Estimation of the oceanic pCO₂ in the North Atlantic from VOS lines in-situ measurements: parameters needed to generate seasonally mean maps

Abstract. Automated instruments on board Volunteer Observing Ships (VOS) have provided high-frequency pCO₂ measurements over basin-wide regions for a decade or so. In order to estimate regional air-sea CO₂ fluxes, it is necessary to interpolate between in-situ measurements to obtain maps of the marine pCO₂. Such an interpolation remains, however, a difficult task because VOS lines are too distant from each other to capture the high pCO₂ variability. Relevant physical parameters available at large scale are thus necessary to serve as a guide to estimate the pCO₂ values between the VOS lines. Satellites do not measure pCO₂ but they give access to parameters related to the processes that control its variability, such as sea surface temperature (SST). In this paper we developed a method to compute pCO₂ maps using satellite data (SST and CHL, the chlorophyll concentration), combined with a climatology of the mixed-layer depth (MLD). Using 15 401 measurements of surface pCO₂ acquired in the North Atlantic between UK and Jamaica, between June 1994 and August 1995, we show that the parameterization of pCO₂ as a function of SST, CHL and MLD yields more realistic pCO₂ values than parameterizations that have been widely used in the past, based on SST, latitude, longitude or SST only. This parameterization was then used to generate seasonal maps of pCO₂ over the North Atlantic. Results show that our approach yields the best marine pCO₂ estimates, both in terms of absolute accuracy, when compared with an independent data set, and of geographical patterns, when compared to the climatology of Takahashi et al. (2002). This suggests that monitoring the seasonal variability of pCO₂ over basin-wide regions is possible, provided that sufficient VOS lines are available.