Revisiting Ocean Dynamical Thermostat Mechanism for the Tropical Pacific SST response to Global Warming

Understanding changes in the pattern of tropical Pacific sea surface temperature (SST), especially its zonal contrast, in response to greenhouse gas forcing is essential for predicting future climate change. Among various mechanisms proposed, the ocean dynamical thermostat (ODT) associated with eastern Pacific upwelling may act to intensify the zonal SST contrast during the early stage of warming. However, its physical processes and efficiency in transient response remain controversial. Here we revisit the ODT mechanism by diagnosing the mixed-layer heat budget in both a simple coupled model (Zebiak–Cane model) and a complex GCM (MIROC6 abrupt 4xCO2 experiment). Following Clement et al. (1996), we decompose the ODT into two processes: ocean dynamical adjustment (ODA) due to mean upwelling and thermocline feedback (THF) due to anomalous upwelling, to investigate their roles in the SST pattern response to imposed surface heating. The SST pattern evolution is very different between the two models: initial eastern Pacific warming in the Zebiak–Cane model is quickly offset by ODA, enhancing the zonal SST contrast and triggering THF and horizontal advection that further cool the east, but MIROC6 exhibits a weakening of the zonal SST contrast from the beginning because the ODA cooling is overwhelmed by other processes. The contrast in the initial response is critical in their long-term response and it is explained mainly by differences in mean ocean currents and the spatial homogeneity of the radiative forcing.