Pacific and Atlantic Modes of Overturning in the Miocene Climatic Optimum

During the Cenozoic Era, the ocean's meridional overturning circulation (MOC) has alternated between North Pacific and North Atlantic sinking modes. The Miocene Climatic Optimum (17.0–14.7 Ma) is a key interval for reconstructing this history because there is partial and inconclusive evidence for both MOC modes during this period. Here we investigate the MOC during the Miocene Climatic Optimum using two different climate models, GFDL CM2.1 and ACCESS-ESM1.5. Simulations are forced with atmospheric CO2 levels of pre-industrial concentration (286 ppm), double (572 ppm) and triple (858 ppm) CO2- the latter two falling within proxy-based estimates for this period.

In the GFDL CM2.1 model, we find either North Pacific overturning or North Atlantic overturning modes at all three CO2 levels, depending on the details of the paleogeography. Arctic-Atlantic gateways are especially important in controlling the freshwater balance, and hence surface density, in the North Atlantic sinking regions. By contrast, in the ACCESS-ESM1.5 model, we find that North Atlantic overturning consistently occurs at pre-industrial CO2 only. At double or triple CO2, the model becomes increasingly stratified, leading to a weakening or collapse of the global overturning circulation. The more stratified regimes are linked to a significantly higher climate sensitivity in ACCESS-ESM1.5, with intensified surface buoyancy changes.  These markedly different overturning regimes have major implications for deep ocean oxygenation, with the stratified cases becoming largely hypoxic in the deep ocean, while cases with active overturning remain well oxygenated.