Menne MJ, Gleason BE, Houston TG, Vose RS (2010) Comprehensive automated quality assurance of daily surface observations. J Appl Meteorol Climatol 49(8):1615–1633. https://doi.org/10.1175/2010JAMC2375.1
Dyson LL (2009) Heavy daily-rainfall characteristics over the Gauteng Province. Water SA 35(5):627–638. https://doi.org/10.4314/wsa.v35i5.49188
Engelbrecht FA, McGregor JL, Engelbrecht CJ (2009) Dynamics of the Conformal-Cubic Atmospheric Model projected climate-change signal over southern Africa. Int J Climatol 29(7):1013–1033. https://doi.org/10.1002/joc.1742
Fahad AA, Burls NJ, Strasberg Z (2020) How will southern hemisphere subtropical anticyclones respond to global warming? Mechanisms and seasonality in CMIP5 and CMIP6 model projections. Clim Dyn 55:703–718. https://doi.org/10.1007/s00382-020-05290-7
Favre A, Hewitson B, Lennard C, Cerezo-Mota R, Tadross M (2013) Cut-off lows in the South Africa region and their contribution to precipitation. Clim Dyn 41:2331–2351. https://doi.org/10.1007/s00382-012-1579-6
George J, Athira P (2020) Long-term changes in climatic variables over the Bharathapuzha river basin, Kerala, India. Theor Appl Climatol 142:269–286. https://doi.org/10.1007/s00704-020-03255-8
Glass IS (2018) The Royal Observatory rainfall records. Mon Notes Astron Soc S Afr 77(5–6):77–80. https://hdl.handle.net/10520/EJC-1342b9d7f0. Accessed 15 Apr 2021
Grise KM, Davis SM, Staten PW, Adam O (2018) Regional and seasonal characteristics of the recent expansion of the Tropics. J Clim 31(17):6839–6856. https://doi.org/10.1175/JCLI-D-18-0060.1
Hekmatzadeh AA, Kaboli S, Torabi Haghighi A (2020) New indices for assessing changes in seasons and in timing characteristics of air temperature. Theor Appl Climatol 140:1247–1261. https://doi.org/10.1007/s00704-020-03156-w
Jury MR (2020) Climate trends in the Cape Town area South Africa. Water SA. 46(3):438–447. https://doi.org/10.17159/wsa/2020.v46.i3.8654
Lakhraj-Govender R, Grab S, Ndebele NE (2017) A homogenized long-term temperature record for the Western Cape Province in South Africa: 1916–2013. Int J Climatol 37(5):2337–2353. https://doi.org/10.1002/joc.4849
Lennard C, Hegerl G (2015) Relating changes in synoptic circulation to the surface rainfall response using self-organising maps. Clim Dyn 44:861–879. https://doi.org/10.1007/s00382-014-2169-6
Li X, Hu ZZ, Jiang X, Li Y, Gao Z, Yang S, Zhu J, Jha B (2016) Trend and seasonality of land precipitation in observations and CMIP5 model simulations. Int J Climatol 36(11):3781–3793. https://doi.org/10.1002/joc.4592
Lim Kam Sian KTC, Wang J, Ayugi BO, Nooni IK, Ongoma V (2021) Multi-Decadal Variability and Future Changes in Precipitation over Southern Africa. Atmosphere 12(6):742. https://doi.org/10.3390/atmos12060742
Lu J, Vecchi GA, Reichler T (2007) Expansion of the Hadley cell under global warming. Geophys Res Lett 34(6):L06805. https://doi.org/10.1029/2006GL028443
MacKellar N, New M, Jack C (2014) Observed and modelled trends in rainfall and temperature for South Africa: 1960–2010. S Afr J Sci 110(7–8):1–13. https://doi.org/10.1590/sajs.2014/20130353
Mahlalela PT, Blamey RC, Reason CJC (2019) Mechanisms behind early winter rainfall variability in the southwestern Cape, South Africa. Clim Dyn 53:21–39. https://doi.org/10.1007/s00382-018-4571-y
Mahlobo DD, Ndarana T, Grab S, Engelbrecht F (2019) Integrated climatology and trends in the subtropical Hadley cell, sunshine duration and cloud cover over South Africa. Int J Climatol 39(4):1805–1821. https://doi.org/10.1002/joc.5917
Muller M (2018) Cape Town’s drought: don’t blame climate change. Nature 559:174–176. https://doi.org/10.1038/d41586-018-05649-1
Ndarana T, Mpati S, Bopape MJ, Engelbrecht F, Chikoore H (2021) The flow and moisture fluxes associated with ridging South Atlantic Ocean anticyclones during the subtropical southern African summer. Int J Climatol 41(S1):E1000–E1017. https://doi.org/10.1002/joc.6745
Ndebele NE, Grab S, Turasie A (2020) Characterizing rainfall in the south-western Cape, South Africa: 1841–2016. Int J Climatol 40(4):1992–2014. https://doi.org/10.1002/joc.6314
Nguyen H, Lucas C, Evans A, Timbal B, Hanson L (2015) Expansion of the southern hemisphere Hadley cell in response to greenhouse gas forcing. J Clim 28(20):8067–8077. https://doi.org/10.1175/JCLI-D-15-0139.1
Odoulami RC, Wolski P, New M (2021) A SOM-based analysis of the drivers of the 2015–2017 Western Cape drought in South Africa. Int J Climatol 41(S1):E1518–E1530. https://doi.org/10.1002/joc.6785
Omar SA, Abiodun BJ (2020) Characteristics of cut-off lows during the 2015–2017 drought in the Western Cape. South Africa Atmos Res 235:104772. https://doi.org/10.1016/j.atmosres.2019.104772
Otto FE, Wolski P, Lehner F, Tebaldi C, van Oldenborgh GJ, Hogesteeger S, Singh R, Holden P, Fučkar NS, Odoulami RC, New M (2018) Anthropogenic influence on the drivers of the Western Cape drought 2015–2017. Environ Res Lett 13(12):124010. https://doi.org/10.1088/1748-9326/aae9f9
Pascale S, Lucarini V, Feng X, Porporato A, ul Hasson S (2016) Projected changes of rainfall seasonality and dryspells in a high greenhouse gas emissions scenario. Clim Dyn 46:1331–1350. https://doi.org/10.1007/s00382-015-2648-4
Pascale S, Kapnick SB, Delworth TL, Cooke WF (2020) Increasing risk of another Cape Town “Day Zero” drought in the 21st century. Proc Natl Acad Sci USA 117(47):29495–29503. https://doi.org/10.1073/pnas.2009144117
Philippon N, Rouault M, Richard Y, Favre A (2012) The influence of ENSO on winter rainfall in South Africa. Int J Climatol 32(15):2333–2347. https://doi.org/10.1002/joc.3403
Picas J, Grab S (2020) Potential impacts of major nineteenth century volcanic eruptions on temperature over Cape Town, South Africa: 1834–1899. Clim Change 159:523–544. https://doi.org/10.1007/s10584-020-02678-6
Picas J, Grab S, Allen R (2019) A 19th century daily surface pressure series for the southwestern Cape region of South Africa: 1834–1899. Int J Climatol 39(3):1404–1414. https://doi.org/10.1002/joc.5890
Reason CJC, Allan RJ, Lindesay JA, Ansell TJ (2000) ENSO and climatic signals across the Indian Ocean Basin in the global context: part I, interannual composite patterns. Int J Climatol 20(11):1285–1327. https://doi.org/10.1002/1097-0088(200009)20:11%3c1285::AID-JOC536%3e3.0.CO;2-R
Reason CJC, Jagadheesa D (2005) Relationships between South Atlantic SST variability and atmospheric circulation over the South African region during austral winter. J Clim 18(16):3339–3355. https://doi.org/10.1175/JCLI3474.1
Reason CJC, Rouault M (2005) Links between the Antarctic Oscillation and winter rainfall over western South Africa. Geophys Res Lett 32(7). https://doi.org/10.1029/2005GL022419
Reason CJC, Rouault M, Melice JL, Jagadheesha D (2002) Interannual winter rainfall variability in SW South Africa and large scale ocean-atmosphere interactions. Meteorol Atmos Phys 80:19–29. https://doi.org/10.1007/s007030200011
Roffe SJ, Fitchett JM, Curtis CJ (2019) Classifying and mapping rainfall seasonality in South Africa: a review. S Afr Geogr J 101(2):158–174. https://doi.org/10.1080/03736245.2019.1573151
Roffe SJ, Fitchett JM, Curtis CJ (2020) Determining the utility of a percentile-based wet-season start- and end-date metric across South Africa. Theor Appl Climatol 140:1331–1347. https://doi.org/10.1007/s00704-020-03162-y
Roffe SJ, Fitchett JM, Curtis CJ (2021a) Investigating rainfall seasonality changes across South Africa: 1987–2016. Int J Climatol 41(S1):E2031–E2050. https://doi.org/10.1002/joc.6830
Roffe SJ, Fitchett JM, Curtis CJ (2021b) Quantifying rainfall seasonality across South Africa on the basis of the relationship between rainfall and temperature. Clim Dyn 56:2431–2450. https://doi.org/10.1007/s00382-020-05597-5
Sneyers R (1991) On the statistical analysis of series of observations. World Meteorological Organization Technical Document 143:192
Sousa PM, Blamey RC, Reason CJ, Ramos AM, Trigo RM (2018) The ‘Day Zero’ Cape Town drought and the poleward migration of moisture corridors. Environ Res Lett 13(12):124025. https://doi.org/10.1088/1748-9326/aaebc7
Vanderwalt AJ, Fitchett JM (2020) Statistical classification of South African seasonal divisions on the basis of daily temperature data. S Afr J Sci 116(9–10):1–15. https://doi.org/10.17159/sajs.2020/7614
Wolski P (2018) How severe is Cape Town’s “Day Zero” drought? Significance 15(2):24–27. https://doi.org/10.1111/j.1740-9713.2018.01127.x
Wolski P, Conradie S, Jack C, Tadross M (2021) Spatio-temporal patterns of rainfall trends and the 2015–2017 drought over the winter rainfall region of South Africa. Int J Climatol 41(S1):1303–1319. https://doi.org/10.1002/joc.6768
Yin JH (2005) A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophys Res Lett 32(18):1–4. https://doi.org/10.1029/2005GL023684
Acknowledgements
We thank the South African Weather Service for providing weather station data for 1933-2020 and Nothabo Ndebele for providing cleaned and quality controlled daily rainfall data for 1841-1899 for the South African Astronomical Observatory.
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SJR acknowledges funding from the University of the Witwatersrand Faculty of Science Research Committee. JMF is funded by the DSI-NRF Centre of Excellence for Palaeoscience.
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SJR, JS, and JMF conceptualized the topic of this paper. SJR conducted all statistical analyses with input from JS and JMF. SJR compiled the manuscript, and JS and JMF provided input on the various drafts.
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Roffe, S.J., Steinkopf, J. & Fitchett, J.M. South African winter rainfall zone shifts: A comparison of seasonality metrics for Cape Town from 1841–1899 and 1933–2020. Theor Appl Climatol 147, 1229–1247 (2022). https://doi.org/10.1007/s00704-021-03911-7
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DOI: https://doi.org/10.1007/s00704-021-03911-7