Water isotopic imprints of Pacific Walker Circulation responses to CO2 decline during the late Pliocene and early Pleistocene

Ocean-atmosphere coupled models predict pronounced weakening of the Pacific Walker Circulation (PWC) with increasing CO₂ concentration associated with the enhanced tropospheric stability and reduced convective mass overturning. However, instrumental observations from the past few decades are inconsistent and do not support a clear weakening of the Walker circulation. The detection of the role 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 mid-Piacenzian boundary conditions, different CO₂ levels, and water tagging of several key 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 is linked to varying rates of moisture convergence change, and 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. Furthermore, coupled simulations are being carried out to understand seawater isotopic signatures of PWC related precipitation changes, and contributions from changing ocean dynamics to PWC changes during this interval.