Tropical–Extratropical Interactions over Southern Africa: Three Cases of Heavy Summer Season Rainfall

Abstract

The synoptic evolution of three tropical-extratropical (TE) interactions, each responsible for extreme rainfall events over southern Africa, is discussed in detail. Along with the consideration of previously studied events, common features of these heavy rainfall producing tropical temperate troughs (TTTs) over southern Africa are discussed. It is found that 2 days prior to an event, northeasterly moisture transports across Botswana, set up by the Angola low, are diverted farther south into the semiarid region of subtropical southern Africa. The TTTs reach full maturity as a TE cloud band, rooted in the central subcontinent, which is triggered by upper-level divergence along the leading edge of an upper-tropospheric westerly wave trough. Convection and rainfall within the cloud band is supported by poleward moisture transports with subtropical air rising as it leaves the continent and joins the midlatitude westerly flow. It is shown that these systems fit within a theoretical framework describing similar TE interactions found globally.Uplift forcing for the extreme rainfall of each event is investigated. Unsurprisingly, quasigeostrophic uplift is found to dominate in the midlatitudes with convective processes strongest in the subtropics. Rainfall in the semiarid interior of South Africa appears to be a result of quasigeostrophically triggered convection.Investigation of TTT formation in the context of planetary waves shows that early development is sometimes associated with previous anticyclonic wave breaking south of the subcontinent, with full maturity of TTTs occurring as a potential vorticity trough approaches the continent from the west. Sensitivity to upstream wave perturbations and effects on anticyclonic wave breaking in the South Indian Ocean are also observed.

A multiscale framework for the origin and variability of the South Pacific Convergence Zone

“…These mechanisms are also likely to be relevant to the weaker diagonal convergence zones found in the other ocean basins: the South Atlantic Convergence Zone (SACZ: Pohl et al, 2009;Hart et al, 2010) and the South Indian Convergence Zone (SICZ: Cook, 2000).…”

Evaluating Climate Models with an African Lens

“…A large proportion of southern African rainfall is associated with TTT systems (Harrison 1984), particularly heavy rainfall (Hart et al 2013). TTTs are therefore a good target for evaluation in terms of impact relevance and might also be an indicator of the credibility of modeled regional dynamics: the location and character of tropical convection, storm tracks, subtropical highs, and the Angola low might all influence the formation of TTCBs (Todd et al 2004;Fauchereau et al 2009;Hart et al 2010;Ratna et al 2013;Macron et al 2014).…”

“…This may indicate that the character of the events in the model, and the mechanisms for generating cloud bands, are different from those found in nature. For example, TTTs are known to be associated with large meridional perturbations in the upper-tropospheric flow in the region (Hart et al 2010;Manhique et al 2011). Hart et al's (2012) tool could be used to unlock the necessary sampling base to analyze these baroclinic waves and other associated circulation features, to better understand the TTT events identified in the model data, and to assess the plausibility of the mechanisms for cloud band formation.…”

A synoptic decomposition of rainfall over the Cape south coast of South Africa

“…Taljaard 1985;Tennant and van Heerden 1994;Katzfey and McInnes 1996;Favre et al 2012), in combination with tropical-temperate troughs (Hart et al 2013), and further, as a COL system evolves, in association with the evolving high pressure system (ridging progressively from the southwest to southeast of South Africa). The typical low-level circulation associated with tropical-temperate troughs-a meridional trough that links the Angola low and a mid-latitude wave/cyclone, is illustrated in Tennant (2003) and Hart et al (2010Hart et al ( , 2012). An objective tracking methodology is therefore used in order to study the effects of these important rainfall producing systems on rainfall attributes over the Cape south coast region.…”