Understanding the present status of the Indian Ocean Acidification and its driving mechanisms

Long-term effects of global climate change and increased anthropogenic CO2 uptake make the Indian Ocean susceptible to ocean acidification. Several studies have projected a decline of upper ocean pH by 0.3-0.4 by the end of the 21st century, which has the potential to reduce oceanic biological production considerably. There is a critical need to understand the present status of Indian Ocean acidification and identify its key drivers. However, the number of spatially and temporally varying available observations to examine the present state of Indian Ocean acidification is limited. The numerical ocean models have a unique ability to integrate our empirical and theoretical understanding of the marine environment. Therefore, the changes in the Indian Ocean seawater pH in response to the changes in sea-surface temperature (SST), sea-surface salinity (SSS), dissolved inorganic carbon (DIC), and total alkalinity (ALK) over the period 1980-2019 and its driving mechanisms has been carried out using a high-resolution regional ocean-ecosystem model outputs. 

The analysis indicates that the rate of change of declining pH in the Arabian Sea (AS), the Bay of Bengal (BoB), and the Equatorial Indian Ocean (EIO) is -0.014 ± 0.002, -0.014 ± 0.001, and -0.015 ± 0.001 unit dec-1, respectively. In the AS (BoB), the highest decadal DIC trend is found in 2000-2009, whereas it is lower in 1990-1999 and 2010-2019, but, in the case of EIO, we find it opposite. Ocean acidification is seen to have accelerated throughout the IO region during 2010-2019 as opposed to the previous decades. Further, our analysis indicates that El Niño, followed by a positive Indian Ocean Dipole, increases acidification in the Indian Ocean. The increasing anthropogenic CO2 uptake by the ocean dominantly controls 79.97% (94.54% and 85.72%) of the net pH trend (1980-2019) in AS (BoB and EIO), whereas ocean warming controls 14.39% (13.38% and 7.02%) of pH trends in AS (BoB and EIO). The changes in ALK contribute to enhancing the pH trend of AS by 5.0%. ALK dominates after DIC in the EIO and, similar to AS, contributes to enhancing ocean acidification by 10.67%. In contrast, it has a buffering effect in the BoB, suppressing the pH trend by -5.4%. In summary, this research work consolidates the current level of understanding about the Indian Ocean acidification based on the available field observations, reconstructed data sets, and model simulations.