Department of Earth System Science

Previous research has shown that the atmospheric pCO₂ sensitivity to changes in low-latitude sea-surface chemistry (“low-latitude sensitivity”) depends on both the volume of the ocean ventilated from low latitudes and on the degree of air-sea disequilibrium at high latitudes. However, it is not clear which effect is more important. In this paper we present a diagnostic framework for quantifying the relative importance of low-latitude ventilation versus high-latitude air-sea disequilibrium in determining the low-latitude sensitivity of ocean carbon cycle models. The diagnostic uses a Green function that partitions the ocean's carbon inventory on the basis of whether the carbon last interacted with the atmosphere in the low latitudes or in the high latitudes. The diagnostic is applied to a simple 3-box model, a box model with a ventilated thermocline, and a suite of OGCM runs meant to capture a range of possible ocean circulations for present and last-glacial-maximum conditions. The diagnostic shows unambiguously that the OGCM has a greater low-latitude sensitivity than the box models because of the greater amount of water ventilated from low latitudes in the OGCM. However, when applied to the suite of OGCM runs, the diagnostic also reveals that the effect of high-latitude air-sea disequilibrium can sometimes dominate the effect of low-latitude ventilation and is highly sensitive to the state of the ocean circulation. In particular, the magnitude of the high-latitude disequilibrium effect correlates strongly with the strength of the Atlantic meridional overturning circulation and the volume of water ventilated from northern high latitudes.