Drivers of future seasonal cycle changes in oceanic pCO2

• Published in: Biogeosciences
• Main Authors: Gallego, M. Angeles, Timmermann, Axel, Friedrich, Tobias, Zeebe, Richard E.
• Format: Text
• Language: English
• Published: 2019
• Subjects: Southern Ocean
• Online Access: https://doi.org/10.5194/bg-15-5315-2018; https://www.biogeosciences.net/15/5315/2018/

Recent observation-based results show that the seasonal amplitude of surface ocean partial pressure of CO 2 ( p CO 2 ) has been increasing on average at a rate of 2–3 µ atm per decade ( Landschützer et al. , 2018 ) . Future increases in p CO 2 seasonality are expected, as marine CO 2 concentration ([ CO 2 ]) will increase in response to increasing anthropogenic carbon emissions ( McNeil and Sasse , 2016 ) . 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 p CO 2 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 p CO 2 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 p CO 2 seasonal amplification we develop a complete analytical Taylor expansion of p CO 2 seasonality in terms of its four drivers: dissolved inorganic carbon (DIC), total alkalinity (TA), temperature ( T ), and salinity ( S ). Using this linear approximation we show that the DIC and T terms are the dominant contributors to the total change in p CO 2 seasonality. To first order, their future intensification can be traced back to a doubling of the annual mean p CO 2 , 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 ∘ S–40 ∘ N) will experience a ≈30 –80 µ atm increase in seasonal cycle amplitude almost exclusively due to a larger background CO 2 concentration that amplifies the T seasonal effect on solubility. This mechanism is further reinforced by an overall increase in the seasonal cycle of T 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 ≈90 –120 µ atm in response to the mean p CO 2 -driven change in the mean DIC contribution and to a lesser extent to the T contribution. However, a decrease in the DIC seasonal cycle amplitude somewhat counteracts this regional amplification mechanism.