Tropical ocean responses to large volcanic eruptions during the last millennium

Volcanic eruptions are known to have been a major natural source of climate modulation during the last millennium. Climate model simulations indicate that large eruptions cause air and ocean surface temperatures to decrease for up to a decade, and increase the likelihood of co-occurring El Niño and positive Indian Ocean Dipole events in the 12 months immediately following an eruption. However, models are known to have biases in their representation of the El Niño-Southern Oscillation and the Indian Ocean Dipole, and the unknown seasonal timing of pre-historic eruptions is also expected to modulate their climate response. Here we use the unambiguous signatures of the three largest eruptions of the last millennium recorded within a precisely dated and monthly resolved spliced fossil coral record of past ocean variability from the equatorial eastern Indian Ocean to assess the seasonal timing of these past eruptions and their impacts on tropical climate. The historic Tambora eruption in April 1815, which caused a well-documented "year without a summer" in Europe, is evident by a sustained ¹³C depletion signal in the coral record, and resulted in peak cooling in the equatorial eastern Indian Ocean 3 years after the eruption. The combination of precise U/Th ages and similar distinctive δ¹³C anomalies allow us to precisely locate the Samalas and Kuwae eruptions in the coral sequences, and to use the monthly-resolved coral data to determine for the first-time the likely seasonal timing of these exceptionally large eruptions. In each case peak cooling in the equatorial eastern Indian Ocean occurs at around 3-years post eruption and the seasonal cycle of coral δ¹⁸O indicates that an El Niño event in the Pacific likely occurred in the year immediately following the eruption. There is no evidence, however, in the coral data of co-occurring positive Indian Ocean Dipole events in the year immediately following large tropical eruptions, suggesting that this feature of climate simulations may be affected by model biases that produce positive Indian Ocean Dipole events too frequently compared with observations.