Recent changes in the accumulation of anthropogenic carbon in mode waters of the Southern Indian Ocean
Authors: Barut Guillaume , Clerc Corentin , Leseurre Coraline , LoMonaco Claire , Metzl Nicolas 
Published: March 2022

&lt;p&gt;The Southern Ocean plays an important role in the climate system by regulating excess CO&lt;sub&gt;2&lt;/sub&gt; in the atmosphere. A large part of the anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; (C&lt;sub&gt;ant&lt;/sub&gt;) absorbed in surface waters of the Southern Ocean is isolated from the atmosphere at mid-latitudes through the sinking of mode waters down to 1500m.&amp;#160;Since&amp;#160;the Southern Ocean CO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;sink and mode waters&amp;#160;formation&amp;#160;vary from&amp;#160;inter-annual to decadal scales,&amp;#160;one would expect the C&lt;sub&gt;ant&lt;/sub&gt; content in these waters&amp;#160;to be also variable. This has been suggested through modeling studies but it is&amp;#160;challenging to detect these changes based on observations.&amp;#160;&amp;#160;This study attempts&amp;#160;to estimate the evolution of C&lt;sub&gt;ant&lt;/sub&gt; accumulation in mode&amp;#160;waters of the Southern Indian Ocean over the period 1985-2019 based on&amp;#160;observations from the Global Ocean Data Analysis Project (GLODAPv2_2020) and the long-term monitoring program OISO (Oc&amp;#233;an Indien Service d&amp;#8217;Observations). The comparison of three data-based diagnostic approaches showed&amp;#160;the strength of the eMLR(C*) method (Clement and Gruber, 2018) for estimating temporal variations in the accumulation of C&lt;sub&gt;ant&lt;/sub&gt;. The increase in C&lt;sub&gt;ant&lt;/sub&gt; estimated between 1985 and 2019 show a relatively good agreement for the three methods in the different types of mode waters identified in the Indian Ocean:&amp;#160;&amp;#160;the&amp;#160;mean trend is&amp;#160;between +1.02 and +1.09 &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; in the Subtropical Mode Water (STMW), between +0.73 and +1.02 &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; in the Subantarctic Mode Water (SAMW) and&amp;#160;between +0.25 and +0.51 &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;&amp;#160;in the Antarctic Intermediate Water (AAIW).&amp;#160;However, on shorter periods we found larger discrepancies between the eMLR(C*) method and the two other techniques (back-calculation and TrOCA), the latter showing larger uncertainties. The mean increase in C&lt;sub&gt;ant&lt;/sub&gt; between 1994 and 2007 estimated using the eMLR(C*) is +1.34 (&amp;#177; 0.18) &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; in the STMW,&amp;#160;+1.05 (&amp;#177; 0.05) &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;&amp;#160;in the SAMW and&amp;#160;+0.60 (&amp;#177; 0.11) &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; in the AAIW,&amp;#160;which is&amp;#160;consistent with previous results obtained over the same time period using the same method (Gruber et al, 2019). Interestingly, the trends estimated with this method in recent years (between 2007 and 2017) weakened by about half in all mode waters, in STMW (+0.73 (&amp;#177; 0.20) &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;), the SAMW (+0.49 (&amp;#177; 0.20) &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;) and AAIW (+0.26 (&amp;#177; 0.42) &amp;#956;mol kg&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;).&amp;#160;Due to the important contribution of mode&amp;#160;waters in&amp;#160;the storage of C&lt;sub&gt;ant&lt;/sub&gt;, these results could significantly reduce the oceanic inventories of C&lt;sub&gt;ant&lt;/sub&gt; in recent years at both the regional and global scales.&amp;#160;The reduction in&amp;#160;C&lt;sub&gt;ant&lt;/sub&gt; trends in mode waters of the Southern Indian Ocean raises questions on the external/internal processes that control mode waters formation and air-sea CO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;exchanges&amp;#160;in the Southern Ocean at a decadal scale.&lt;/p&gt;