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Biogeosciences An interactive open-access journal of the European Geosciences Union
Journal topic
Volume 15, issue 17
Biogeosciences, 15, 5315-5327, 2018
https://doi.org/10.5194/bg-15-5315-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Biogeosciences, 15, 5315-5327, 2018
https://doi.org/10.5194/bg-15-5315-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Sep 2018

Research article | 03 Sep 2018

Drivers of future seasonal cycle changes in oceanic pCO2

M. Angeles Gallego1, Axel Timmermann2,3,4, Tobias Friedrich2, and Richard E. Zeebe1 M. Angeles Gallego et al.
  • 1Department of Oceanography, School of Ocean and Earth Sciences and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA
  • 2International Pacific Research Center, School of Ocean and Earth Sciences and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA
  • 3Center for Climate Physics, Institute for Basic Science (IBS), Busan, South Korea
  • 4Pusan National University, Busan, South Korea

Abstract. Recent observation-based results show that the seasonal amplitude of surface ocean partial pressure of CO2 (pCO2) has been increasing on average at a rate of 2–3µatm per decade (Landschützer et al. 2018). Future increases in pCO2 seasonality are expected, as marine CO2 concentration ([CO2]) 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 pCO2 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 pCO2 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 pCO2 seasonal amplification we develop a complete analytical Taylor expansion of pCO2 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 pCO2 seasonality. To first order, their future intensification can be traced back to a doubling of the annual mean pCO2, 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  ≈ 30–80µatm increase in seasonal cycle amplitude almost exclusively due to a larger background CO2 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  ≈ 90–120µatm in response to the mean pCO2-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.

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It is projected that the summer–winter difference in pCO2 levels will be larger in the future. In this paper, we study the causes of this seasonal amplification of pCO2. We found that anthropogenic CO2 enhances the effect of seasonal changes in temperature (T) and dissolved inorganic carbon (DIC) on pCO2 seasonality. This is because the oceanic pCO2 becomes more sensitive to seasonal T and DIC changes when the CO2 concentration is higher.
It is projected that the summer–winter difference in pCO2 levels will be larger in the future....
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