Tropical Lows in Southern Africa: Tracks, Rainfall Contributions, and the Role of ENSO.

Southern African tropical lows are synoptic‐scale cyclonic vortices that propagate westward across southern Africa in the Austral summer. They strongly influence local rainfall and aggregate in the climatological December, January, and February mean to form the Angola Low. In this study, tropical lows are identified and tracked using an objective feature tracking method. The statistics of tropical low tracks over southern Africa are presented and compared across three reanalysis products. Findings are compared to the literature of tropical low‐pressure areas elsewhere in the world, where it is found that most tracking statistics compare well but that the tendency of tropical lows to become semistationary over Angola is unique to southern Africa. The hypothesis that tropical lows in Angola have a causal relationship with Tropical Temperate Troughs is tested, and a correlation between occurrence frequencies is found at interannual but not daily time scales. Precipitation is attributed to the tropical lows, and it is found that tropical lows are associated with 31% of rainfall across tropical southern Africa, based on gridded precipitation products. The interannual variability of the number of tropical lows that form per year (σ=6 events/year) is linked to El Niño–Southern Oscillation (ENSO) and the tropical easterly jet. The mean latitude of tropical lows is shifted northward during El Niño and southward during La Niña. Much of the interannual precipitation variability maximum in Angola is attributed to rainfall associated with tropical lows. These results provide insights into the southern African response to ENSO and into the mechanisms of rainfall in the southern African tropical edge. Plain Language Summary: Tropical lows are storms that are a bit like very weak tropical cyclones. They occur in summer in tropical regions. This paper tracks individual tropical lows across southern Africa in weather records from the last 39 years. We then look at how they behave, how much they rain, whether they interact with other storms, and how they vary from year to year. About 33 tropical lows are found each year, and they tend to cluster in eastern Angola. We find that 31% of rainfall in the region surrounding Angola and Zambia comes from tropical lows. The number of tropical lows that form in a summer is really important for whether that summer will be unusually dry or wet, particularly along the northern border of Namibia. The number of tropical lows that form in each year is related to the El Niño–Southern Oscillation and also to the winds high up in the atmosphere. In the El Niño phase of El Niño–Southern Oscillation cycle, which usually causes drought in southern Africa, tropical lows form less frequently and further north. In the La Niña phase, which is usually wet in southern Africa, tropical lows form more frequently and further south. Key Points: Vorticity tracking and rainfall attribution algorithms are applied to reanalysis tropical lows in southern AfricaTropical lows track north (south) in El Niño (La Niña); this behavior dominates the ENSO rainfall signal west of 30°EOn a synoptic time scale tropical temperate troughs and Angola tropical lows have statistically independent timing [ABSTRACT FROM AUTHOR]