The flow and moisture fluxes associated with ridging South Atlantic Ocean anticyclones during the subtropical southern African summer

Abstract

Using 38 years of ERA-Interim reanalyses, this study examines the flow and associated moisture fluxes induced by ridging South Atlantic Ocean anticyclones over South Africa. The flow and moisture fluxes are divided into their geostrophic and ageostrophic components. Composite analysis reveals that ridging high pressure systems are modulated by Rossby wave trains that develop upstream in the middle latitudes near South America and the South Atlantic Ocean near the Greenwich Meridian. The wave trains are similar to those that are associated with tropical temperate troughs (TTTs) and nearly all TTTs are followed by ridging, whilst 20% of the ridging events are linked to TTTs. Composite analysis also shows that ridging-induced moisture fluxes affecting South Africa originate from different areas of the surrounding oceans at different times during the evolution of ridging highs. In the early stages, moisture enters the country along the southeastern coast through ageostrophic processes from a moisture divergence region located adjacent to the coast and remains largely south of 30°S latitude line. The moisture fluxes have a northeastern orientation on land, following the geometry of the eastern coast. These ageostrophic fluxes contribute to the occurrence of rainfall during the ridging process in the region located between Lesotho and Swaziland. Ridging events are associated with about 60% of rainfall days in summer over southern Africa whilst TTTs contribute about 21% summer rainfall days. During the later stages of the evolution of ridging, geostrophic fluxes enter the southern parts of Mozambique and contribute the accumulation of moisture in that region. The associated moisture divergence region is always located ahead of its ageostrophic counterpart, with a local maximum eventually forming in the Mozambique Channel.