Spatial and temporal distribution of physical and CO2 properties in the English Channel based on voluntary observing ships between 2006 and 2021

Spatial and temporal evolution of sea surface temperature, salinity and CO₂ properties are studied in the English Channel (EnC) (48.8°N-5.2°W and 51.2°N-1.5°E) from 2006 to 2021. In situ measurements are collected using voluntary observing ships (VOS) as part of the ICOS program, during repeated transects every year, providing a good temporal coverage to study monthly to interannual variability in the area. The analysis of the longitudinal distribution of the parameters highlights a strong east-west difference. SST decreases from West to East in winter and spring, while the opposite gradient is observed at the end of summer and in autumn. During the month of July, a strong SST gradient up to 3 °C is observed around 3°W. Along the transect, the salinity slightly decreases from West to East, with a higher variability in the Eastern EnC. Mean SST in the EnC varies between 9 in March and 17°C in August-September and the mean difference between sea-water fCO₂ and atmospheric fCO₂ (ΔfCO₂) ranges from -45 µatm at the end of spring/beginning of summer to 40 µatm in autumn. Differences in seasonality and variability are observed between the Western and Eastern EnC. For example, a strong sink of CO₂ is observed in summer in the Western EnC, while in the Eastern EnC, the strongest sink occurs in spring. These CO₂ sinks are associated with a rise in biological activity as shown by the very high surface Chl-a concentrations observed from satellite images. To better understand the physical and biological processes behind the fCO₂ variations, relations between parameters are examined. Finally, interannual evolution of SST, SSS and CO₂ properties are discussed to assess the long-term changes in this region.