Summer trends and drivers of sea surface fCO 2 and pH changes observed in the southern Indian Ocean over the last two decades (1998–2019)

[摘要] The decadal changes in the fugacity of CO 2 ( fCO 2 ) and pH in surface waters are investigated in the southern Indian Ocean(45–57 ∘  S) using repeated summer observations, including measurements of fCO 2 , total alkalinity ( A T ) and totalcarbon ( C T ) collected over the period 1998–2019 in the frame of the French monitoring programme OISO ( Océan Indien Service d'Observation ). We used three datasets (underway fCO 2 , underway A T – C T and station A T – C T ) to evaluate the trends of fCO 2 and pH and theirdrivers, including the accumulation of anthropogenic CO 2 ( C ant ). The study region is separated into six domains based on thefrontal system and biogeochemical characteristics: (i) high-nutrient low-chlorophyll (HNLC) waters in the polar front zone (PFZ) and (ii) north partand (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 insurface fCO 2 and pH using the fCO 2 or A T – C T datasets. Over the period 1998–2019, we observed an increase insurface fCO 2 and a decrease in pH ranging from + 1.0 to + 4.0  µ atm yr −1 and from − 0.0015 to − 0.0043  yr −1 , respectively. South of the PF, the fCO 2 trend is close to theatmospheric CO 2 rise ( + 2.0  µ atm 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 theincrease in C T mainly due to the accumulation of C ant (around + 0.6  µ mol kg −1  yr −1 ). In the PFZ, our datashow slower fCO 2 and pH trends (around + 1.3  µ atm yr −1 and − 0.0013  yr −1 , respectively) associated with anincrease in A T (around + 0.4  µ mol kg −1  yr −1 ) that limited the impact of a more rapid accumulation of C ant north of the PF (up to + 1.1  µ mol kg −1  yr −1 ). In the fertilised waters near Crozet and Kerguelen islands, fCO 2 increased and pH decreased faster than in the other domains, between + 2.2 and + 4.0  µ atm yr −1 and between − 0.0023 and − 0.0043  yr −1 . The fastest trends of fCO 2 and pH are found around Kerguelen Island north and south of the PF. These trendsresult from both a significant warming (up to + 0.07  ∘ C yr −1 ) and a rapid increase in C T (up to + 1.4  µ mol kg −1  yr −1 ) mainly explained by the uptake of C ant . Our data also show rapid changes in short periods anda relative stability of both fCO 2 and pH in recent years at several locations both north and south of the PF, which leaves many openquestions, notably the tipping point for the saturation state of carbonate minerals that remains highly uncertain. This highlights the need tomaintain observations in the long-term in order to explore how the carbonate system will evolve in this region in the next decades.

[学科分类] 大气科学