Multi-decadal oscillations in the hydro-climate of the Okavango River system during the past and under a changing climate

SummaryThe focus of this paper is to understand the multi-decadal oscillatory component of variability in the Okavango River system, in southwestern Africa, and its potential evolution through the 21st century under climate change scenarios. Statistical analyses and hydrological modelling are used to show that the observed multi-decadal wet and dry phases in the Okavango River and Delta result from multi-decadal oscillations in rainfall, which are likely to be related to processes of internal variability in the climate system, rather than external natural or anthropogenic forcing. Analyses of changes in this aspect of variability under projected climate change scenarios are based on data from a multi-model ensemble of 19 General Circulation Models, which are used to drive hydrological models of the Okavango River and Delta. Projections for the 21st century indicate a progressive shift towards drier conditions attributed to the influence of increasing temperatures on water balance. It is, however, highly likely that multi-decadal oscillations, possibly of similar magnitude to that of 20th century, will be superimposed on the overall trend. These may periodically offset or amplify the mean drying trend. This effect should be accounted for in water and catchment management and climate change adaptation strategies.

► Multi-decadal oscillations in the Okavango River basin, southwestern Africa.
► Weak multi-decadal rainfall signal magnified by catchment processes.
► GCMs simulate multi-decadal oscillations in 20th century.
► Oscillations in 21st century similar in magnitude but superimposed on drying trend.
► Multi-decadal oscillations affect climate change assessments and require alternative water management approaches.