Characteristics of cut-off lows during the 2015-2017 drought in the Western Cape, South Africa

Cut-Off Lows (COLs) are known for heavy rainfall in the Western Cape, but there is a dearth of information on COL rainfall characteristics over the Cape. To bridge this gap we analysed three types of datasets (observation, satellite and reanalysis) to study the characteristics of COLs that occurred in the vicinity of the Western Cape over 37 years (1981-2017) and applied a self-organising Map (SOM) to classify the COLs into groups based on their rainfall patterns. The results show about 10 COLs (per year) occur over the Western Cape and contribute about 11% of the annual rainfall in the cape but with a large inter-annual variability. In 2015 and 2016, the COLs occurred more frequently and contributed more rainfall than normal which reduced the drought severity. But, in 2017, the frequency and rainfall contribution of the COLs were below normal. Nevertheless, more COLs does not always mean higher COL rainfall as > 45% of the COLs over the Western Cape produce little or no rainfall. The SOM results reveal that the spatial distribution of COL rainfall can be grouped into four major patterns. The first pattern indicates rainfall over the entire Western Cape, while the second shows little or no rainfall; the third and fourth patterns feature rainfall over the south-east coast and south-west coast, respectively. Wet COLs are associated with more southward transport of warm, moist tropical air into the Western Cape and greater contrasts between the warm and cold air masses compared to the dry COLs.