Identifying the Fingerprints of Early Pliocene-like Sea Surface Temperature Gradients with an Isotope-Enabled General Circulation Model

The Pliocene epoch is frequently studied to gain insights about future climate change, due to its atmospheric CO2 levels near anthropogenically elevated values, similar geography, and a global mean surface temperature estimated at 3-4C above pre-industrial values. Using an atmospheric general circulation model forced with a sea surface temperature (SST) climatology that resembles Early Pliocene proxy SST data, Burls & Fedorov (2017) show that Early Pliocene SST patterns could have supported wetter subtropics, rather than the drying predicted for modern warming. The discrepancy in the hydrological response seen between simulations of future global warming and this Pliocene simulation is due to the greatly reduced SST gradients in the Early Pliocene, which reduce the strength of the Hadley circulation and subtropical moisture divergence. However, the interpretation of some Pliocene SST proxies is still debated, placing some uncertainty on the magnitude of large-scale Pliocene SST gradients. One avenue toward constraining this uncertainty in Pliocene warming patterns is to establish the degree of dynamical consistency between Pliocene SST reconstructions and hydrological cycle reconstructions. To this end, hydrological cycle reconstructions are needed in regions whose water isotopic signals are predicted to be the most unique to given Pliocene conditions. Here, we attempt to identify such regions by using an isotope-enabled GCM, iCESM1.2, to model the distribution of water isotopes in precipitation in response to four climatological SST and sea-ice fields representing modern, abrupt 4xCO­­2, Late Pliocene and Early Pliocene climates. We identify two regions by their isotopic signals in precipitation as distinct dynamical fingerprints of Early Pliocene SST gradients. The first region, the West Pacific tropical warm pool, is characterized by isotopic enrichment due to weakened convection and a reduced amount effect. The second region, the Sahel, is characterized by isotopic depletion due to more intense and widespread precipitation. A limited model-data comparison with precipitation proxies provides promising initial results. However, more data are needed in both of our identified regions to provide robust evidence of dynamical consistency with current Early Pliocene SST reconstructions.