A variable and decreasing sink for atmospheric CO2 in the North Atlantic

A time series of observations from merchant ships between the U.K. and the Caribbean is used to establish the variability of sea surface pCO 2 and air‐to‐sea flux from the mid‐1990s to early 2000s. We show that the sink for atmospheric CO 2 exhibits important interannual variability, which is in phase across large regions from year to year. Additionally, there has been an interdecadal decline, evident throughout the study region but especially significant in the northeast of the area covered, with the sink reducing >50% from the mid‐1990s to the period 2002–2005. A review of available observations suggests a large region of decrease covering much of the North Atlantic but excluding the western subtropical areas. We estimate that the uptake of the region between 20°N and 65°N declined by ∼0.24 Pg C a −1 from 1994/1995 to 2002–2005. Declining rates of wintertime mixing and ventilation between surface and subsurface waters due to increasing stratification, linked to variation in the North Atlantic Oscillation, are suggested as the main cause of the change. These are exacerbated by a contribution from the changing buffer capacity of the ocean water, as the carbon content of surface waters increases.