Regional Precipitation Influenced by Stationary Wave Changes in Model of Mid-Pliocene Climate

Abstract:

The mid-Pliocene warm period roughly 3 million years ago, during which atmospheric CO₂ concentrations were comparable to those of the present, is often invoked as our best recent analog for a future warmer climate. To understand this period’s hydroclimate, we analyze simulations of the mid-Pliocene warm period climate with the University of Toronto Version of the Community Climate System Model Version 4 (UofT-CCSM4). We find that two areas where atmospheric rivers (ARs) have a strong influence on local weather and climate in the present day — the Pacific coast of North America and the Pacific coast of Chile — experience opposite patterns of change in extreme precipitation (compared to the pre-industrial simulation) in the mid-Pliocene simulation. We link the precipitation changes to dynamical shifts in AR behavior, especially in the Northern Hemisphere winter. In particular, a Northern Hemisphere stationary wave train present in the pre-industrial control nearly disappears in the mid-Pliocene simulation, corresponding with fewer ARs making landfall over western North America. We analyze the sensitivity of stationary wave patterns to model boundary conditions to better understand the response of Northern Hemisphere stationary waves to mid-Pliocene ice sheets and orography, independent of CO₂ forcing. Further, we compare model output to an updated network of proxy reconstructions of Pliocene hydroclimate, and identify factors that may contribute to apparent discrepancies between proxy evidence and model simulations of midlatitude regional hydroclimate patterns. In addition to exploring the links between regional responses to past and future climate changes, this study has implications for the design of future modeling studies of mid-Pliocene climate, and for future efforts to improve reconstructions of the cryosphere, continental configuration, and hydroclimate.