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

Abstract

The decadal changes in the fugacity of CO2 (fCO2) and pH in surface waters are investigated in the southern Indian Ocean (45-57ggS) 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 programme OISO (Ocean Indien Service d'Observation). 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 six domains based on the frontal system and biogeochemical characteristics: (i) high-nutrient low-chlorophyll (HNLC) waters in the polar front zone (PFZ) and (ii) north part and (iii) south part of HNLC waters south of the polar front (PF), as well as the highly productive zones in fertilised waters near (iv) Crozet Island and (v) north and (vi) south of Kerguelen Island. 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 hfrom +1.0 to +4.0gμatmgyr-1 and from -0.0015 to -0.0043gyr-1, respectively. South of the PF, the fCO2 trend is close to the atmospheric CO2 rise (+2.0gμatmgyr-1), and the decrease in pH is in the range of the mean trend for the global ocean (around -0.0020gyr-1); these trends are driven by the warming of surface waters (up to +0.04ggCgyr-1) and the increase in CT mainly due to the accumulation of Cant (around +0.6gμmolgkg-1gyr-1). In the PFZ, our data show slower fCO2 and pH trends (around +1.3gμatmgyr-1 and -0.0013gyr-1, respectively) associated with an increase in AT (around +0.4gμmolgkg-1gyr-1) that limited the impact of a more rapid accumulation of Cant north of the PF (up to +1.1gμmolgkg-1gyr-1). In the fertilised waters near Crozet and Kerguelen islands, fCO2 increased and pH decreased faster than in the other domains, between +2.2 and +4.0gμatmgyr-1 and between -0.0023 and -0.0043gyr-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 +0.07ggCgyr-1) and a rapid increase in CT (up to +1.4gμmolgkg-1gyr-1) mainly explained by the uptake of Cant. Our data also show rapid changes in 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 in the long-term in order to explore how the carbonate system will evolve in this region in the next decades.