Drivers of future seasonal cycle changes in oceanic p CO 2
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 µatm per decade (Landschützer et al. 2018). Future increases in p CO 2 seasonality are expected, as marine CO 2 concentration ([CO 2...
| Published in: | Biogeosciences |
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| Main Authors: | , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
Copernicus Publications
2018
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| Subjects: | |
| Online Access: | https://doi.org/10.5194/bg-15-5315-2018 https://doaj.org/article/cd70e9c4efbb4f62baac606b08e677d5 |
| Summary: | 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 µatm 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 µatm 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 ... |
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