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

Highlights

• Air-sea CO2 fluxes follow the upwelling intensity throughout the year.
• Circulation is the dominant mechanism driving variability in the nearshore area.
• Biology and solubility effects partially damp upwelling-driven pCO2 variability.
• High coastal pCO2 values are due to a spatiotemporal decoupling between circulation and biology.
pCO2 is more sensitive to changes in dissolved inorganic carbon and temperature.

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 (pCO2) and associated air-sea CO2 fluxes in the Northern Humboldt Current System (NHCS). Simulated pCO2 is in good agreement with available observations with an average absolute error of, approximately, 24 μatm. The highly productive upwelling region, 300 km from the shore and between 5°S-17°S, is shown to be a strong CO2 source with an averaged flux of 5.60  ±  2.94 mol C m−2 yr−1, which represents an integrated carbon flux of 0.028  ±  0.015 Pg C yr−1 . Through a series of model experiments we show that the high pCO2 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 pCO2 up to levels around 1100 μatm. Even though biological production is high, it reduces pCO2 only by about 300 μatm. We show that this relatively low degree of biological compensation, which implies an inefficient biological pump in 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 outgassing and the processes affecting CO2 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 pCO2 is more sensitive to changes in DIC and sea surface temperature, while alkalinity plays a minor role.

Mogollón R. & Calil P. H. R., in press. Modelling the mechanisms and drivers of the spatiotemporal variability of pCO2 and air-sea CO2 fluxes in the Northern Humboldt Current System. Ocean Modelling. Article (subscription required).

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