700 Years of Indian Ocean Basin and Dipole Modes Reconstructed Through Data Assimilation

Sea surface temperature (SST) variability in the Indian Ocean significantly influences global and regional climate, economy, and livelihoods, characterized by two primary modes: (a) the Indian Ocean Basin‐Wide mode (IOBW) and (b) the Indian Ocean Dipole (IOD). However, previous coral‐based SST reconstructions have been limited by the short temporal span of proxy records, leading to issues of discontinuity and insufficient coverage. This study utilizes a data assimilation approach to integrate model outputs with targeted selected and rigorously validated paleoclimate records from the Indian Ocean region and adjacent areas. By concentrating on proxies specifically related to Indian Ocean SST variability, this method enables the reconstruction of annually resolved IOD and IOBW indices for the period 1300–2000. The results reveal significant interannual variability in IOD and IOBW over the past 700 years, with variability intensifying since the Industrial Revolution and nearing peak levels associated with natural variability. Statistical results show a significant increase in IOD events in the late 20th century, with positive IOD (pIOD) frequency being the second highest in the past 700 years, potentially linked to anthropogenic global warming. Superposed Epoch Analysis indicates that volcanic activity has led to a cooling in the Indian Ocean SST during eruption years (negative IOBW anomaly), followed by an increased likelihood of pIOD events the following year. These continuous reconstructions extend instrumental records and provide important insights into the mechanisms of Indian Ocean interannual variability.