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Abstract. Recentobservation-based results show that the seasonal amplitude of surface oceanpartial pressure of CO2 ( pCO2 ) has been increasingon average at a rate of 2–3  µ atm per decade ( Landschutzer et al. ,  2018 ) . Future increases in pCO2 seasonalityare 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 coupledatmosphere–ocean–carbon cycle–ecosystem model simulations conducted aspart of the Coupled Model Intercomparison Project Phase 5 (CMIP5) to studyfuture projections of the pCO2 annual cycle amplitude and toelucidate the causes of its amplification. We find that for the RCP8.5emission scenario the seasonal amplitude (climatological maximum minusminimum) of upper ocean pCO2 will increase by a factor of1.5 to 3 over the next 60–80 years. To understand the drivers and mechanismsthat control the pCO2 seasonal amplification we develop acomplete analytical Taylor expansion of pCO2 seasonality interms of its four drivers: dissolved inorganic carbon (DIC), totalalkalinity (TA), temperature ( T ), and salinity ( S ). Using this linearapproximation we show that the DIC and T terms are the dominantcontributors to the total change in pCO2 seasonality. Tofirst order, their future intensification can be traced back to a doubling ofthe annual mean pCO2 , which enhances DIC and alters theocean carbonate chemistry. Regional differences in the projected seasonalcycle amplitude are generated by spatially varying sensitivity terms. Thesubtropical and equatorial regions (40 ∘  S–40 ∘  N) willexperience a ≈30 –80  µ atm increase in seasonal cycleamplitude almost exclusively due to a larger background CO2 concentration that amplifies the T seasonal effect on solubility. Thismechanism is further reinforced by an overall increase in the seasonal cycleof T as a result of stronger ocean stratification and a projected shoalingof mean mixed layer depths. The Southern Ocean will experience a seasonalcycle amplification of ≈90 –120  µ atm in response to themean pCO2 -driven change in the mean DIC contribution and toa lesser extent to the T contribution. However, a decrease in the DICseasonal cycle amplitude somewhat counteracts this regional amplificationmechanism.

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

Drivers of future seasonal cycle changes in oceanic <i>p</i>CO<sub>2</sub>