Abstract
Walker circulations near East Africa are identified and their influence on the interannual variability of East African rainfall is explored in multiple reanalyses and observational precipitation datasets. The robustness of methodology for identifying 2-dimensional overturning circulations in a three-dimensional flow is investigated. Three Walker circulations with potential relevance to East African rainfall are identified, namely, the East African, the Congo Basin, and the Indian Ocean Walker circulations. Consistent anti-correlations across the reanalyses exist for the upward and downward branches of the Indian Ocean Walker Circulation during September–March, with strongest connections during October–December; they do not emerge for the other two Walker circulations. Less (more) precipitation occurs over East Africa in the short-rains season when the Indian Ocean Walker Circulation is anomalously strong (weak). These rainfall variations are associated with anomalous mid-level moisture divergence that primarily results from variations in wind divergence rather than atmospheric moisture. The anomalous horizontal advection of moist static energy can be important in modulating convection, but this argument is not conclusive considering the unclosed budget and the variations within the composites. The associations above do not indicate causality. Caution is advisable when using the concept of Walker circulations because a two-dimensional stream function can over-simplify the complexity of the three-dimensional circulation.
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Acknowledgements
Support from NSF Award #1701520 is gratefully acknowledged. The HPC and database resources needed for the analysis were supplied by the Texas Advanced Computing Center (TACC) at The University of Texas at Austin. URL: http://www.tacc.utexas.edu. The authors thank Dr. Edward K. Vizy for general support and helpful discussions.
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Zhao, S., Cook, K.H. Influence of Walker circulations on East African rainfall. Clim Dyn 56, 2127–2147 (2021). https://doi.org/10.1007/s00382-020-05579-7
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DOI: https://doi.org/10.1007/s00382-020-05579-7