El Nino-Southern Oscillation and internal sea surface temperature variability in the tropical Indian Ocean since 1675

The dominant modes of climate variability on interannual timescales in the tropical Indian Ocean are the El Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole. El Nino events have occurred more frequently during recent decades, and it has been suggested that an asymmetric ENSO teleconnection (warming during El Nino events is stronger than cooling during La Nina events) caused the pronounced warming of the western Indian Ocean. In this study, we test this hypothesis using coral Sr/Ca records from the central Indian Ocean (Chagos Archipelago) to reconstruct past sea surface temperatures (SSTs) in time windows from the mid-Little Ice Age (1675-1716) to the present. Three sub-fossil massive Porites corals were dated to the 17-18th century (one coral) and the 19-20th century (two corals). Their records were compared with a published modern coral Sr/Ca record from the same site. All corals were subsampled at a monthly resolution for Sr/Ca measurements, which were measured using a simultaneous inductively coupled plasma optical emission spectrometer (ICP-OES). Wavelet coherence analysis shows that interannual variability in the four coral records is driven by ENSO, suggesting that the ENSO- SST teleconnection in the central Indian Ocean has been stationary since the 17th century. To determine the symmetry of El Nino and La Nina events, we compiled composite records of positive and negative ENSO-driven SST anomaly events. We find similar magnitudes of warm and cold anomalies, indicating a symmetric ENSO response in the tropical Indian Ocean. This suggests that ENSO is not the main driver of central Indian Ocean warming.

Automated summary

The study reconstructs past sea surface temperatures in the central Indian Ocean using coral Sr/Ca records from the Chagos Archipelago, showing that ENSO drives interannual variability in coral records and the ENSO-SST teleconnection has been stable since the 17th century. Composite records of positive and negative ENSO-driven SST anomaly events reveal a symmetric ENSO response in the tropical Indian Ocean.