Climate and Climate Sensitivity to Greenhouse Gas Forcing Under a Range of Ocean Circulations

The ocean shapes Earth's climate and its response to greenhouse gas forcing. Previous studies have shown that different continental geometries produce vastly different climate states through their influence on ocean circulation. Meanwhile, ocean circulation governs the climate response to greenhouse gas forcing through both ocean heat uptake and the sea-surface temperature pattern effect, wherein effective climate sensitivity increases over time owing to the fact that the regions of more positive feedbacks tend to be delayed in their warming. However, a clear theory for why positive feedback regions are slow to warm, or the robustness of this finding, is lacking. Moreover, the total meridional heat transport and its partitioning is projected to change under CO2 forcing due to changes in top of atmosphere and surface radiative fluxes. Using a novel ocean-atmosphere-sea-ice coupled idealized model configuration (MOM6-AM2), we study the climate and its response to greenhouse gas forcing under four idealized continental geometries: Aqua, which has no continents and thus no ocean gyres; Ridge, which has one narrow strip of land from pole to pole; Drake, which has a narrow land boundary from the north pole to 50oS; and NPac DD, which has two ridges that form a narrow basin and a wide basin connected by a Southern Ocean, and an Atlantic-like meridional overturning circulation in the narrow basin. Each of these configurations is run to steady state and then forced with doubled atmospheric CO2. With this ensemble of idealized geometry configurations, we first examine the mean-state pole-to-equator temperature gradient and partitioning of ocean and atmosphere heat transport. We then analyze how the transient response and equilibrium climate sensitivity under greenhouse gas forcing differ between these four configurations. We consider how the efficiency of ocean heat uptake depends background ocean circulation and test the hypothesis that regions of ocean upwelling drive the time-evolution of effective climate sensitivity as they experience delayed warming and positive cloud and lapse-rate feedbacks.