Abstract
AbstractSouthern 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.
Highlights
The southern “tropical edge” of the African rain band (16–22◦ S) is a hot spot of rainfall variability
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 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
Summary
The southern “tropical edge” of the African rain band (16–22◦ S) is a hot spot of rainfall variability. TTTs are elongated cloud bands that form over southern Africa and provide a large proportion of the subtropical summer rainfall to the south of the tropical edge, where annual rainfall is typically low (Harrison, 1984). Hart et al (2010) identify the importance of tropical lows in three case studies, while Macron et al (2014) highlight an increase in moisture flux around the Angola Low in lagged TTT composites. This relationship has not been studied on a synoptic time scale. A sensitivity analysis of the tropical low detection thresholds is provided in Appendix A
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