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Africa: Coupled regional modelling of land-atmosphere-ocean interactions over western-southern Africa and climate change

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The climate system of western-southern Africa is highly coupled and exhibits intricate interactions between the land, atmosphere and ocean. The dominating biome is the African savannahs, where complex tree-grass interactions are shaped by fire and land-use.

It has been postulated that under future climate change rising CO2 levels and increasing temperatures may favor trees over grasses in the savannah, leading to bush encroachment at the expense of biodiversity and grazing potential. However, given that that much of southern Africa is likely to become drier and significantly warmer under climate change (a conclusion of the Africa Chapter of AR5) it is plausible that mega-fire events may occur more frequently in savannahs, effectively favoring grasses over trees.

Complex dynamic vegetation models including realistic descriptions of savannah fire events, coupled to RCMs, are required to gain more insight into the future of the African savannahs under climate change. Fire in the savannahs, and the consequent release of biomass-burning aerosols into the atmosphere, are potentially important towards reducing uncertainties regarding one of the most intriguing open questions in climate science: climate sensitivity. The role of these aerosols on the stratocumulus (Sc) cloud deck over the Atlantic remains largely unexplored, but given that the southern African region is the largest source of biomass-burning aerosols in the world, it is a potentially important role.

Prognostic aerosols schemes in RCMs, combined with suitably sophisticated microphysical schemes will enable the study of both aerosol direct and indirect effects on the Sc cloud deck. Of particular interest is the role of climate change and land-use change on biomass-burning, with potential consequences for the Sc cloud deck and therefore climate sensitivity, through altered aerosol attributes and aerosol transport patterns.

Another fascinating aspect of the western-southern African climate system is the Benguela upwelling regime on the western coastal areas of South Africa and Namibia, and the related occurrence of Benguela-Niño events along the African west coast. Potential changes in this upwelling regime under climate change will be of crucial importance to ocean productivity and fisheries along these coastal areas.

Moreover, the rich biodiversity of the Namib desert depends largely on the inland penetration of fog as a source of water, and the formation of such fog events in turn depend on the location of upwelling patterns. High-resolution and coupled RCMs are needed to project the effects of regional climate change on the prevailing southeasterlies that blow along the west coast of southern Africa, and how such changes may impact on the formation of upwelling zones and fog. In fact, the more realistic simulation of African west-coast upwelling may be important towards reducing the significant biases most GCMs exhibit in simulating SSTs along the African west coast.

Similarly, the more realistic simulation of aerosol and Sc radiative effects may result in reducing this problem that has plagued GCMs in both AR4 and AR5. The above arguments build a convincing case to apply coupled RCMs to the western-southern African region, towards tackling a key open problem in climate science, reducing one of the most important biases common to current climate models, and to better anticipate and prepare for climate change impacts on this fascinating and complex coupled climate regime.

Contact person:
Francois Engelbrecht  francois.engelbrecht@wits.ac.za

Reports:

Presentations:
Presentation from ICRC-CORDEX2019

SAT-member:
Fredolin Tangang