Coherent Indian Summer Monsoon and Sahel Rainfall Variability Revealed by Ethiopian Rainfall δ¹⁸O

Coherent fluctuations in Sahel and Indian summer monsoon (ISM) rainfall influence the socio-economic welfare of several densely populated regions. Here we identify a common mode of interannual rainfall variability over the Sahel and Indian regions and demonstrate the influence of both Pacific and Atlantic sea surface temperatures (SST) in inducing such a large-scale response. The principal component of the leading mode of precipitation variability (PC1) over these regions correlates independently well with both ISM and Sahel rainfall indices. Here we show that this global SST-driven dominant mode of Indo-Sahel rainfall variability is well captured in present-day rainfall δ¹⁸O and speleothem δ¹⁸O of the Ethiopian region. We use a ∼40-year-long monthly observational record of precipitation stable isotopes at Addis Ababa, Ethiopia to show that episodic isotopic depletions in rainfall at this site occur during the wet phases of PC1. These isotopic anomalies are associated with eastward intrusions of moisture initiating from the tropical Atlantic Ocean. Finally, we demonstrate decadal-scale co-variability between PC-1 and a century-long δ¹⁸O record from an actively growing Ethiopian speleothem to highlight the potential of rainfall isotopic records from this region to capture fluctuations in the coupled variability of the Indo-African boreal monsoons.