Modelling the mechanisms and drivers of the spatiotemporal variability of pCO2 and air–sea CO2 fluxes in the Northern Humboldt Current System

Abstract We use a coupled physical–biogeochemical model to investigate the drivers and mechanisms responsible for the spatiotemporal variability of the partial pressure of carbon dioxide in seawater ( p CO 2 ) and associated air–sea CO 2 fluxes in the Northern Humboldt Current System (NHCS). Simulated p CO 2 is in good agreement with available observations with an average absolute error of, approximately, 24 µatm. The highly productive upwellingregion, 300 km from the shore and between 5 and 17 °S, is shown to be a strong CO 2 source with an averaged flux of 5.60  ±  2.94 mol C m − 2 year − 1 , which represents an integrated carbon flux of 0.028  ±  0.015 Pg C year − 1 . Through a series of model experiments we show that the high p CO 2 is primarily the result of coastal upwelling, which is incompletely compensated by biology. Specifically, the supply of dissolved inorganic carbon (DIC)-rich waters to the surface pushes p CO 2 up to levels around 1100 µatm. Even though biological production is high, it reduces p CO 2 only by about 300 µatm. We show that this relatively low degree of biological compensation, which implies an inefficient biological pumpin the nearshore domain, results from a spatiotemporal decoupling between the counteracting effects of biological production and the transport and mixing of DIC. The contribution of the outgassingand the processes affecting CO 2 solubility, associated with the seasonal cycle of heating and cooling, are minor. Across the whole domain, the balance of mechanisms is similar, but with smaller amplitudes. We demonstrate that seawater p CO 2 is more sensitive to changes in DIC and sea surface temperature, while alkalinity plays a minor role.