17th May 2017

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  • The Western Cape winter rainfall region is experiencing a critical drought with significant social and economic impacts.  Extremely low reservoir levels are at least partly a result of an accumulated 2 to 3 years of relatively low winter rainfall (characterised by an unusual distribution of rainfall in space and time) and which has limited the extent to which reservoirs have been replenished during the winter season.  The role of demand management and allocations is outside of our remit however there is no doubt that this has had an influence on the current crisis.

    The scientific challenge we face is whether a dependable and consistent seasonal climate forecast can be produced in the sub-region which would assist with short to medium term planning and longer term water security. The science of Seasonal Climate Prediction (SCP) has been applied in South Africa with moderate success in the north east of the country (summer rainfall region), but is limited and constrained in its efficacy to only certain climate conditions and periods. Given that the Western Cape is a winter rainfall region (with dry summers) the challenge is specifically to develop a robust system for prediction of the extent of winter rainfall and its likely departure (positive or negative) from long term averages.

    The scientific community do not unanimously agree on how to select, assess the performance of and interpret the signal of the wide range of SCP climate modelling systems available which provide long range forecasts for the winter month in the Western Cape. This is due to the fact that there is a range of methods to make these determinations in a highly complex climate system. Some scientists argue that there is sufficient skill (i.e. a technical term meaning accuracy and consistency) to venture a statistical probability of a range of possibilities (i.e. below average, average or above average rainfall) while others content that no existing forecast system, either locally or internationally, has sufficient skill for predicting winter rainfall for the Western Cape operationally. The meeting has considered this debate and agree that a working group will be set up to develop a consensus on how to address this challenge.

    While the research literature (including work completed here in South Africa) details some complex relationships with various regional and global scale climate drivers, these do not necessarily translate into scientifically defensible predictive skill (though with further investment in the area they may).  Therefore whether it is currently possible to produce a scientifically defensible and robust prediction of the seasonal winter rainfall for the Western Cape is disputed.

    For the purposes of this meeting forecasts from those researchers who have opted to publish their individual seasonal forecast can be accessed in the presentations below:

    University of Pretoria

    South African Weather Service

    Given this situation, the Western Cape needs to consider the full range of possibilities which comprise  that the 2017 winter season may be drier than normal (which may greatly exacerbate the current situation) or normal (which may fail to relieve the current situation) or wetter than normal (which may bring relief to the current situation).  At the rate of rainfall replenishment equal to that of an average winter rainfall season, it will take more than one season to completely replenish the deficit of surface water reserves in the region. It should be noted that the above are not intended as forecasts for the coming winter season, but merely outlines the different possible outcomes. Users interested in specific outlooks for the upcoming winter season are advised to contact SAWS, the CSIR or UP.

    On the longer time scale of climate change (decades and beyond) it is important to note that  seasonal cycles and natural cycles of relatively wetter and driers winters (including multi-year drought) will continue.  However, there is a weight of evidence provided by a large body of published research, adequately assessed in the most recent Intergovernmental Panel on Climate Change (United Nations) assessment, as well as information provided by the CMIP5 climate model archive, to support the reasonable expectation of a relatively drier future Western Cape by the 2030s to 2040s. This means that while seasonal and inter-annual variability will remain a feature of our climate, a shift in the range of that variability is anticipated. This is well detailed in numerous reports including the final draft of the  United Nations Framework Convention on Climate Change (Third National Communication) and recent reports for the City of Cape Town.  There is little doubt that the Western Cape needs to prepare, in the longer term, for a drier climate.  Research does continue on the finer details of these long term changes and in particular on aspects of large-scale modes of variability and mountain climate which are relevant, but currently this work remains in the domain of research rather than informing decision making.

    *The ACCESS programme is a NRF/DST funded national partnership among a variety of research institutions and universities whose aim it is to bring together the respective researchers and role-players in climate and earth systems science.