Annual carbon fluxes in the upper Greenland Sea based on measurements and a box-model approach

Abstract

Measurements of nitrate and the carbonate system parameters performed mainly from 1993 to 1997 have been used to estimate the evolution of the concentration fields over the year in the surface and underlying waters of the central Greenland Sea. This, together with synoptic surface wind data from the NCEP/NCAR reanalysis project, is used to evaluate the vertical mixing, the biological production and decay, as well as the air-sea exchange of CO2 in the region. In the winter season, the vertical mixing dominates the change of the nitrate concentration in the surface water. The mixing factor estimated for this season is used to compute the addition of chemical constituents to the surface water from below. The residual nitrate concentration change, after the mixing contribution has been subtracted, is attributed to biological production or decay. The computations are performed with a 1-day resolution and initially the advective contribution is neglected, as the horizontal gradients in the central Greenland Sea gyre are small. Following this approach, the air-sea flux of CO2 is directed into the sea all year around, with an annual uptake of 53 ± 4 g C m-2 yr-1 for the years 93 to 97. The carbon flux as driven by biology shows a strong primary production peak around Julian day 140 followed by a decrease which turns into decay of organic matter at about day 200. Summarizing the biological activity in the surface water over the year gives a new production of 34 g C m-2 yr-1. The vertical flux of dissolved inorganic carbon into the surface water from below amounts to 11 g C m-2 yr-1. The build up of carbon in the surface water, 30 g C m-2 yr-1, is explained by that the temperature of the outflowing water is approximately 2°C colder than the inflowing water, giving a higher dissolved inorganic carbon concentration as a result of the increase in the solubility of carbon dioxide with lower temperatures. The uncertainties in the above stated numbers are ± 20%.