The solubility pump of carbon in the subtropical gyre of the North Atlantic

The subduction of carbon is examined using abiotic models of the solubility pump in the subtropical gyre of the North Atlantic. The importance of the seasonal cycle of the mixed layer, and advection of carbon, is examined using sensitivity experiments with a Lagrangian model of the carbon system. The rate of subduction of carbon is found to be strongly influenced by the gradients in mixed-layer thickness over the gyre and, to a lesser extent, modified by the end of winter bias in the properties of subducted fluid. A seasonally-cycling geochemical model of the carbon system is then developed for the North Atlantic. The model is diagnosed to examine the seasonal exchange in carbon between the atmosphere and ocean induced by the seasonal warming and cooling. There is a net annual air-sea flux of carbon into the subtropical gyre of the model due to undersaturation of p(CO2) with respect to the local equilibrium with the atmosphere. The undersaturation is due to advection of carbon by the circulation. Along the path of the Gulf Stream, northward advection and cooling of the low latitude waters is so rapid that the surface waters are significantly undersaturated in carbon. Due to its long equilibration period, there is a resultant air-sea flux of carbon dioxide over the northern flank and interior of the subtropical gyre. Warm, low carbon water from the tropics is fluxed into the southern flank of the subtropical gyre in the Ekman layer, inducing an oceanic uptake of carbon there. The model experiments suggest that it is necessary to account for advection to close the carbon budget in the observed time-series measurements at Bermuda.