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

Abstract

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.

Using seasonal rainfall clusters to explain the interannual variability of the rain belt over the Greater Horn of Africa

“…Howard and Washington (2018) have shown that the characteristics of the AL can be separated into heat and tropical low phases. Howard et al (2019) further investigated the relationship between the tropical low activity and precipitation over southern Africa. Their two circulation patterns of tropical low activity across the southern African continent agree with the two dominating clusters during the Msimu Rains in our work.…”

Attributing correlation skill of dynamical GCM precipitation forecasts to statistical ENSO teleconnection using a set-theory-based approach

“…The green color indicates grid cells where significantly positive anomaly correlation is attributable to negative ENSO teleconnection. They appear in northern South America and southern Africa, where negative ENSO teleconnection is known to exist (Cai et al, 2020;Howard et al, 2019) and also in parts of Far East and Alaska. There are also grey areas where significantly positive anomaly correlation is not attributable to ENSO teleconnection.…”

Attributing correlation skill of dynamical global precipitation forecasts to statistical ENSO teleconnection using a set theory based approach

“…The green color indicates grid cells where significantly positive anomaly correlation is attributable to negative ENSO teleconnection. They appear in northern South America and southern Africa, where negative ENSO teleconnection is known to exist (Howard et al, 2019;Cai et al, 2020) and also in parts of Far East and Alaska. There are also gray areas where significantly positive anomaly correlation is not attributable to ENSO teleconnection.…”