Water Mass Analysis of Effect of Climate Change on Air–Sea CO₂ Fluxes: The Southern Ocean

Abstract

Impacts of climate change on air-sea CO 2 exchange are strongly region dependent, particularly in the Southern Ocean. Yet, in the Southern Ocean the role of water masses in the uptake of anthropogenic carbon is still debated. Here, a methodology is applied that tracks the carbon flux of each Southern Ocean water mass in response to climate change. A global marine biogeochemical model was coupled to a climate model, making 140-yr Coupled Model Intercomparison Project phase 5 (CMIP5)-type simulations, where atmospheric CO 2 increased by 1% yr 21 to 4 times the preindustrial concentration (4 3 CO 2 ). Impacts of atmospheric CO 2 (carbon-induced sensitivity) and climate change (climate-induced sensitivity) on the water mass carbon fluxes have been isolated performing two sensitivity simulations. In the first simulation, the atmospheric CO 2 influences solely the marine carbon cycle, while in the second simulation, it influences both the marine carbon cycle and earth's climate. At 4 3 CO 2 , the cumulative carbon uptake by the Southern Ocean reaches 278 PgC, 53% of which is taken up by modal and intermediate water masses. The carbon-induced and climate-induced sensitivities vary significantly between the water masses. The carbon-induced sensitivities enhance the carbon uptake of the water masses, particularly for the denser classes. But, enhancement strongly depends on the water mass structure. The climate-induced sensitivities either strengthen or weaken the carbon uptake and are influenced by local processes through changes in CO 2 solubility and stratification, and by large-scale changes in outcrop surface (OS) areas. Changes in OS areas account for 45% of the climateinduced reduction in the Southern Ocean carbon uptake and are a key factor in understanding the future carbon uptake of the Southern Ocean.

The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges

“…Though global models typically cannot afford to resolve the small‐scale dynamics that idealized and regional models are able to reproduce, they are valuable in determining the significance of the WG, especially with regard to air‐sea fluxes and interbasin exchanges, including ventilation of the abyssal global ocean (e.g., Marinov et al, ; Renner et al, ; Sarmiento & Orr, ; Séférian et al, ). Bernardello et al () emphasized the role of Weddell Sea convection in governing ocean carbon uptake.…”

Inconsistent strategies to spin up models in CMIP5: implications for ocean biogeochemical model performance assessment

“…It needs to be better constrained in order to draw robust conclusions on the impact of climate change on the carbon cycle as well as on climate feedback (e.g., Arora et al, 2013;Friedlingstein et al, 2013;Roy et al, 2011;Schwinger et al, 2014;Séférian et al, 2012) and on marine ecosystems (e.g., Bopp et al, 2013;Boyd et al, 2015;Cheung et al, 2012;Doney et al, 2012;Gattuso et al, 2015;Lehodey et al, 2006). So far, the most frequently used approach relies on long preindustrial control simulations running parallel to a transient simulations, allowing the "removal" of the drift in the simulated fields over the historical period or future projections (e.g., Bopp et al, 2013;Cocco et al, 2013;Friedlingstein et al, 2013Friedlingstein et al, , 2006Frölicher et al, 2014;Gehlen et al, 2014;Keller et al, 2014;Steinacher et al, 2010;Tjiputra et al, 2014).…”

The Southern Ocean as a constraint to reduce uncertainty in future ocean carbon sinks

“…To address this issue, we also perform additional analyses where we use a dynamic boundary separating the mid-and high-latitude Southern Ocean applying a surface density of 26.5 kg m −3 . For instance, Séférian et al (2012) apply this density line to separate the subtropical mode water (TMW; region of weak increase in future CO 2 uptake) and the subantarctic model water (MW; region of strong increase in future CO 2 uptake).…”