Pronounced volcanic cooling and freshening in the northeastern Indian Ocean during the early 19th century

The Indian Ocean climate response to natural external forcing, such as volcanic eruptions, is still uncertain due to potential model biases and lack of validation from observations and subannual-resolution marine palaeorecords. Here we present monthly temperature and hydrology reconstructions derived from coral Sr/Ca and oxygen isotopes in the northeastern Indian Ocean back to 1774. Our reconstructions reveal anomalous and prolonged cooling and freshening during the early 19^th century (~1809-1824), which we attribute to a cluster of tropical volcanic eruptions that includes the unidentified 1809 and Tambora 1815 eruptions. The regional cooling and freshening were unusually strong compared to the wider Indian Ocean. The eruptions forced negative Indian Ocean Dipole (IOD)-like conditions in our reconstructions, followed by positive IOD-like conditions in subsequent years, regardless of eruption magnitude. Our results and other palaeorecords suggest positive IOD-like mean conditions during the early 19^th century, accompanied by stronger summer rainfall over areas of India and a negative Interdecadal Pacific Oscillation state, were associated with the regional cooling and freshening. Our findings highlight the sensitivity of the northeastern Indian Ocean to external forcing and that available observations, proxy records, and climate model simulations do not capture the full range of regional climate variability, complicating climate change projections for this highly populated region vulnerable to future climate extremes.