Dr. Ran Feng, University of Connecticut: Do Pliocene records support weakened Pacific Walker Circulation with elevated CO2?

Ocean-atmosphere coupled models predict pronounced weakening of the Pacific Walker Circulation (PWC) with increasing CO2 associated with the enhanced tropospheric stability and reduced convective mass overturning. However, observational evidence is inconclusive and do not confirm a clear weakening signal. Understanding the role of CO2 in driving PWC changes is in part impeded by substantial internal variability and anthropogenic aerosol forcings incorporated in instrumental records. Here, we explore the possibility of using a paleoclimatic analogue, i.e., the interval from the mid-Piacenzian (3.3 – 3.0 Ma) to early Pleistocene (~2.4 Ma),  to understand the contemporary PWC sensitivity to CO2 changes. We focus on identifying imprints of PWC changes on water isotope archives. Existing research has shown high potential for developing proxy constraints for PWC variability during the last millennium using water isotope archives.

A set of isotope-tracking enabled and atmosphere-land-ocean coupled iCESM1.2 simulations were developed to explore this question. These simulations utilized the water tagging technique to track life-cycles of different water species (H216O, H218O and HD16O), and ran with mid-Piacenzian boundary conditions and a range of CO2 values. They revealed a linear relationship between PWC strength and precipitation δD contrast between the eastern and western equatorial Pacific as a result of weakened convection primarily in the western equatorial Pacific. Moreover, varying east-west contrast of seawater δ18O across the equatorial Pacific strongly scales with varying equatorial zonal ocean current and wind stress, suggesting an important role of PWC in modulating seawater δ18O through changing surface wind stress. Estimated seawater δ18O evolution during the past ~3.5 Myrs, derived from published carbonate δ18O and SST estimates, is qualitatively consistent with our simulated seawater δ18O changes with mid-Piacenzian and preindustrial CO2 levels, and hence, supporting a weaker PWC with elevated CO2 during the mid-Piacenzian. These results therefore lend support to the predicted future weakening of the PWC by many atmosphere-ocean coupled models. Ongoing work will further test the effect of glaciation and uncertainties in prescribed ice sheets on simulated water isotope responses.

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