An Assessment of the Role of the North Atlantic as a CO2 Sink

A numerical interpolation scheme based upon the lateral diffusive and advective transport of ocean surface waters has been developed to interpolate measurements made in irregular time and space over the oceans. This has been applied to about 2700 surface ocean pCO₂ measurements observed during the period 1972-1992 in the North Atlantic to give the distribution of sea-air pCO₂ difference (Δ pCO₂) over the Atlantic. Although the atmospheric CO₂ concentration has increased by about 28 ppm over this period, the pCO₂ values in the surface waters of subarctic regions have increased little because they are dictated primarily by the properties of underlying deep waters through vertical mixing. Accordingly, Δ pCO₂ values measured north of 50 degrees N have been corrected to the year 1990 using the secular increase of atmospheric CO₂. Because the surface water pCO₂ value in temperate waters tracks the secular increase in atmospheric CO₂ with a time lag of about two years, no correction was applied to the warm water Δ pCO₂ data. It has been assumed that seasonal variations are the same for each year. The net CO₂ flux across the sea surface has been computed over a 4 degrees latitude × 5 degrees longitude grid using the mean monthly Δ pCO₂ values and the gas transfer coefficients estimated using the mean monthly wind speed. It has been found that the areas of the high latitude North Atlantic and the adjacent seas north of 42 degrees N are net CO₂ sinks of 0.2 to 0.5 Gt C a^-1. The total sink flux of CO₂ over the temperate North Atlantic areas between 18 degrees N and 42 degrees N is balanced approximately by the source flux over the tropical Atlantic between 18 degrees N and 18 degrees S.