Decreasing Rainfall to Year 2100 - Role of the Congo Air Boundary (DRY-CAB)

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Climate change is expected to impose a considerable burden on many regions of the planet including semi-arid areas and where rainfall is vital for subsidence agriculture, hydropower and livelihoods. There are two large, land-based projected drying trends towards the end of the 21st century, one over the Mediterranean basin and the other over southern Africa. This proposal focuses on southern Africa. In early summer, the southern edge of the African tropical rain belt moves southwards from the Congo Basin bringing with it deep convective clouds which displace the clear skies and stable atmosphere that dominates the dry southern subtropical African winter. Climate models simulate a future delay in rainfall onset of several weeks and an associated drying in the early summer months of 30% over southern Africa. The drying is expected to occur over south eastern Angola, western Zambia, north east Namibia and much of the South African interior. There are several problems bound up with the projected southern African drying that require urgent attention so that confidence in the emergence of the change can be matched with timely adaptation measures. First, the atmospheric dynamics take place in a region with extremely sparse observations, particularly in the lower few kilometres of the atmosphere. Secondly, related to the sparse observations, our grasp of the dynamics is very poor compared with other parts of the inhabited tropics and subtropics. Despite being the third largest region of tropical convection, our understanding of the Congo Basin climate system is, at best, rudimentary. How the Congo Basin connects with and potentially controls the climate of the southern subtropics is not known. Meanwhile, climate change is afoot. The frequency of rain-bearing systems is currently in decline and is projected to decline in future decades. Exactly how these systems function at their tropical edge near the Congo Basin southern boundary is unknown. Third, what we do know about the dynamics of early summer drying depends entirely on numerical model simulations. This situation reduces the confidence with which the early summer drying ought to be embraced in adaptation measures given that we have no means of comparing the numerical models with the real world. Fourth, while there has been a step-change in model capability over Africa, e.g. through the 4.5 km convection-permitting model developed as part of the IMPALA project, observations with which to assess the new view of an old circulation problem, are not in place. This delays the adoption of novel, emerging views. At the end of the DRY-CAB project we will have the means to assess whether the tools we use for climate projection have the fidelity to simulate realistically the dynamical elements that control the progression of rainfall southwards in early summer. This proposal is an urgently needed and vital programme that focuses on the early summer drying dynamics through an ambitious observational campaign and matching suite of model analyses. The research concentrates on the interface between the tropical deep convection at the southern edge of the Congo Basin and the dry, stable air to the south. This juncture is the Congo Air Boundary (CAB). We propose a coordinated and flexible international field programme in order to gain the observations of the CAB urgently needed to confront the numerical models which are the sole source of our current understanding. The overall goal is to bring the matching confidence to the climate science that is required for bold adaptation decision making. The work takes place in one of two places in the world where widespread future drying is projected to occur. We expect that insights gained from the observed dynamics of the evolving edge of the tropical rain belt will be of value in other regions of the tropics.