A regional pCO₂ climatology of the Baltic Sea from in situ pCO₂ observations and a model-based extrapolation approach

Abstract. Ocean surface pCO₂ estimates are of great interest for the calculation of air-sea CO₂ fluxes, oceanic uptake of anthropogenic CO₂, and eventually the Global Carbon Budget. They are accessible from direct observations, which are discrete in space and time and thus always sparse, or from biogeochemical models, which only approximate reality. Here, a combined method for the extrapolation of pCO₂ observations is presented that uses (1) model-based patterns of variability from an EOF analysis of variability with (2) observational data to constrain EOF pattern amplitudes in (3) an ensemble approach, which locally adjusts the spatial scale of the mapping to the density of the observations. Thus, data-constrained, gap- and discontinuity-free mapped fields including local error estimates are obtained without the need for or dependence on ancillary data (like, e.g., satellite sea surface temperature maps). This extrapolation approach is generic in that it can be applied to any oceanic or coastal region covered by a suitable model and observations. It is used here to establish a regional pCO₂ climatology of the Baltic Sea, largely based on ICOS-DE SOOP Finnmaid surface pCO₂ observations between Lübeck-Travemünde (Germany) and Helsinki (Finland). The climatology can serve as improved input for atmosphere-ocean CO₂ flux estimation in this coastal environment.