Summer trends and drivers of sea surface fCO2 and pH changes observed in the southern Indian Ocean over the last two decades (1998–2019)

The decadal changes of the fugacity of CO2 (fCO2) and pH in surface waters are investigated in the Southern Indian Ocean (45° S–57° S) using repeated summer observations, including measurements of fCO2, total alkalinity (AT) and total carbon (CT) collected over the period 1998–2019 in the frame of the French monitoring program OISO. We used three datasets (underway fCO2, underway AT-CT and station AT-CT) to evaluate the trends of fCO2 and pH and their drivers, including the accumulation of anthropogenic CO2 (Cant). The study region is separated into three domains based on the frontal system and biogeochemical characteristics: (i) High Nutrients Low Chlorophyll (HNLC) waters in the Polar Front Zone (PFZ), (ii) HNLC waters south of the Polar Front (PF) and (iii) the highly productive zones in fertilized waters near Crozet and Kerguelen Islands. Almost everywhere, we obtained similar trends in surface fCO2 and pH using the fCO2 or AT-CT datasets. Over the period 1998–2019, we observed an increase in surface fCO2 and a decrease in pH ranging from +1.0 to +4.0 µatm yr−1 and from −0.0015 to −0.0043 yr−1, respectively. South of the PF, the fCO2 trend is close to the atmospheric CO2 rise (+2.0 µatm yr−1) and the decrease in pH is in the range of the mean trend for the global ocean (around −0.0020 yr−1); these trends are driven by the warming of surface waters (up to +0.04 °C yr−1) and the increase in CT, mainly due to the accumulation of Cant (around +0.6 µmol kg−1 yr−1). In the PFZ, our data show slower fCO2 and pH trends (around +1.3 µatm yr−1 and −0.0013 yr−1, respectively) associated with an increase in AT (around +0.4 µmol kg−1 yr−1) that limited the impact of a more rapid accumulation of Cant north of the PF (up to +1.1 µmol kg−1 yr−1). In the fertilized waters near Crozet and Kerguelen Islands, fCO2 increased and pH decreased faster than in the other domains, between +2.2 and +4.0 µatm yr−1 and between −0.0023 yr−1 and −0.0043 yr−1. The fastest trends of fCO2 and pH are found around Kerguelen Island north and south of the PF. These trends result from both a significant warming (up to +0.07 °C yr−1) and a rapid increase in CT (up to +1.4 µmol kg−1 yr−1), mainly explained by the uptake of Cant. Our data also show rapid changes on short periods and a relative stability of both fCO2 and pH in recent years at several locations both north and south of the PF, which leaves many open questions, notably the tipping point for the saturation state of carbonate minerals that remains highly uncertain. This highlights the need to maintain observations on the long-term in order to explore how the carbonate system will evolve in this region in the next decades