Session: Benchmarking & cross-cutting Group 1 (Isotope modelling, COMPARE)
Author: Samantha Stevenson / firstname.lastname@example.org / National Center for Atmospheric Research
Co-author: Bette Otto-Bliesner, National Center for Atmospheric Research;
Esther Brady, National Center for Atmospheric Research;
Clay Tabor, National Center for Atmospheric Research;
Jesse Nusbaumer, NASA Goddard Institute for Space Studies;
The response of climate variability in the tropical Pacific (i.e. the El Nino/Southern Oscillation, or ENSO) to external forcing is a major outstanding question. Since the instrumental record is too short to mitigate internal ENSO variability, observational targets often include multi-century oxygen isotopic records from tropical corals. However, model/proxy comparison is complicated by the fact that most climate models do not directly simulate seawater oxygen isotopic composition. Here we present first results from the newly completed Last Millennium simulation with the isotope-enabled Community Earth System Model (CESM), covering the 850-2005 period with all anthropogenic and natural forcings included. CESM simulates ENSO variability quite well, albeit with an amplitude stronger than observed, and this simulation constitutes an isotope-enabled complement to the CESM Last Millennium Ensemble (LME). The structure of seawater oxygen isotopic signals (seawater d18O) associated with ENSO variability is investigated, and compared with forward-modeled coral d18O patterns which include the effects of temperature. Unforced ENSO variability appears to dominate over much of the past millennium, but effects from volcanic eruptions and greenhouse gas forcing do appear to be detectable in some circumstances. The implications for evaluating the fidelity of Last Millennium model simulations using coral proxy records are discussed.