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

• Published in: Journal of geophysical research. Atmospheres 2022-11, Vol.127 (22), p.n/a
• Main Authors: Midhun, M., Lekshmy, P. R., Thirumalai, Kaustubh, Ramesh, R.
• Format: Article
• Language: English
• Subjects: Anomalies; Coherence; Fluctuations; Geophysics; Global temperatures; isotope; Isotope ratios; Isotopes; Moisture effects; monsoon; Monsoon rainfall; Monsoons; Northern Hemisphere; Oxygen; Oxygen isotopes; Population density; Precipitation; Precipitation variability; Rainfall; Rainfall variability; Remote control; Sea surface; Sea surface temperature; speleothem; Stable isotopes; Summer; Summer monsoon; Summer rainfall; Surface temperature; Variability; Wind
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2022JD037160

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. Plain Language Summary Indian summer monsoon and Sahel summer rainfall exhibit substantial co‐variability on year‐to‐year and longer timescales. In this study, we identified a coherent mode of variability in Northern Hemisphere summer rainfall across these regions, which is remotely controlled by global‐scale sea surface temperature (SST) variability. Using long‐term (>40 years) rainfall oxygen isotope observations from Addis Abada, Ethiopia, we show that oxygen isotopic ratios in Ethiopian rainfall record these SST‐related variations in Indian and African boreal summer monsoons. We demonstrate the application of our hypothesis using a century‐long speleothem oxygen isotope record from an Ethiopian cave. These new findings offer possibilities for reconstructing this particular mode of variability of Indo‐African boreal summer monsoons from oxygen isotope‐based proxies from the Ethiopian region. Key Points Ethiopian rainfall δ18O captures coherent Indo‐African rainfall variability Enhanced westerly moisture transport depletes Ethiopian rainfall δ18O during strong monsoon years Century‐long speleothem δ18O from Ethi