Drivers of future seasonal cycle changes in oceanic &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

M Angeles Gallego,Richard E Zeebe,Axel Timmermann,Tobias Friedrich

doi:10.5194/bg-15-5315-2018

Abstract

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 2–3 µatm 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 ≈30–80 µatm 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 ≈90–120 µatm 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.

Highlights

Owing to its large chemical capacity to resist changes in [CO2], the ocean has absorbed nearly half of the anthropogenic CO2 produced by fossil fuel burning and cement production since the industrial revolution (Sabine et al, 2004)
Our analysis focuses on 21st century evolution using the Representative Concentration Pathway 8.5 (RCP8.5) scenario
To gain more insight into the processes causing the amplification of δpCO2 we introduce a method based on a second Taylor series expansion described below

Summary

Introduction

Owing to its large chemical capacity to resist changes in [CO2] (referred to as buffering capacity), the ocean has absorbed nearly half of the anthropogenic CO2 produced by fossil fuel burning and cement production since the industrial revolution (Sabine et al, 2004). While the ocean’s absorption of CO2 lowers the atmospheric concentration, it increases the ocean’s [CO2] and in turn lowers its buffering capacity. This leads to a reduction in the oceanic uptake of CO2 and an intensification of the pCO2 seasonal cycle ( on referred to as δpCO2) (Völker et al, 2002; McNeil and Sasse, 2016). An intensification of the δpCO2 amplitude could produce seasonal hypercapnia conditions (McNeil and Sasse, 2016), which together with increased [H+] seasonality (Hagens and Middelburg, 2016; Kwiatkowski and Orr, 2018) and aragonite undersaturation events (Shaw et al, 2013; Hauri et al, 2015; Sasse et al, 2015) could expose marine life to harmful seawater conditions earlier than expected if considering only annual mean values. A projected amplification of δpCO2 might increase the net CO2 uptake in some regions, such as the Southern Ocean, thereby further accelerating the decrease in the buffering capacity in that region (Hauck and Völker, 2015)

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Conclusion