Abstract
During 2016–2017, the Indian Ocean experienced a pronounced increase in dissolved inorganic carbon (∼0.39 PgC/yr), approximately four times greater than the annual mean air–sea CO2 flux. Using a reconstructed data product and a state-of-the-art ocean biogeochemical model, we attribute this anomaly to an enhanced Southern Ocean inflow and a weakened Indonesian Throughflow associated with an El Niño event accompanied by a positive Indian Ocean Dipole (IOD), and followed by a negative IOD during the El Niño-to-La Niña transition. The resulting carbon accumulation leads to a decline in aragonite saturation and a shoaling of the aragonite saturation horizon in the southeastern Indian Ocean. This subsurface acidification may pose risks to deep-water calcifying organisms. Our findings demonstrate that ocean carbon storage and acidification are strongly modulated by circulation-driven transport processes, highlighting the need for improved subsurface observations and model capabilities to better capture the interior carbon response to climate variability.
Plain Language Summary
Between 2016 and 2017, the Indian Ocean stored a much larger amount of carbon than usual—about four times more than the typical annual exchange of carbon between the ocean and atmosphere. Using reconstructed observations and an advanced ocean model, we show that this unusual carbon buildup was caused by stronger inflow from the Southern Ocean and a weaker Indonesian Throughflow, driven by El Niño and negative Indian Ocean Dipole events. This extra carbon made the water more acidic and caused the depth at which aragonite (a mineral important for shell-building organisms) remains stable to rise by nearly 20 m in the southeastern Indian Ocean. These chemical changes could threaten deep-water organisms that rely on stable chemical conditions. Our results highlight how ocean currents can strongly affect carbon storage and acidification, and point to the need for better subsurface measurements and models to understand how climate variability impacts the ocean interior.
Key Points
- Indian Ocean carbon storage varied unprecedentedly in 2016–2017, driven by circulation anomalies linked to climate variability
- Anomalous dissolved inorganic carbon inventory was mainly due to increased Southern Ocean inflow and weakened Indonesian Throughflow
- Anomalous carbon redistribution caused subsurface acidification, shoaling aragonite saturation depth by ∼20 m in the southeast Indian Ocean
Zhang L. & Liao E., 2025. Unprecedented carbon accumulation in the Indian Ocean during 2016–2017. Geophysical Research Letters 52(20): e2025GL117199. doi: 10.1029/2025GL117199. Article.
