Effects of Tide-Induced Mixing on the Equator-to-Pole Surface Temperature Gradient during the Middle Miocene Climate Optimum

Abstract Based on reconstructions from proxy records, the Middle Miocene Climate Optimum (MMCO, 16.9–14.7 Ma) was a relatively warm interval of amplified polar warmth and a weaker-than-present equator-to-pole meridional temperature gradient. However, simulating the exact weaker-than-present gradient reconstructed from proxy records is still challenging. Several studies of both past and present oceans have shown that tidal mixing strongly influences the climate. However, none of the previous climate simulations of the MMCO have considered the impact of tidal dissipation thus far. This study estimates the tidal dissipation and mixing in the deep ocean during the MMCO using the coupled model FGOALS-g3. A set of sensitivity experiments was conducted to investigate the effects of tidal mixing on the equator-to-pole meridional temperature gradient during the MMCO. The results show that compared to the experiment without tidal mixing, the experiment with tidal mixing featured an increase in the surface air temperature (SAT) of only 0.23°C at low latitudes and as much as 1.98°C at high latitudes during the MMCO, leading to a 1.75°C reduction in the simulated equator-to-pole meridional surface temperature gradient. These results are related to the enhancement of meridional overturning circulation and oceanic meridional heat transport from the equatorial zones to the poles with increasing ocean vertical mixing when tidal mixing is considered. Overall, our study shows that considering tidal mixing can improve the simulation of the meridional temperature gradient during the MMCO, which provides a feasible method for improving the simulation of the MMCO climate. Significance Statement Almost all simulations during the Middle Miocene Climate Optimum (MMCO, 16.9–14.7 Ma) have a surface meridional temperature gradient between the equator and the poles that is at least ∼10°C larger than that reconstructed from proxies, and none of these simulations have considered the impact of tidal dissipation and mixing. Hence, this study aims to investigate the effect of tide-induced mixing in the open deep ocean on the simulation of the equator-to-pole temperature gradient during the MMCO using the coupled model FGOALS-g3. We show that when considering the influence of tidal mixing, the equatorial and high-latitude mean surface air temperatures increase by 0.23° and 1.98°C during the MMCO, respectively, leading to a 1.75°C reduction in the simulated equator-to-pole meridional surface temperature gradient. Our results show that considering the effect of tidal mixing can improve the agreement between climate model results and proxy records. This provides a feasible method for improving the simulation of the MMCO climate.