Isotopic Fingerprints of Early Pliocene-like Sea Surface Temperature Gradients

The Pliocene epoch offers insights into future climate change, with near-modern atmospheric pCO₂ and global mean surface temperature estimated to be 3-4°C above pre-industrial. The discrepancy in the hydrological response seen between simulations of future global warming and early Pliocene simulations is hypothesized to result from reduced SST gradients in the early Pliocene. However, the interpretation of Pliocene SST proxies is still debated, generating uncertainty about the reduced gradient scenario. One avenue toward reducing 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 where water isotopic signals are predicted to be uniquely sensitive to Pliocene SST gradient changes. Here, we seek to identify these regions using an isotope-enabled GCM, iCAM5, 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 with distinct precipitation isotope fingerprints resulting from early Pliocene SST gradients. The first region, the Indo-Pacific 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 model-proxy comparison with available precipitation proxies in Africa provides promising initial results. However, additional proxy reconstructions are needed in both target regions to provide robust tests of dynamical consistency with current early Pliocene SST reconstructions.