Response of the Hydrological Cycle to Early Eocene Warmth: Insights from DeepMIP-Eocene

Investigating how the early Eocene (∼56–48 million years ago) hydrological cycle operated under elevated atmospheric CO2 concentrations and globally higher temperatures can provide important insights into understanding of current climate change and projects of future climate. Here, we investigate the global and zonal-mean rainfall patterns during the early Eocene using an integrated data-model approach. We leverage insights from the DeepMIP-Eocene suite of model simulations in combination with a compilation of paleobotanical proxies of precipitation. In short, the mid- and high latitudes, as well as the tropical band, are characterized by a thermodynamically-dominated hydrological response to warming, and overall wetter conditions (“wet-gets-wetter”). A more complex picture is painted for the subtropics. Although these are overall characterized by negative precipitation-evaporation anomalies (“dry-gets-drier”) in the DeepMIP models, there is surprisingly large inter-model variability in mean annual precipitation. Intriguingly, we find that models with weaker meridional temperature gradients (e.g., CESM, GFDL) are characterized by a reduction in subtropical moisture divergence, leading to an increase in MAP. These model simulations agree more closely with our new proxy-derived precipitation reconstructions and other key climate metrics and imply that the early Eocene was characterized by reduced subtropical moisture divergence. If the meridional temperature gradient was even weaker than suggested by those DeepMIP models, circulation-induced changes may have outcompeted thermodynamic changes, leading to wetter subtropics, thus going against the “wet-gets-wetter, dry-gets-drier” paradigm. This highlights the importance of evaluating multiple climate metrics against sets of simulations and can provide food for thought for DeepMIP phase two.