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Drivers of future seasonal cycle changes in oceanic <i>p</i>CO<sub>2</sub>
Gallego, M. Angeles,Timmermann, Axel,Friedrich, Tobias,Zeebe, Richard E. Copernicus GmbH 2018 Biogeosciences Vol.15 No.17
<P><p><strong>Abstract.</strong> Recent observation-based results show that the seasonal amplitude of surface ocean partial pressure of <span class='inline-formula'>CO<sub>2</sub></span> (<span class='inline-formula'><i>p</i>CO<sub>2</sub></span>) has been increasing on average at a rate of 2-3<span class='thinspace'></span><span class='inline-formula'>µ</span>atm per decade <span class='cit' id='xref_paren.1'>(<a href='#bib1.bibx16'>Landschützer et al.</a>, <a href='#bib1.bibx16'>2018</a>)</span>. Future increases in <span class='inline-formula'><i>p</i>CO<sub>2</sub></span> seasonality are expected, as marine <span class='inline-formula'>CO<sub>2</sub></span> concentration ([<span class='inline-formula'>CO<sub>2</sub></span>]) will increase in response to increasing anthropogenic carbon emissions <span class='cit' id='xref_paren.2'>(<a href='#bib1.bibx19'>McNeil and Sasse</a>, <a href='#bib1.bibx19'>2016</a>)</span>. Here we use seven different global coupled atmosphere-ocean-carbon cycle-ecosystem model simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5) to study future projections of the <span class='inline-formula'><i>p</i>CO<sub>2</sub></span> annual cycle amplitude and to elucidate the causes of its amplification. We find that for the RCP8.5 emission scenario the seasonal amplitude (climatological maximum minus minimum) of upper ocean <span class='inline-formula'><i>p</i>CO<sub>2</sub></span> will increase by a factor of 1.5 to 3 over the next 60-80 years. To understand the drivers and mechanisms that control the <span class='inline-formula'><i>p</i>CO<sub>2</sub></span> seasonal amplification we develop a complete analytical Taylor expansion of <span class='inline-formula'><i>p</i>CO<sub>2</sub></span> seasonality in terms of its four drivers: dissolved inorganic carbon (DIC), total alkalinity (TA), temperature (<span class='inline-formula'><i>T</i></span>), and salinity (<span class='inline-formula'><i>S</i></span>). Using this linear approximation we show that the DIC and <span class='inline-formula'><i>T</i></span> terms are the dominant contributors to the total change in <span class='inline-formula'><i>p</i>CO<sub>2</sub></span> seasonality. To first order, their future intensification can be traced back to a doubling of the annual mean <span class='inline-formula'><i>p</i>CO<sub>2</sub></span>, which enhances DIC and alters the ocean carbonate chemistry. Regional differences in the projected seasonal cycle amplitude are generated by spatially varying sensitivity terms. The subtropical and equatorial regions (40<span class='inline-formula'><sup>∘</sup></span><span class='thinspace'></span>S-40<span class='inline-formula'><sup>∘</sup></span><span class='thinspace'></span>N) will experience a <span class='inline-formula'>≈30</span>-80<span class='thinspace'></span><span class='inline-formula'>µ</span>atm increase in seasonal cycle amplitude almost exclusively due to a larger background <span class='inline-formula'>CO<sub>2</sub></span> concentration that amplifies the <span class='inline-formula'><i>T</i></span> seasonal effect on solubility. This mechanism is further reinforced by an overall increase in the seasonal cycle of <span class='inline-formula'><i>T</i></span> as a result of stronger ocean stratification and a projected shoaling of mean mixed layer depths. The Southern Ocean will experience a seasonal cycle amplification of <span class='inline-formula'>≈90</span>-120<span class='thinspace'></span><span class='inline-formula'>µ</span>atm in response to the mean <span class='inline-formula'><i>p</i>CO<sub>2</sub></span>-driven change in the mean DIC contribution and to a lesser extent to the <span class='inline-formula'><i>T</i></span> contribution. However, a decrease in the DIC seasonal cycle amplitude somewhat counteracts this regional amplification mechanism.</p> </P>