Coral extension rates in the NW Indian Ocean II: reconstruction of 20th century Indian monsoon strength and rainfall over India
To date, coral proxy studies have proved largely unsuccessful in reconstructing temporal variability of the Indian monsoon system in the NW Indian Ocean. In a recent publication in this journal, Storz and Gischler demonstrated that extension rates in corals can be used to fill this gap. Those authors found a link between decadal and interannual variations of the SW monsoon current velocities and Porites lutea extension rates in the lagoon of Rasdhoo Atoll (4°N/73°W) in the central Maldives for the period 1917–2007. This sister paper shows that this extension-rate record can be used to reconstruct decadal variations of summer monsoon rainfall over peninsular India and adjacent areas. The amount of monsoon rainfall and current velocities during summer are both affected by the strength of the Indian monsoon system. Assessments of coral extension rates and the mean May–September All India Monsoon Rainfall index, a measure of monsoon strength over the Indian subcontinent, revealed a significant spectral coherence within periods of 18–19 and 6–7 years. Almost 92% of variance is shared between both time series in the former band, and 85% in the latter. A correlation between the 5-year running mean extension rate and rainfall records from the Western Ghats (grid of 73–76°E/13–15°N) is significant especially for 1958–2006 (r = −0.82, p < 0.05). These findings imply that coral growth characteristics can serve as a new marine archive to reconstruct past variations of the Indian monsoon system on interannual to decadal timescales.
KeywordsMonsoon Rainfall Pacific Decadal Oscillation Coral Growth Simple Ocean Data Assimilation India Monsoon Rainfall
We thank the Deutsche Forschungsgemeinschaft for supporting this project with grant Gi 222/14. We are grateful to Dr. Harold Hudson (Miami), who drilled the cores and helped with all sorts of solutions to technical problems. Captain Haneef of Rasdhoo Island and his dhoni crew did a great job of boatmanship in Maldivian waters. We thank Dr. Reinhard Kikinger (Kuramathi Island Biostation) and Mr. Bill Allison (Male) for their invaluable help with logistics. Ms. Fareesha Adam and Dr. Shiham Adam of the Maldives government gave administrative support and helped with permit requirements. Ms. Eva Behrens and Mr. Renald Gless (Frankfurt) assisted during sample preparations. Finally, we thank Dr. Geert Jan van Oldenborgh, who is responsible for the development and upkeeping of the KNMI climate explorer in De Bilt, The Netherlands. Constructive reviews by C. Betzler, L. Montaggioni, and an anonymous referee proved useful in improving the original manuscript.
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