Modelling the seasonal course of the upper ocean pCO2 (I). Development of a one-dimensional model

A one-dimensional model is developed to simulate the seasonal course of the mixed-layer CO₂ partial pressure (pCO₂) at a given oceanic site. This rather detailed numerical tool is particularly designed to quantify the relative role of physical and biological processes in the modulation of the pCO₂. It accounts for (i) the variations in the physical environment, i.e., mixed-layer depth, temperature and eddy diffusivity, in response to the external forcing, (ii) the photosynthetic carbon fixation, (iii) the fate of the organic carbon produced through photosynthesis, either locally recycled or exported down to deep waters, and which is parameterised by using the temporal variations of the chlorophyll content combined with a f-ratio, (iv) the chemistry of CO₂ in seawater, allowing the pCO₂ inside the mixed layer to be computed from the total inorganic carbon (μCO₂) and total alkalinity (TA) contents. The carbon fluxes resulting from air-sea exchange, upward transport, and net primary production are thus simultaneously assessed. The model is purposely developed under the constraint of using (almost exclusively) remotely, sensed data, namely those about chlorophyll, wind, temperature, and incident irradiance; nevertheless, an a priori knowledge of the oceanic zone under consideration and corresponding variables is required when initializing the model. If the photosynthetic carbon uptake is explicitely accounted for, the model, however, cannot predict the algal biomass evolution. On the contrary, it is driven by this evolution and the chlorophyll concentration, as detectable from space, is the main input into the carbon-based biological compartment of the code.