The spatiotemporal dynamics of the sources and sinks of CO2 in the global coastal ocean

The spatio-temporal variability and the underlying drivers of the carbon dioxide (CO₂) exchange at the air-water interface (FCO₂) of the global coastal ocean are still poorly understood and their quantification remains highly uncertain. Here, we present an analysis of the spatial and seasonal variability of FCO₂ using a high-resolution (0.25 degree) monthly climatology (1998-2015 period) for coastal sea surface partial pressure in CO₂ (pCO₂), globally.

Overall, a clear latitudinal pattern emerges from our analysis regarding sources/sinks distribution of atmospheric CO₂ and we find that in most regions, annual mean CO₂ flux densities are comparable in sign and magnitude to those of the adjacent open ocean except for river dominated systems. Globally, coastal regions act as a CO₂ sink with a more intense uptake occurring in summer because of the disproportionate influence of high latitude coastal seas in the Northern Hemisphere. The majority of the coastal seasonal FCO₂ variations stems from the air-sea pCO₂ gradient, although changes in wind speed and sea-ice cover can also be significant regionally. To investigate further the drivers of the spatio-seasonal variability, our observation-based pCO₂ climatology is used in conjunction with global ocean biogeochemistry model MOM6-COBALT. The model outputs allow us to quantify the respective contributions of thermal effects, biology, and non-thermal physical processes (circulation and freshwater inputs) to seasonal variations in coastal pCO₂. Generally, biological activity is the dominant driver of the pCO₂ seasonal variability in temperate and high latitudes while thermal and non-thermal physical processes dominate in low latitudes.