Evaluating future changes in the South Indian Ocean Convergence Zone projected by CMIP5 models and associated uncertainty

Abstract

Southern Africa relies heavily on precipitation for agricultural purposes; therefore, spatial and temporal changes in precipitation are crucial to identify and understand. The South Indian Ocean Convergence Zone (SIOCZ), a large-scale, austral summer rainfall feature extending across southern Africa into the southwest Indian Ocean, is evaluated in future projections. Using a best-fit algorithm, future projections of the SIOCZ are determined, which indicate a northward shift of approximately 120 km in CMIP5 models under RCP8.5. A dipole pattern of precipitation wetting/drying is evident, where wetting occurs to the north of the climatological axis of maximum rainfall, implying a northward shift of the Inter Tropical Convergence Zone, consistent with the SIOCZ shift. Common drivers responsible for model changes include enhanced warming in the northern Indian Ocean in line with the ‘warmest-get-wetter’ sea surface temperature hypothesis, which impacts circulation by transporting moisture away from the SIOCZ towards the equator. Most CMIP5 models exhibit drying trends over the SIOCZ region, with mechanisms driving uncertainty related to diverse warming trends across models. Empirical orthogonal function patterns of future precipitation changes across CMIP5 models exhibit a pattern much like the SIOCZ, which is related to inter-model changes in future temperature changes. Reductions in model spread are established in SIOCZ projections, whereby model processes of change exhibit agreement, despite differing initial SIOCZ conditions. Therefore, model process convergence and coherence are established with respect to projected changes in the SIOCZ, irrespective of initial climatology biases. Understanding future changes in this feature will help inform decision-making for water and agriculture adaptation planning in southern Africa.