Asymmetric carbon response to the 2019 extreme positive Indian Ocean Dipole

In 2019, one of Indonesia’s most devastating bushfires burned about 3.1 million hectares of forests and peatlands, releasing approximately 708 million tonnes of CO2. The fires were driven by an extreme positive Indian Ocean Dipole (pIOD), which strengthened easterly winds and brought cooler and drier air to the region, creating severe drought conditions. These conditions triggered widespread forest fires across Indonesia. Simultaneously, the same easterly winds displaced warm, nutrient-poor surface waters westward in the eastern equatorial Indian Ocean (EEIO), enhancing upwelling of nutrient-rich deep waters. This process fueled a massive phytoplankton bloom extending over 1000 km. In addition, nutrient deposition from aerosols emitted by the Indonesian fires further enriched the bloom. Together, oceanic upwelling and atmospheric deposition boosted the biomass of large phytoplankton. Satellite estimates suggest that the bloom sequestered approximately 40.19 Tg of carbon, equivalent to 10.1% of the CO2 released by the Indonesian fires, into the deeper ocean. In contrast, air-sea CO2 flux anomalies indicate negligible net atmospheric CO2 uptake, with only 0.05% (0.33 Tg CO2) of the emissions reabsorbed, due to the upwelling-driven dissolved inorganic carbon (DIC)-rich waters. Together, these results highlight a coupled but asymmetric response to climate extremes. The pIOD can simultaneously intensify terrestrial carbon losses through fires, while upwelling enhances the biological pump through blooms. However, the strength of the sequestration was offset by the DIC-rich waters from the upwelling. Understanding this dual impact is crucial for modelling future climate scenarios and assessing the long-term impacts of climate variability on global carbon cycles.