Variability and trends of carbon parameters at a time series in the eastern tropical Atlantic

Abstract

Hourly fCO₂ is recorded at a time series at the PIRATA buoy located at 6°S 10°W in the eastern tropical Atlantic since June 2006. This site is located south and west of the seasonal Atlantic cold tongue and is affected by its propagation from June to September. Using an alkalinity–salinity relationship determined for the eastern tropical Atlantic and the observed fCO₂, pH and the inorganic carbon concentration are calculated. The time series is investigated to explore the intraseasonal, seasonal and interannual timescales for these parameters, and to detect any long-term trends. At intraseasonal timescales, fCO₂ and pH are strongly correlated. On seasonal timescales, the correlation still holds between fCO₂ and pH and their variations are in agreement with those of sea surface salinity. At interannual timescales, some important differences appear in 2011–2012: lower fCO₂ and fluxes are observed from September to December 2011 and are explained by higher advection of salty waters at the mooring, in agreement with the wind. In early 2012, the anomaly is still present and associated with lower sea surface temperatures. No significant long-term trend is detected over the period 2006–2013 on CO₂ and any other physical parameter. However, as atmospheric fCO₂ is increasing over time, the outgassing of CO₂ is reduced over the period 2006–2013 as the flux is mainly controlled by the difference of fCO₂ between the ocean and the atmosphere. A longer time series is required to determine if any significant trend exists in this region.

• fugacity of CO₂
• ocean acidification
• time series
• eastern tropical Atlantic

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6. Acknowledgements

We acknowledge support from the European Integrated Projects CARBOOCEAN (contract 511176-2) and CARBOCHANGE (grant agreement 264879), the Institut de Recherche pour le Développement (IRD) and the national program LEFE CYBER. We are very grateful to US IMAGO of IRD, and especially to Jacques Grelet and Fabrice Roubaud for their technical support at sea and the deployment of the sensor. We thank the DT INSU for preparation and calibration of the CARIOCA sensor. Seawater samples were analysed for TCO₂ and TA by the SNAPO-CO₂ at LOCEAN in Paris. Data management for PIRATA moorings is conducted by the TAO project office at NOAA/PMEL in collaboration with many research institutes listed on the PIRATA website (www.pmelnoaa.gov/pirata). Precipitation data from the GPCP were downloaded from the Giovanni online data system, developed and maintained by the NASA Goddard Earth Sciences (GES) Data and Information Services Center (DISC). We also acknowledge the TRMM mission scientists and associated NASA personnel for production of the data used in this research effort. TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science and REASoN DISCOVER project. They are available at www.remss.com. We thank Fabrice Hernandez from IRD for making available to us the products of the Mercator Ocean (Toulouse, France) GLORYS2V3 global ocean reanalysis. We acknowledge the OSCAR Project Office for providing the ocean surface current analyses. The fCO₂ data at 6°S 10°W presented here are publicly available at the Carbon Dioxide Information and Analysis Center (CDIAC), www.cdiac.ornl.gov/oceans/ and in the SOCAT database (www.socat.info). We are grateful to Rik Wanninkhof and an anonymous reviewer for their comments that improve the manuscript.

Notes

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