Abstract
Although on average, the southwestern Cape (SWC) of South Africa is a winter rainfall dominated region, almost 30% of the total rainfall occurs during the extended summer (October-March). A previous study showed that anomalously wet summers may help mitigate the effects of severe winter drought. Apart from that study, very little work has been done on summer rainfall variability over the SWC or the mechanisms associated with it. Here, station data and ERA5 reanalyses are used to investigated summer rainfall day variability and associated mechanisms. Interannual variability in summer rainfall day frequencies appears related to that in the South Atlantic High Pressure (SAHP) and westerly moisture fluxes across the midlatitude South Atlantic. Increased rainfall days are associated with cyclonic anomalies over the region and enhanced westerly moisture fluxes. Some of these circulation changes are related to the Southern Annular Mode, and in late summer, also to ENSO and changes in the zonal wavenumber 3 pattern. Significant decreasing trends in rainfall days were found in the mid- and late summer for the southern part of the region where most of the population lives and the main water supply dams are located. These trends seem associated with significant trends found in the southern boundary of the SAHP and in decreasing (increasing) South Atlantic storm counts in the 35°–45° S (50°–60° S) latitude bands.
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Data availability statement
Station rainfall data used in this study are available upon request from the South African Weather Service (SAWS) (http://www.weathersa.so.za). ERA5 reanalysis was sourced from the Copernicus Climate Data Store (https://cds.climate.copernicus.eu/). The SAM and ENSO indices were obtained from the Climate Explorer portal (http://climexp.knmi.nl/ ). The Gramcianinov et al. (2020) extratropical cyclone track data is available at (https://data.mendeley.com/datasets/kwcvfr52hp/3). The OISSTv2 is available on the NOAA website (https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html) to calculate the SAOD and SIOD.
References
Archer E, Landman WA, Malherbe J, Tadross M, Pretorius S (2019) South Africa’s winter rainfall region drought: a region in transition? Clim Risk Manag 25:100188. https://doi.org/10.1016/j.crm.2019.100188
Behera SK, Yamagata T (2001) Subtropical SST dipole events in the southern Indian Ocean. Geophys Res Lett 28(2):327–330. https://doi.org/10.1029/2000GL011451
Blamey R, Reason CJC (2007) Relationships between Antarctic sea-ice and South African winter rainfall. Clim Res 33(2):183–193. https://doi.org/10.3354/cr033183
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
Burls NJ, Blamey RC, Cash BA, Swenson ET, Fahad A, Bopape MJM, Straus DM, Reason CJ (2019) The Cape Town “Day Zero” drought and Hadley cell expansion. NPJ Clim Atmos Sci 2(1):1–8. https://doi.org/10.1038/s41612-019-0084-6
Colberg F, Reason CJC, Rodgers K (2004) South Atlantic response to El Niño-Southern Oscillation induced climate variability in an ocean general circulation model. J Geophys Res Oceans. https://doi.org/10.1029/2004JC002301
De Coning E (2013) Optimizing satellite-based precipitation estimation for nowcasting of rainfall and flash flood events over the South African domain. Remote Sens 5(11):5702–5724. https://doi.org/10.3390/rs5115702
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
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
Engelbrecht CJ, Landman WA, Engelbrecht FA, Malherbe J (2015) A synoptic decomposition of rainfall over the Cape south coast of South Africa. Clim Dyn 44(9–10):2589–2607. https://doi.org/10.1007/s00382-014-2230-5
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(3):703–718. https://doi.org/10.1007/s00382-020-05290-7
Fauchereau N, Trzaska S, Richard Y, Roucou P, Camberlin P (2003) Sea-surface temperature co‐variability in the Southern Atlantic and Indian Oceans and its connections with the atmospheric circulation in the Southern Hemisphere. Int J Climatol 23(6):663–677. https://doi.org/10.1002/joc.905
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(9–10):2331–2351. https://doi.org/10.1007/s00382-012-1579-6
Gillett NP, Stott PA (2009) Attribution of anthropogenic influence on seasonal sea level pressure. Geophys Res Lett. https://doi.org/10.1029/2009GL041269
Gillett NP, Zwiers FW, Weaver AJ, Stott PA (2003) Detection of human influence on sea-level pressure. Nature 422(6929):292–294. https://doi.org/10.1038/nature01487
Gilliland JM, Keim BD (2018) Position of the South Atlantic anticyclone and its impact on surface conditions across Brazil. J Appl Meteorol Climatol 57(3):535–553. https://doi.org/10.1175/JAMC-D-17-0178.1
Gramcianinov CB, Campos RM, de Camargo R, Hodges KI, Soares G, da Silva Dias C, P.L (2020) Analysis of Atlantic extratropical storm tracks characteristics in 41 years of ERA5 and CFSR/CFSv2 databases. Ocean Eng 216 C:108111. https://doi.org/10.1016/j.oceaneng.2020.108111
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
Hartmann DL, Lo F (1998) Wave-driven zonal flow vacillation in the Southern Hemisphere. J Atmos Sci 55(8):1303–1315. https://doi.org/10.1175/1520-0469(1998)055<1303:WDZFVI>2.0.CO;2
Hermes JC, Reason CJC (2005) Ocean model diagnosis of interannual coevolving SST variability in the South Indian and South Atlantic Oceans. J Clim 18(15):2864–2882. https://doi.org/10.1175/JCLI3422.1
Hersbach H, Bell B, Berrisford P, Hirahara S, Horányi A, Muñoz-Sabater J, Nicolas J, Peubey C, Radu R, Schepers D, Simmons A (2020) The ERA5 global reanalysis. Q J R Meteorol 146(730):1999–2049
Jones DA, Simmonds I (1993) A climatology of Southern Hemisphere extratropical cyclones. Clim Dyn 9(3):131–145
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
Kendall MG (1975) Rank correlation methods, 4th edn. Charles Griffin, London
Kruger AC, Nxumalo MP (2017) Historical rainfall trends in South Africa: 1921–2015. Water SA 43(2):285–297. https://doi.org/10.4314/wsa.v43i2.12
Mächel H, Kapala A, Flohn H (1998) Behaviour of the centres of action above the Atlantic since 1881. Part I: characteristics of seasonal and interannual variability. Int J Climatol 18:1–22. https://doi.org/10.1002/(SICI)1097-0088(199801)18:1<1::AID-JOC225>3.0.CO;2-A
Mahlalela PT, Blamey RC, Reason CJC (2019) Mechanisms behind early winter rainfall variability in the southwestern Cape. South Africa. Clim Dyn 53(1):21–39. https://doi.org/10.1007/s00382-018-4571-y
Mann HB (1945) Nonparametric tests against trend. Econometrica. https://doi.org/10.2307/1907187
Marshall GJ (2003) Trends in the southern annular mode from observations and reanalyses. J Clim 16(24):4134–4143
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
Murray RJ, Simmonds I (1991) A numerical scheme for tracking cyclone centres from digital data. Aust Meteorol Mag 39(3):155–166
Nguyen H, Evans A, Lucas C, Smith I, Timbal B (2013) The Hadley circulation in reanalyses: climatology, variability, and change. J Clim 26(10):3357–3376. https://doi.org/10.1175/JCLI-D-12-00224.1
Nnamchi HC, Li J, Anyadike RN (2011) Does a dipole mode really exist in the South Atlantic Ocean? J Geophys Res Atmos. https://doi.org/10.1029/2010JD015579
Pezza AB, Simmonds I, Renwick JA (2007) Southern Hemisphere cyclones and anticyclones: recent trends and links with decadal variability in the Pacific Ocean. Int J Climatol 27(11):1403–1419. https://doi.org/10.1002/joc.1477
Pezza AB, Durrant T, Simmonds I, Smith I (2008) Southern Hemisphere synoptic behavior in extreme phases of SAM, ENSO, sea ice extent, and southern Australia rainfall. J Clim 21(21):5566–5584. https://doi.org/10.1175/2008JCLI2128.1
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
Ramos AM, Blamey RC, Algarra I, Nieto R, Gimeno L, Tomé R, Reason CJ, Trigo RM (2018) From Amazonia to southern Africa: atmospheric moisture transport through low-level jets and atmospheric rivers. Ann N Y Acad Sci 1436:217–230. https://doi.org/10.1111/nyas.13960
Raphael MN (2004) A zonal wave 3 index for the Southern Hemisphere. Geophys Res Lett. https://doi.org/10.1029/2004GL020365
Reason CJC (2001) Subtropical Indian Ocean SST dipole events and southern African rainfall. Geophys Res Lett 28(11):2225–2227. https://doi.org/10.1029/2000GL012735
Reason CJC (2002) Sensitivity of the southern African circulation to dipole sea-surface temperature patterns in the south Indian Ocean. Int J Climatol 22(4):377–393. https://doi.org/10.1002/joc.744
Reason CJC, Allan RJ, Lindesay JA, Ansell TJ (2000) ENSO and climatic signals across the Indian Ocean basin in the globalcontext: Part I, interannual composite patterns. Int J Climatol 20:1285–1327. https://doi.org/10.1002/1097-0088(200009)20:11<1285::AID-JOC536>3.0.CO;2-R
Reason CJC, Jagadheesha 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. https://doi.org/10.1029/2005GL022419
Reason CJC, Landman W, Tennant W (2006) Seasonal to decadal prediction of southern African climate and its links with variability of the Atlantic Ocean. Bull Am Meteorol Soc 87(7):941–956. https://doi.org/10.1175/BAMS-87-7-941
Reboita MS, Ambrizzi T, Silva BA, Pinheiro RF, Da Rocha RP (2019) The South Atlantic subtropical anticyclone: present and future climate. Front Earth Sci 7:1–15. https://doi.org/10.3389/feart.2019.00008
Scheff J, Frierson DM (2012) Robust future precipitation declines in CMIP5 largely reflect the poleward expansion of model subtropical dry zones. Geophys Res Lett. https://doi.org/10.1029/2012GL052910
Sen PK (1968) Estimates of the regression coefficient based on Kendall’s tau. J Am Stat Assoc 63(324):1379–1389. https://doi.org/10.1080/01621459.1968.10480934
Shepherd N (2019) Making sense of “Day Zero”: slow catastrophes, anthropocene futures, and the story of Cape Town’s water crisis. Water 11(9):1744. https://doi.org/10.3390/w11091744
Sinclair MR (1997) Objective identification of cyclones and their circulation intensity, and climatology. Weather Forecast 12(3):595–612. https://doi.org/10.1175/1520-0434(1997)012<0595:OIOCAT>2.0.CO;2
Singleton AT, Reason CJC (2007) Variability in the characteristics of cut-off low pressure systems over subtropical southern Africa. Int J Climatol 27(3):295–310. https://doi.org/10.1002/joc.1399
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
Staten PW, Lu J, Grise KM, Davis SM, Birner T (2018) Re-examining tropical expansion. Nat Clim 8(9):768–775. https://doi.org/10.1038/s41558-018-0246-2
Sun X, Cook KH, Vizy EK (2017) The South Atlantic subtropical high: climatology and interannual variability. J Clim 30(9):3279–3296. https://doi.org/10.1175/JCLI-D-16-0705.1
Theil H (1950) A rank-invariant method of linear and polynimical regression analysis. Proc R Neth Acad Sci 53:1397–1412
Van Wilgen BW (1984) Fire climates in the southern and western Cape Province and their potential use in fire control and management. S Afr J Sci 80(8):358
Van Wilgen BW (2009) The evolution of fire and invasive alien plant management practices in fynbos. S Afr J Sci 105(9):335–342. https://hdl.handle.net/10520/EJC96966
Weldon D, Reason CJC (2014) Variability of rainfall characteristics over the South Coast region of South Africa. Theor Appl Climatol 115(1):177–185. https://doi.org/10.1007/s00704-013-0882-4
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 41:1303–1319. https://doi.org/10.1002/joc.6768
Zarrin A, Ghaemi H, Azadi M, Farajzadeh M (2010) The spatial pattern of summertime subtropical anticyclones over Asia and Africa: a climatological review. Int J Climatol 30(2):159–151. https://doi.org/10.1002/joc.1879
Acknowledgements
WDK was funded by the National Research Foundation (NRF) of South Africa through the ACSyS and SANAP projects. RCB acknowledges support of the FLAIR programme, a partnership between the African Academy of Sciences and the Royal Society funded by the UK Government’s Global Challenges Research Fund. The authors thank all the listed institutions above for making the data available.
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De Kock, W.M., Blamey, R.C. & Reason, C.J.C. Large-scale mechanisms linked to anomalously wet summers over the southwestern Cape, South Africa. Clim Dyn 59, 3503–3517 (2022). https://doi.org/10.1007/s00382-022-06280-7
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DOI: https://doi.org/10.1007/s00382-022-06280-7