Revisiting the low-gradient problem with weather-resolving atmosphere-ocean coupled simulations

The reduction in the tropical to mid-latitude sea surface temperature gradient, as shown by proxy records across the Pacific, is a common feature of past warm intervals but remains difficult for climate models to replicate. This model-data discrepancy (termed “the low-gradient problem”) is potentially tied to the parameterizations of cloud and moist convection in the models, which remain highly uncertain and largely limit the confidence of predicted future climate change. Here, focusing on the mid-Pliocene (4 - 3 Ma) interval, for which climate forcing conditions are relatively well constrained and global SSTs are well sampled, two sets of atmosphere-dynamical ocean coupled simulations with (at 25 km) and without (at 100 km) weather-resolving atmospheric resolution using Community Earth System Model version 1.3 are compared to identify whether better resolved cloud and moist convection can ameliorate the low-gradient problem. Preliminary results show more extensive mid-Pliocene warming in the North Pacific relative to the preindustrial simulated in the high-resolution simulations compared to the low-resolution simulations, whereas tropical Pacific warming is similar in both sets of simulations. The approximate partial radiative perturbation method and heat transport decomposition will be implemented to quantify differences in the shortwave cloud effects and responses of atmospheric dry static and latent energy and ocean heat transport between the high- and low-resolution simulations. The contributions from better resolved clouds and moist convection to the amplified North Pacific warming in the high-resolution simulations will be further quantified.