Trends and drivers of sea surface fCO2 an pH changes observed in the Southern Indian Ocean over the last two decades (1998-2019)
Authors: Leseurre Coraline , Lo Monaco Claire , Reverdin Gilles , Metzl Nicolas , Fin Jonathan , Mignon Claude , Benito Léa 
Published: March 2022

&lt;p&gt;The decadal changes of the fugacity of CO&lt;sub&gt;2&lt;/sub&gt; (fCO&lt;sub&gt;2&lt;/sub&gt;) and pH in surface waters are investigated in the Southern Indian Ocean (45&amp;#176;S-57&amp;#176;S) using repeated summer observations, including measurements of fCO&lt;sub&gt;2&lt;/sub&gt;, total alkalinity (A&lt;sub&gt;T&lt;/sub&gt;) and total carbon (C&lt;sub&gt;T&lt;/sub&gt;) collected over the period 1998-2019 in the frame of the French monitoring program OISO. We used three datasets (underway fCO&lt;sub&gt;2&lt;/sub&gt;, underway A&lt;sub&gt;T&lt;/sub&gt;-C&lt;sub&gt;T&lt;/sub&gt; and station A&lt;sub&gt;T&lt;/sub&gt;-C&lt;sub&gt;T&lt;/sub&gt;) to evaluate the trends of fCO&lt;sub&gt;2&lt;/sub&gt; and pH and their drivers, including the accumulation of anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; (C&lt;sub&gt;ant&lt;/sub&gt;). 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 fCO&lt;sub&gt;2&lt;/sub&gt; and pH using the fCO&lt;sub&gt;2&lt;/sub&gt; or A&lt;sub&gt;T&lt;/sub&gt;-C&lt;sub&gt;T&lt;/sub&gt; datasets. Over the period 1998-2019, we observed an increase in surface fCO&lt;sub&gt;2&lt;/sub&gt; and a decrease in pH ranging from +1.0 to +4.0 &amp;#181;atm yr&lt;sup&gt;-1&lt;/sup&gt; and from -0.0015 to -0.0043 yr&lt;sup&gt;-1&lt;/sup&gt;, respectively. South of the PF, the fCO&lt;sub&gt;2&lt;/sub&gt; trend is close to the atmospheric CO&lt;sub&gt;2&lt;/sub&gt; rise (+2.0 &amp;#181;atm yr&lt;sup&gt;-1&lt;/sup&gt;) and the decrease in pH is in the range of the mean trend for the global ocean (around -0.0020 yr&lt;sup&gt;-1&lt;/sup&gt;). These trends are driven by the warming of surface waters (up to +0.04&amp;#176;C yr&lt;sup&gt;-1&lt;/sup&gt;) and the increase in C&lt;sub&gt;T&lt;/sub&gt;, mainly due to the accumulation of C&lt;sub&gt;ant&lt;/sub&gt; (around +0.6 &amp;#181;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;). In the PFZ, our data show slower fCO&lt;sub&gt;2&lt;/sub&gt; and pH trends (around +1.3 &amp;#181;atm yr&lt;sup&gt;-1&lt;/sup&gt; and -0.0013 yr&lt;sup&gt;-1&lt;/sup&gt;, respectively) associated with an increase in A&lt;sub&gt;T&lt;/sub&gt; (around +0.4 &amp;#181;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;)&lt;sup&gt;&lt;/sup&gt;that limited the impact of a more rapid accumulation of C&lt;sub&gt;ant&lt;/sub&gt; north of the PF (up to +1.1 &amp;#181;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;)&lt;sub&gt;.&lt;/sub&gt; In the fertilized waters near Crozet and Kerguelen Islands, fCO&lt;sub&gt;2&lt;/sub&gt; increased and pH decreased faster than in the other domains, between +2.2 and +4.0 &amp;#181;atm yr&lt;sup&gt;-1&lt;/sup&gt; and between -0.0023 yr&lt;sup&gt;-1&lt;/sup&gt; and -0.0043 yr&lt;sup&gt;-1&lt;/sup&gt;. The fastest trends of fCO&lt;sub&gt;2&lt;/sub&gt; 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&amp;#176;C yr&lt;sup&gt;-1&lt;/sup&gt;) and a rapid increase in C&lt;sub&gt;T&lt;/sub&gt; (up to +1.4 &amp;#181;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;), mainly explained by the uptake of C&lt;sub&gt;ant&lt;/sub&gt;.&lt;/p&gt;