Trends and drivers of sea surface fCO2 an 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 (A_T) and total carbon (C_T) collected over the period 1998-2019 in the frame of the French monitoring program OISO. We used three datasets (underway fCO₂, underway A_T-C_T and station A_T-C_T) to evaluate the trends of fCO₂ and pH and their drivers, including the accumulation of anthropogenic CO₂ (C_ant). 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 A_T-C_T 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 C_T, mainly due to the accumulation of C_ant (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 A_T (around +0.4 µmol kg^-1 yr^-1)that limited the impact of a more rapid accumulation of C_ant 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 C_T (up to +1.4 µmol kg^-1 yr^-1), mainly explained by the uptake of C_ant.