Mechanisms behind early winter rainfall variability in the southwestern Cape, South Africa
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The southwest region of South Africa is the only part of southern Africa that predominantly receives its total annual rainfall during the austral winter months (April–September). In 2015–2017, this part of the country experienced extreme dry conditions which led to the severe water shortages experienced in the city of Cape Town. In this study, focused is placed on understanding the contribution of the early winter period (April–May) to wet and dry years in the southwestern part of South Africa. This period is of particular interest given its key role in the recent drought, the lack of previous work on this season, and climate change projections that the winter rainy season may shorten in duration. The early winter is found to be prone to dry conditions in recent decades, such that five of the six driest April–May in recent record have occurred after the year 2000. The dry early winters in particular tend to be associated with a weaker subtropical jet, less moisture flowing into the domain and a more stable atmosphere. It is found that although there is a moderate relationship between the Southern Annular Mode and early winter rainfall, it is not as strong as that compared to the full winter period. An analysis of CMIP5 models find that the projections portray the winter rainfall region in South Africa as being exposed to an increased likelihood of early winter dry conditions into the future (2040–2060). However, it remains a challenge for these models to reasonably capture the onset of winter rainfall in South Africa.
KeywordsSouth Africa Early winter rainfall Drought SAM CMIP5
The authors thank the South African Weather Service (SAWS) for providing the rainfall data. NCEP, GPCP, CMAP and OISST data were obtained from the NOAA/ESRL Physical Sciences Division, Boulder Colorado website at http://www.esrl.noaa.gov/psd/. We also acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling responsible for CMIP5 model data, which was provided by the Program for Climate Model Diagnosis and Intercomparison (PCMDI; https://pcmdi.llnl.gov/). MSLP observation data for the SAM analysis was provided by Gareth Marshall’s website (https://legacy.bas.ac.uk/met/gjma/sam.html). We thank Joseph Woohoo for her comments and the reviewers for their comments that helped improve the manuscript. And thanks to Nicolas Barrier for making his nbtools Python library available. Precious Mahlalela was supported by funding from the South African NRF and UCT Postgrad Funding Office for her MSc thesis. Ross Blamey and Chris Reason were supported by the Natural Environment Research Council (NERC) Future Climate For Africa (FCFA) regional consortium project ‘UMFULA’ (NE/M020223/1).
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