Water isotopic imprints of the Pliocene Pacific Walker Circulation

Ocean-atmosphere coupled models predict pronounced weakening of the Pacific Walker Circulation (PWC) with increasing CO₂ concentration due to enhanced tropospheric stability and reduced convective mass overturning. However, current observational results are inconsistent and do not confirm a clear weakening signal. The detection of the signature of increasing CO₂ is in part impeded by substantial internal variability and anthropogenic aerosol forcings. Here we explore the possibility of using a paleoclimatic analogue to understand the contemporary PWC sensitivity to CO₂ changes. We focus on the interval from mid-Piacenzian (MP, 3.3 – 3.0 Ma) to early Pleistocene (~2.4 Ma). The MP had elevated CO₂ concentrations (~400ppm) and geography, topology, and vegetation similar to today. Following the MP global CO₂ and temperature decreased, leading to the intensification of the Northern hemisphere glaciation. We seek to identify potential proxy constraints on model simulated PWC sensitivity to CO₂ forcing by focusing on changes in the hydroclimatology during this time interval. We developed several sets of isotope-tracking enabled CESM version 1.2 simulations, which utilize pre-Industrial and Pliocene boundary conditions, different CO₂ levels, and water tagging of 11 oceanographic regions to track the life cycles of various water species (H216O, H218O and HD16O). Preliminary results show that Pliocene boundary conditions have little impact on the relationship between the CO₂ forcing and the intensity of PWC. The precipitation δD contrast between the eastern and western tropical Pacific, scales well with the PWC strength, suggesting high potential for developing PWC strengths proxy with precipitation isotopic records from both sides of the tropical Pacific. Our ongoing work will further identify physical processes responsible for the simulated precipitation isotopic signals: i.e., whether they reflect changes in the moisture source, moisture transport, or moist convection at the destination. Additionally, prescribed-SST simulations will also be conducted to quantify the isotopic imprints of changing tropospheric instability from SST changes.