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)

• Published in: Biogeosciences
• Main Authors: Leseurre, Coraline, Lo Monaco, Claire, Reverdin, Gilles, Metzl, Nicolas, Mignon, Claude, Benito, Léa
• Other Authors: Cycles biogéochimiques marins : processus et perturbations (CYBIOM), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité), Processus et interactions de fine échelle océanique (PROTEO), LEFEINSU programme KER-Trend
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: [SDU]Sciences of the Universe [physics]; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Kerguelen Islands
• Online Access: https://insu.hal.science/insu-03746452; https://insu.hal.science/insu-03746452/document; https://insu.hal.science/insu-03746452/file/bg-19-2599-2022.pdf; https://doi.org/10.5194/bg-19-2599-2022

International audience 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 total carbon (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 their drivers, including the accumulation of anthropogenic CO 2 (C ant ). 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 fCO 2 and pH using the fCO 2 or A T -C T datasets. Over the period 1998-2019, we observed an increase in surface fCO 2 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 fCO 2 trend is close to the atmospheric CO 2 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 fCO 2 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 fertilised waters near Crozet and Kerguelen islands, fCO 2 increased and pH ...