Seasonal variabiltiy of the surface ocean carbon cycle: a global synthesis

Here we present a synthesis of surface ocean pCO₂ and air-sea CO₂ flux seasonality for a modern climatology and      their decadal trends between the 1980s and 2010s, as part of the REgional Carbon Cycle Assessment and Processes Phase 2 (RECCAP2) project. Working with both surface ocean pCO₂-observation products (pCO₂ products) and global ocean biogeochemistry models (GOBMs), our main findings are: (i) Over biome scales, both pCO₂ products and GOBMs confirm increases in the seasonal amplitude of pCO₂ and integrated CO₂ fluxes between 1985-1989 and 2014-2018. (ii) For the 2014-2018 climatology, GOBMs exhibit a systematic bias with too-weak biologically-driven seasonal variability in surface dissolved inorganic carbon (DIC), such that the pCO₂ seasonal cycle in subtropical biomes is spuriously large and both the amplitude and phase of seasonal pCO₂ variations diverge from those in the pCO₂ products in subpolar and circumpolar biomes. (iii) Decadal increases in pCO₂ seasonal cycle amplitude in subtropical biomes are attributed to being largely driven by reducing CO₂ buffering capacity and increasing sensitivity to temperature due to increasing anthropogenic carbon (C_ant) content insurface waters for both the pCO₂ products and GOBMs. In subpolar and circumpolar biomes, the seasonality change for GOBMs is dominated by C_ant invasion, whereas for pCO₂ products modulations of the climate state are equally important. (iv) Considered together, the subtropical biomes exhibit decadal increases in CO₂ flux seasonality that are larger during winter than summer, consistent with the mechanism described by Fassbender et al. (2022) and potentially promoting a negative feedback in the climate system by increasing the CO₂ uptake in winter, by virtue of surface winds being stronger in winter than summer. (v) Large ensemble simulations with ESMs were applied to confirm the validity of biomes as aggregation domains for identifying forced signals. Despite compromises to DIC seasonality impacting pCO₂ seasonality, the chosen biome-scale is appropriate for representing the decadal rate of increase of pCO₂ seasonality for both GOBMs  and pCO₂ products.