Two types of ridging South Atlantic Ocean anticyclones over South Africa and the associated dynamical processes

Abstract

Using 41 years of ERA5 reanalysis, two types of ridging South Atlantic Ocean high pressure systems were identified in the South African domain. Type-N events have a zonal structure and the ridging component breaks off from the parent South Atlantic Ocean anticyclone, after extending across the South African mainland. Type-S events extend south of the mainland and then break off. The Type-N (Type-S) ridging component is weaker (stronger) leaving behind a stronger (weaker) South Atlantic Ocean high. The two types of ridging events are associated with different configurations of Rossby wave packets that propagate across the South Atlantic Ocean. Surface and upper tropospheric anomalies associated with Type-S wave packets are stronger than those associated with Type-N events and the vertical coupling of the anomalies is much stronger during Type-S events. Type-N events are associated with a double jet streak structure, with the downstream jet contributing to upward motion over the landmass by means of its thermally direct circulation at its jet entrance. The upstream jet during Type-N events induces downward motion over the southern half of South Africa as it propagates eastward. The Type-S upstream jet streak, which only appears during winter, has limited zonal extent and does not induce downward motion over the country. Type-N ridging is associated with stronger ageostrophic moisture fluxes along the southern coast leading to higher moisture content and precipitation along the south eastern and eastern coasts of South Africa.