A framework to evaluate and elucidate the driving mechanisms of coastal sea surface pCO₂ seasonality using an ocean general circulation model (MOM6-COBALT)

The temporal variability of the sea surface partial pressure of CO2 (pCO2) and the underlying processes driving this variability are poorly understood in the coastal ocean. In this study, we tailor an existing method that quantifies the effects of thermal changes, biological activity, ocean circulation and fresh water fluxes to examine seasonal pCO2 changes in highly-variable coastal environments. We first use the Modular Ocean Model version 6 (MOM6) and biogeochemical module Carbon Ocean Biogeochemistry And Lower Trophics version 2 (COBALTv2) at a half degree resolution to simulate the coastal CO2 dynamics and evaluate it against pCO2 from the Surface Ocean CO2 Atlas database (SOCAT) and from the continuous coastal pCO2 product generated from SOCAT by a two-step neuronal network interpolation method (coastal-SOM-FFN, Laruelle et al., 2017). The MOM6-COBALT model not only reproduces the observed spatio-temporal variability in pCO2 but also in sea surface temperature, salinity, nutrients, in most coastal environments except in a few specific regions such as marginal seas. Based on this evaluation, we identify coastal regions of ‘high’ and ‘medium’ model skill where the drivers of coastal pCO2 seasonal changes can be examined with reasonable confidence. Second, we apply our decomposition method in three contrasted coastal regions: an Eastern (East coast of the U.S) and a Western (the Californian Current) boundary current and a polar coastal region (the Norwegian Basin). Results show that differences in pCO2 seasonality in the three regions are controlled by the balance between ocean circulation, biological and thermal changes. Circulation controls the pCO2 seasonality in the Californian Current, biological activity controls pCO2 in the Norwegian Basin, while the interplay between biology, thermal and circulation changes is key in the East coast of the U.S. The refined approach presented here allows the attribution of pCO2 changes with small residual biases in the coastal ocean, allowing future work on the mechanisms controlling coastal air-sea CO2 exchanges and how they are likely to be affected by future changes in sea surface temperature, hydrodynamics and biological dynamics.