Recent trends in air-sea CO2 fluxes and ocean acidification in the Indian Ocean

Abstract

<p>Recent observations and modeling evidence indicate that the Arabian Sea (AS) is a net source of carbon to the atmosphere, whereas the Bay of Bengal (BoB) and the southern Indian Ocean (SIO) act as net carbon sinks. Yet, the inter-annual variability modulating the air-sea CO<sub>2</sub> fluxes in the Indian Ocean, as well as their long-term trends, remains poorly understood. Furthermore, while the rising atmospheric concentration of CO<sub>2</sub> is causing surface ocean pH to drop globally, little is known about local and regional acidification trends in the Indian Ocean. Here, we simulate the evolution of air-sea CO<sub>2</sub> fluxes and reconstruct the progression of ocean acidification in the Indian Ocean from 1980 through 2018 using an eddy-resolving ocean biogeochemical model forced with observation-based winds and heat and freshwater fluxes. Additionally, using a set of sensitivity simulations that vary in terms of physical forcing we disentangle the contributions of climate variability and rising atmospheric CO<sub>2</sub> concentrations to long-term trends in air-sea CO<sub>2</sub> fluxes and acidification. Our analysis reveals a strong variability in the air-sea CO<sub>2</sub> fluxes and pH on a multitude of timescales ranging from the intra-seasonal to the decadal. Furthermore, a relatively fast progression of ocean acidification is simulated in the SIO, as well as in the Arabian/Persian Gulf and locally in the BoB. Our analysis indicates that in addition to the increasing anthropogenic CO2 concentrations in the atmosphere, wind variability - and more prominently- the recent fast surface warming of the Indian Ocean have substantially modulated trends in air-sea CO<sub>2</sub> fluxes and acidification at local and regional scales.</p>