Variation of pCO2 along a North Atlantic shipping route (U.K. to the Caribbean): A year of automated observations

Underway measurements of the partial pressure of carbon dioxide (pCO2) in the ocean surface and overlying atmosphere have been made using a fully automated instrument installed on a merchant ship. This ship travelled between the U.K. and the Caribbean with an unattended pCO2 instrument operating between June 1994 and August 1995. Each voyage was five weeks long with slightly different, repeated routes for the outbound and return transects. Seasonal variability in the ocean surface saturation state of carbon dioxide has been quantified from the two resultant pCO2 time series. The large scale seasonal changes in a broad region of the mid-Atlantic are consistent with the thermodynamic change in pCO2 due to seasonal temperature fluctuation. However, to the north and east of the region studied the pCO2 variation was in antiphase to the temperature signal, due to deep winter mixing and spring–summer biological activity. Large variations in oceanic pCO2 were also observed on relatively small spatial scales, particularly in coastal and shelf waters, where the effects of phytoplankton activity are more obvious than in the open ocean. Our data return demonstrates the feasibility of unattended data acquisition, a cost effective method of substantially expanding global oceanographic and biogeochemical databases. The design, installation and technical details of the pCO2 instrument are also described in this paper.