• Blamey, R. C., A. M. Ramos, R. M. Trigo, R. Tomé, and C. J. C. Reason, 2018: The influence of atmospheric rivers over the South Atlantic on winter rainfall in South Africa. J. Hydrometeor., 19, 127142, https://doi.org/10.1175/JHM-D-17-0111.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Burls, N. J., R. C. Blamey, B. A. Cash, E. T. Swenson, A. al Fahad, M. J. M. Bopape, D. M. Straus, and C. J. Reason, 2019: The Cape Town “Day Zero” drought and Hadley cell expansion. npj Climate Atmos. Sci., 2, 27, https://doi.org/10.1038/s41612-019-0084-6.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and et al. , 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597, https://doi.org/10.1002/qj.828.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Dettinger, M. D., F. M. Ralph, T. Das, P. J. Neiman, and D. R. Cayan, 2011: Atmospheric rivers, floods and the water resources of California. Water, 3, 445478, https://doi.org/10.3390/w3020445.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • de Waal, J. H., A. Chapman, and J. Kemp, 2017: Extreme 1-day rainfall distributions: Analysing change in the Western Cape. S. Afr. J. Sci., 113, 4350, https://doi.org/10.17159/sajs.2017/20160301.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Durre, I., M. J. Menne, B. E. Gleason, T. G. Houston, and R. S. Vose, 2010: Comprehensive automated quality assurance of daily surface observations. J. Appl. Meteor. Climatol., 49, 16151633, https://doi.org/10.1175/2010JAMC2375.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Eiras-Barca, J., S. Brands, and G. Miguez-Macho, 2016: Seasonal variations in North Atlantic atmospheric river activity and associations with anomalous precipitation over the Iberian Atlantic Margin. J. Geophys. Res. Atmos., 121, 931948, https://doi.org/10.1002/2015JD023379.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Favre, A., B. Hewitson, C. Lennard, R. Cerezo-Mota, and M. Tadross, 2013: Cut-off lows in the South Africa region and their contribution to precipitation. Climate Dyn., 41, 23312351, https://doi.org/10.1007/s00382-012-1579-6.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gimeno, L., R. Nieto, M. Vázquez, and D. A. Lavers, 2014: Atmospheric rivers: A mini-review. Front. Earth Sci., 2, 2, https://doi.org/10.3389/feart.2014.00002.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gimeno, L., and et al. , 2016: Major mechanisms of atmospheric moisture transport and their role in extreme precipitation events. Annu. Rev. Environ. Resour., 41, 117141, https://doi.org/10.1146/annurev-environ-110615-085558.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hart, N. C., C. J. Reason, and N. Fauchereau, 2013: Cloud bands over southern Africa: Seasonality, contribution to rainfall variability and modulation by the MJO. Climate Dyn., 41, 11991212, https://doi.org/10.1007/s00382-012-1589-4.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hart, R., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic-scale events objectively. Mon. Wea. Rev., 129, 24262442, https://doi.org/10.1175/1520-0493(2001)129<2426:UNCATR>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • IPCC, 2013: Climate Change 2013: The Physical Science Basis. Cambridge University Press, 1535 pp., https://doi.org/10.1017/CBO9781107415324.

    • Crossref
    • Export Citation
  • Jakobson, E., T. Vihma, T. Palo, L. Jakobson, H. Keernik, and J. Jaagus, 2012: Validation of atmospheric reanalyses over the central Arctic Ocean. Geophys. Res. Lett., 39, L10802, https://doi.org/10.1029/2012GL051591.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kishore, P., and et al. , 2011: Global (50°S–50°N) distribution of water vapor observed by COSMIC GPS RO: Comparison with GPS radiosonde, NCEP, ERA-Interim, and JRA-25 reanalysis data sets. J. Atmos. Sol.-Terr. Phys., 73, 18491860, https://doi.org/10.1016/j.jastp.2011.04.017.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lu, J., C. Deser, and T. Reichler, 2009: Cause of the widening of the tropical belt since 1958. Geophys. Res. Lett., 36, L03803, https://doi.org/10.1029/2008GL036076.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lucas, C., B. Timbal, and H. Nguyen, 2014: The expanding tropics: A critical assessment of the observational and modeling studies. Wiley Interdiscip. Rev.: Climate Change, 5, 89112, https://doi.org/10.1002/wcc.251.

    • Search Google Scholar
    • Export Citation
  • Mahlalela, P. T., R. C. Blamey, and C. J. C. Reason, 2019: Mechanisms behind early winter rainfall variability in the southwestern Cape, South Africa. Climate Dyn., 53, 2139, https://doi.org/10.1007/s00382-018-4571-y.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Molekwa, S., C. J. Engelbrecht, and C. D. Rautenbach, 2014: Attributes of cut-off low induced rainfall over the Eastern Cape Province of South Africa. Theor. Appl. Climatol., 118, 307318, https://doi.org/10.1007/s00704-013-1061-3.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Neiman, P. J., F. M. Ralph, A. B. White, D. E. Kingsmill, and P. O. G. Persson, 2002: The statistical relationship between upslope flow and rainfall in California’s coastal mountains: Observations during CALJET. Mon. Wea. Rev., 130, 14681492, https://doi.org/10.1175/1520-0493(2002)130<1468:TSRBUF>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Newell, R. E., N. E. Newell, Y. Zhu, and C. Scott, 1992: Tropospheric rivers? – A pilot study. Geophys. Res. Lett., 19, 24012404, https://doi.org/10.1029/92GL02916.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Nieto, R., and et al. , 2005: Climatological features of cutoff low systems in the Northern Hemisphere. J. Climate, 18, 30853103, https://doi.org/10.1175/JCLI3386.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Nieto, R., M. Sprenger, H. Wernli, R. M. Trigo, and L. Gimeno, 2008: Identification and climatology of cut-off lows near the Tropopause. Ann. N. Y. Acad. Sci., 1146, 256290, https://doi.org/10.1196/annals.1446.016.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pienaar, A., A. C. Brent, J. K. Musango, and I. H. De Kock, 2017: Water resource infrastructure implications of a green economy transition in the Western Cape Province of South Africa: A system dynamics approach. S. Afr. J. Ind. Eng., 28, 7894, https://doi.org/10.7166/28-2-1639.

    • Search Google Scholar
    • Export Citation
  • Poli, P., S. B. Healy, and D. P. Dee, 2010: Assimilation of global positioning system radio occultation data in the ECMWF ERA-Interim reanalysis. Quart. J. Roy. Meteor. Soc., 136, 19721990, https://doi.org/10.1002/qj.722.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ralph, F. M., P. J. Neiman, and G. A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the eastern North Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev., 132, 17211745, https://doi.org/10.1175/1520-0493(2004)132<1721:SACAOO>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ralph, F. M., T. Coleman, P. J. Neiman, R. J. Zamora, and M. D. Dettinger, 2013: Observed impacts of duration and seasonality of atmospheric-river landfalls on soil moisture and runoff in coastal Northern California. J. Hydrometeor., 14, 443459, https://doi.org/10.1175/JHM-D-12-076.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ramos, A. M., R. M. Trigo, M. L. Liberato, and R. Tomé, 2015: Daily precipitation extreme events in the Iberian Peninsula and its association with atmospheric rivers. J. Hydrometeor., 16, 579597, https://doi.org/10.1175/JHM-D-14-0103.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ramos, A. M., R. C. Blamey, I. Algarra, R. Nieto, L. Gimeno, R. Tomé, C. J. Reason, and R. M. Trigo, 2018: From Amazonia to southern Africa: Atmospheric moisture transport through low-level jets and atmospheric rivers. Ann. N. Y. Acad. Sci., 1436, 217230, https://doi.org/10.1111/nyas.13960.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Reason, C. J. C., and M. R. Jury, 1990: On the generation and propagation of the southern African coastal low. Quart. J. Roy. Meteor. Soc., 116, 11331151, https://doi.org/10.1002/qj.49711649507.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Reason, C. J. C., and M. Rouault, 2002: ENSO-like decadal variability and South African rainfall. Geophys. Res. Lett., 29, 1638, https://doi.org/10.1029/2002GL014663.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Reason, C. J. C., and D. Jagadheesha, 2005: Relationships between South Atlantic SST variability and atmospheric circulation over the South African region during austral winter. J. Climate, 18, 33393355, https://doi.org/10.1175/JCLI3474.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Reason, C. J. C., M. Rouault, J. L. Melice, and D. Jagadheesha, 2002: Interannual winter rainfall variability in SW South Africa and large scale ocean–atmosphere interactions. Meteor. Atmos. Phys., 80, 1929, https://doi.org/10.1007/s007030200011.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Rutz, J. J., W. J. Steenburgh, and F. M. Ralph, 2014: Climatological characteristics of atmospheric rivers and their inland penetration over the western United States. Mon. Wea. Rev., 142, 905921, https://doi.org/10.1175/MWR-D-13-00168.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Scheff, J., and D. Frierson, 2012: Twenty-first-century multimodel subtropical precipitation declines are mostly midlatitude shifts. J. Climate, 25, 43304347, https://doi.org/10.1175/JCLI-D-11-00393.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Singleton, A. T., and C. J. C. Reason, 2006: Numerical simulations of a severe rainfall event over the Eastern Cape coast of South Africa: Sensitivity to sea surface temperature and topography. Tellus, 58A, 335367, https://doi.org/10.1111/j.1600-0870.2006.00180.x.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Singleton, A. T., and C. J. C. Reason, 2007a: Variability in the characteristics of cut-off low pressure systems over subtropical southern Africa. Int. J. Climatol., 27, 295310, https://doi.org/10.1002/joc.1399.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Singleton, A. T., and C. J. C. Reason, 2007b: A numerical model study of an intense cutoff low pressure system over South Africa. Mon. Wea. Rev., 135, 11281150, https://doi.org/10.1175/MWR3311.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sousa, P. M., R. C. Blamey, C. J. Reason, A. M. Ramos, and R. M. Trigo, 2018: The ‘Day Zero’ Cape Town drought and the poleward migration of moisture corridors. Environ. Res. Lett., 13, 124025, https://doi.org/10.1088/1748-9326/aaebc7.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sousa, P. M., D. Barriopedro, A. M. Ramos, R. García-Herrera, F. Espírito-Santo, and R. M. Trigo, 2019: Saharan air intrusions as a relevant mechanism for Iberian heatwaves: The record breaking events of August 2018 and June 2019. Wea. Climate Extremes, 26, 100224, https://doi.org/10.1016/j.wace.2019.100224.

    • Search Google Scholar
    • Export Citation
  • Taljaard, J. J., 1985: Cut-off lows in South African region. South African Weather Bureau Tech. Paper 14, 153 pp.

  • Tyson, P. D., and R. A. Preston-Whyte, 2000: The Weather and Climate of Southern Africa. Oxford University Press, 396 pp.

  • Usman, M. T., and C. J. C. Reason, 2004: Dry spell frequencies and their variability over southern Africa. Climate Res., 26, 199211, https://doi.org/10.3354/cr026199.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhu, Y., and R. E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev., 126, 725735, https://doi.org/10.1175/1520-0493(1998)126<0725:APAFMF>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zilli, M. T., L. M. Carvalho, and B. R. Lintner, 2019: The poleward shift of South Atlantic Convergence Zone in recent decades. Climate Dyn., 52, 25452563, https://doi.org/10.1007/s00382-018-4277-1.

    • Crossref
    • Search Google Scholar
    • Export Citation
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Large Summer Rainfall Events and Their Importance in Mitigating Droughts over the South Western Cape, South Africa

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  • 1 Department of Oceanography, University of Cape Town, Rondebosch, South Africa
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Abstract

Although the south Western Cape receives most of its rainfall between May and September, there are substantial rainfall events in some summers. These events are of interest in themselves as well as for their possible role in mitigating the frequent winter droughts that the region suffers from. Most recently, greater Cape Town suffered a devastating drought during 2015–18 known as the Day Zero drought due to the high risk of urban areas running out of piped water supply. Estimated data from the city show that major dam levels in the south Western Cape increased more than 1% in some cases after large rainfall events (LREs) in the summer of 2018/19. This increase is significant as dam levels often decrease by several percent per month during the hot summer. In this study, LREs over the south Western Cape during the summer (October–March) are investigated together with dam level data. Most summer LREs result from atmospheric rivers (ARs) or cutoff lows (COLs). ARs have not been previously studied in the South African region except for one study for winter that showed they are responsible for almost all the heavy rainfall events in the Western Cape. Although COLs are most common in the transition months, they can also occur in midwinter and summer. COLs tend to last longer and cover larger areas than ARs, which typically yield relatively short bursts of intense rainfall mostly concentrated around greater Cape Town. After each summer LRE, average dam levels increase by up to 5%, suggesting they are very important for drought recovery. In particular, summer LREs following the anomalously dry winters of 1980, 1984, 2003, 2004, and 2015–18 played an important role in mitigating those droughts.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Wade De Kock, dkcwad001@myuct.ac.za

Abstract

Although the south Western Cape receives most of its rainfall between May and September, there are substantial rainfall events in some summers. These events are of interest in themselves as well as for their possible role in mitigating the frequent winter droughts that the region suffers from. Most recently, greater Cape Town suffered a devastating drought during 2015–18 known as the Day Zero drought due to the high risk of urban areas running out of piped water supply. Estimated data from the city show that major dam levels in the south Western Cape increased more than 1% in some cases after large rainfall events (LREs) in the summer of 2018/19. This increase is significant as dam levels often decrease by several percent per month during the hot summer. In this study, LREs over the south Western Cape during the summer (October–March) are investigated together with dam level data. Most summer LREs result from atmospheric rivers (ARs) or cutoff lows (COLs). ARs have not been previously studied in the South African region except for one study for winter that showed they are responsible for almost all the heavy rainfall events in the Western Cape. Although COLs are most common in the transition months, they can also occur in midwinter and summer. COLs tend to last longer and cover larger areas than ARs, which typically yield relatively short bursts of intense rainfall mostly concentrated around greater Cape Town. After each summer LRE, average dam levels increase by up to 5%, suggesting they are very important for drought recovery. In particular, summer LREs following the anomalously dry winters of 1980, 1984, 2003, 2004, and 2015–18 played an important role in mitigating those droughts.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Wade De Kock, dkcwad001@myuct.ac.za
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