Circulation anomalies associated with mid- to late summer rainfall differences during multiyear La Niña events over subtropical southeastern Africa

Abstract More than half of La Niña occurrences are multiyear events, lasting for at least two consecutive summers and tend to be associated with above average summer rainfall over southern Africa. Over southeastern Africa, it is found that double year events exhibit wet conditions much less consistently than triple year events during mid to late austral summer. Here, seasonal and sub-seasonal composites of atmospheric fields are used to examine the circulation anomalies associated with differences in various rainy season characteristics between the double and triple summer La Niña events over two domains in southeastern Africa. During double summers, recurving of the northeasterly monsoon towards the tropical Indian Ocean leads to less moisture transport into southeastern Africa while the presence of a strong Mozambique Channel Trough traps moisture in the northern Mozambique Channel, leading to drier conditions on the mainland. By contrast, triple summers are characterised by a stronger Mascarene High that weakens the monsoonal recurving and strengthens easterly moisture fluxes into subtropical southeastern Africa. Furthermore, a stronger Angola low and widespread cyclonic anomaly over the subcontinent draw moisture from the southwest Indian Ocean resulting in wet conditions. On sub-seasonal scales, particular severe weather systems make major contributions to the anomalously wet triple summers in some cases (e.g., April 2022) for eastern South Africa and February 2023 for southern Mozambique).