Coupled climate model simulation of Tropical-Extratropical Cloud Bands over southern Africa

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ABSTRACT

There are increasing efforts to use climate model output for adaptation planning, meanwhile there is often limited understanding of how models represent regional climate. Here we analyse the simulation in global coupled climate models of a key rainfall-generating mechanism over southern Africa: Tropical-Temperate Troughs (TTTs). An image-processing algorithm is applied to outgoing longwave radiation data from satellites and models to create TTT event sets. All models investigated produce TTTs with similar circulation features to observed. However, there are large differences among models in the number, intensity, and preferred longitude of events. Five groups of models are identified. The first group generates too few TTTs, and relatively dry conditions over southern Africa compared to other models. A second group generates more TTTs and wet biases. The contrast between these two groups suggests that the number of TTTs could explain inter-model variations in climatological rainfall. However, there is a third group of models which simulate up to 92% more TTTs than observed, but do not have large rainfall biases, as each TTT event is relatively weak. Finally, there are a further two groups which concentrate TTTs over the subcontinent or the ocean respectively. These distinctions between models are associated with the amount of convective activity in the Congo Basin, the magnitude of moisture fluxes into southern Africa, and the degree of zonal asymmetry in upper-level westerly flow. Model development focused on tropical convection and the representation of orography is needed for improved simulation of TTTs, and therefore southern African rainfall.