A Climatology of Drylines in the Interior of Subtropical Southern Africa

Abstract

Abstract A climatology of synoptic drylines on the subtropical southern African interior plateau (SAP) is developed using ERA5 reanalysis specific humidity and surface temperature gradients and an objective detection algorithm. Drylines are found to occur regularly during spring and summer (September–March), and almost daily during December of that period, but rarely in winter. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November with a mean of 10 drylines and then over the central (mean of 12) and western SAP (mean of 20) in December. During midsummer, drylines over the eastern SAP are negatively correlated with drylines in the west. Between 1980 and 2020, a significant correlation exists between ENSO and dryline days over the eastern (r = 0.44; p value = 0.004) and central (r = 0.41; p value = 0.008) SAP with fewer drylines (up to 10) occurring during years with increased surface moisture and more drylines (up to 45) occurring during years with decreased surface moisture. Drylines forming over the eastern parts of the SAP were more likely to move westward than drylines over the central and western parts. Onset times across the SAP show that drylines have a tendency to form during either the late morning to early afternoon (1100 and 1400 LST) or during the early evening hours (1700 and 2000 LST), suggesting that the surface heat trough (Kalahari heat low) and westward moisture transport mechanisms, such as the Limpopo low-level jet and ridging highs, are responsible for the formation of most drylines across the SAP. Significance Statement “Drylines” are used to describe boundaries separating regions of very dry air from those with much higher moisture content. The importance of these drylines is that they tend to act as a trigger for thunderstorms, which can produce severe weather. In this study, we build a long-term climatological description of drylines in subtropical southern Africa. We find that drylines are most frequent over eastern South Africa during the early summer, a time when storms with large hail and damaging winds are most likely to occur. Drylines are sensitive to moisture circulation patterns and respond differently during El Niño and La Niña years, with generally more drylines during El Niño over eastern South Africa and fewer during La Niña.