Observing recent trends in the carbon cycle of the Northeast Pacific

On the basis of the Line P program, we investigate the processes driving and modulating the long-term changes observed in the carbon cycle of the Northeast Pacific. Analysis of continuous observations (approximately 3 times per year) of dissolved inorganic carbon (DIC) from 1990 to 2018 reveal significant positive trends in surface DIC, in line with that of atmospheric pCO₂. Even though the main driver at the surface is the increase in anthropogenic CO₂, the magnitude of the DIC trend is affected by changes in surface salinity that tend to have an opposing effect at the coastal and oceanic ends of the transect. The positive DIC trend at the surface becomes weaker with depth and increases again below the thermocline, i.e., in the realm of the North Pacific Intermediate Water (150 – 400 m depth). However, the vertical profile of the trends and their magnitude is variable along line P. Here we contrast this vertical and spatial variations of the long-term DIC trends along line P and discuss the potential physical and biological processes driving such variability.