Abstract
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.
Highlights
The burning of fossil fuels, the intensification of land-use, and the production of cement over the last 250 years have increased the atmospheric carbon dioxide (CO2) concentration from 280 parts per million prior to the industrial revolution (Neftel et al, 1985) to the current level of more than 380 ppm (Keeling and Whorf, 2000; Houghton et al, 2001)
Using 15 401 measurements for the years 1994 and 1995, we have developed a new parameterization of pCOn2est × σ (pCO2), based on sea surface temperature (SST), CHL and mixed-layer depth (MLD) for the North Atlantic (10◦ N–58◦ N)
This new parameterization has been applied to satellite data, to provide pCO2 maps of the North Atlantic
Summary
The burning of fossil fuels, the intensification of land-use, and the production of cement over the last 250 years have increased the atmospheric carbon dioxide (CO2) concentration from 280 parts per million (ppm) prior to the industrial revolution (Neftel et al, 1985) to the current level of more than 380 ppm (Keeling and Whorf, 2000; Houghton et al, 2001). While the water temperature is primarily regulated by physical processes (i.e. solar energy input, sea-air heat exchanges, physical mixing and mixed-layer depth), the total CO2 concentration and the alkalinity are primarily controlled by biological processes. The North Atlantic, in particular, is the subject of intensive pCO2 measurements, using Volunteer Observing ships (VOS) in the framework of the former CAVASSOO (2001–2003) and of the ongoing CARBOOCEAN (2005–2009) European projects These data sets can either be used to consolidate numerical models, which can be used, in turn, to compute the carbon budget, or, as in this study, to develop relationships between oceanic pCO2 and other parameters available globally, such as the satellite sea surface temperature (SST), to interpolate maps of pCO2 for the North Atlantic. We aim at generalizing this multi-parameter approach to basin-wide regions, such as the North Atlantic, using in situ measurements from VOS lines
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