The wind-driven, subtropical gyres and the solubility pump of CO2

Abstract

Using a suite of abiotic, ocean-atmosphere, carbon cycle models we demonstrate that the representation of the ventilated thermocline leads to a significant enhancement of the sensitivity of atmospheric pCO2 to subtropical surface ocean properties. In particular, we study an idealized sector, ocean circulation and abiotic carbon cycle model with a coupled atmospheric reservoir and examine the solubility pump of CO2 in the subtropical oceans. We compare solutions for atmospheric pCO2 when driven only by buoyancy forces to those with both buoyancy and wind stress forcing. Introducing the wind stress leads to the formation of a subtropical gyre and the warm lens of the ventilated thermocline. This lens is depleted in carbon relative to the surrounding waters since its properties are inherited from the warm, subtropical surface ocean. It is undersaturated in carbon since subduction quickly follows the strong cooling in the western boundary current before equilibration with the overlying atmosphere can occur. Plausible wind stress patterns increase atmospheric pCO2, relative to the case without wind forcing, and double the sensitivity of atmospheric pCO2 to perturbations of the low latitude surface carbon system properties. We suggest that it is the resolution of the ventilated thermocline in global, three-dimensional, ocean circulation models that enhances their sensitivity of atmospheric pCO2 to warm surface water properties relative to highly idealized box models.