Rainfall dynamics and change in East Africa

East Africa is vulnerable to changes in rainfall because of its reliance on agriculture, which supports livelihoods and drives economic development, combined with reduced adaptive capacity. Historical rainfall conditions have included an overall drying trend in the March to May season, called the long rains, as well as extreme flooding events. However, the mechanisms controlling rainfall variability in East Africa are not yet fully understood, and global climate models do not have adequate representation of present-day conditions. The models on average underestimate rainfall totals during the long rains and overestimate them during the short rains of October to November. Future projections from these models are also uncertain; the models disagree on the sign and the magnitude of changes, and the ensemble mean rainfall increase in the long rains is in opposition to recent observed drying. It is therefore difficult to plan for climate change adaptation, as neither a wetter nor a drier future can be confidently predicted. To address these issues, this thesis investigates the physical processes that affect East African rainfall on different spatial scales, and their representation in global climate models. The Indian Ocean Walker Circulation (IOWC) is an important large-scale feature which drives descending air over East Africa, and the strength of this descent is negatively correlated with rainfall totals in both rainy seasons (r = -0.57 - -0.85, p<0.05). It is also a source of model rainfall biases, as models with more (less) intense Walker Circulations produce less (more) rainfall over Kenya (r ~ 0.8, p<0.05). The Turkana Low-Level Jet (LLJ) is a meso-scale feature which affects regional rainfall through a variety of mechanisms. Its strength is negatively correlated with rainfall in Kenya, but positively correlated with rainfall in the continental interior (r = ± 0.6, p<0.1). Correlations between the LLJ strength and the large-scale zonal pressure gradient imply a link between the IOWC and the LLJ (r =± 0.5, p<0.1) The LLJ may be decreasing in strength on decadal timescales (-0.4 - -0.8 ms -1 decade-1, p<0.05), consistent with observed climate change, but reanalysis representations of its variability and annual cycle differ. It is not adequately simulated in models as a result of simplified topography representation, and thus further contributes to the uncertainty in projections; given that the IOWC is also constrained by topography, this is suggested as a key focus for future model development. Future projections of increasing rainfall over Kenya are associated with increasing ascent over East Africa (r ~-0.8, p<0.05) and enhanced easterly winds over the Indian Ocean (r~-0.5, p<0.05), reflecting a weakening of the IOWC. Using an assessment of the ability of models to represent the IOWC to constrain future projections, it is found that models with more realistic regional dynamics only feature significant increasing rainfall trends in April, November, and December. It is therefore concluded that the ensemble mean rainfall increase across the long rains may not be plausible, that the long rains may contract in future, and that rainfall increases in the short rains are more likely.

Keywords: Kenya
East Africa
rainfall
climate science
CMIP5
climate
climate modelling
Advisor: Washington, R
Engelstaedter, S