Summer trends and drivers of sea surface fCO<sub>2</sub> 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 surface waters are investigated southern Indian Ocean (45–57∘ S) using repeated summer observations, including measurements fCO2, total alkalinity (AT) carbon (CT) collected over period 1998–2019 frame French monitoring programme OISO (Océan Indien Service d'Observation). We used three datasets (underway underway AT–CT station AT–CT) to evaluate trends fCO2 their drivers, accumulation anthropogenic (Cant). study region is separated into six domains based on frontal system biogeochemical characteristics: (i) high-nutrient low-chlorophyll (HNLC) polar front zone (PFZ) (ii) north part (iii) south HNLC (PF), as well highly productive zones fertilised near (iv) Crozet Island (v) (vi) Kerguelen Island. Almost everywhere, we obtained similar or datasets. Over 1998–2019, observed an increase a decrease ranging from +1.0 +4.0 µatm yr−1 −0.0015 −0.0043 yr−1, respectively. South PF, trend close atmospheric rise (+2.0 yr−1), range mean for global ocean (around −0.0020 yr−1); these driven by warming (up +0.04 ∘C yr−1) CT mainly due Cant +0.6 µmol kg−1 yr−1). In PFZ, our data show slower +1.3 −0.0013 respectively) associated with AT +0.4 that limited impact more rapid PF +1.1 islands, increased decreased faster than other domains, between +2.2 −0.0023 yr−1. fastest found around PF. These result both significant +0.07 +1.4 explained uptake Cant. Our also short periods relative stability recent years at several locations which leaves many open questions, notably tipping point saturation state carbonate minerals remains uncertain. This highlights need maintain observations long-term order explore how will evolve this next decades.