Evaluating the ENSO Impact on Last Millennium Megadroughts Using Improved Coral Forward Models

Wednesday, 17 December 2014
Samantha L Stevenson1,2, Brian Powell3, Kim M Cobb4, Bette L Otto-Bliesner2, Mark A Merrifield1,5, David C Noone6,7, Jesse M Nusbaumer7,8, Esther C Brady9, John Fasullo2, Andrew Mai2 and Nan A Rosenbloom2, (1)University of Hawaii, Honolulu, HI, United States, (2)National Center for Atmospheric Research, Boulder, CO, United States, (3)Univ. of Hawaii, Honolulu, HI, United States, (4)EAS Georgia Tech, Atlanta, GA, United States, (5)Sch Ocean & Earth Sci & Tech, Honolulu, HI, United States, (6)Oregon State University, College of Earth, Ocean and Atmospheric Sciences, Corvallis, OR, United States, (7)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (8)University of Colorado at Boulder, Atmospheric and Oceanic Sciences, Boulder, CO, United States, (9)National Center for Atmospheric Research, Boulder, Boulder, CO, United States
Extreme "megadroughts" of decadal or longer duration are observed in the proxy record, and the persistence/severity of such events are thought to be influenced by the cold phase of the El Nino/Southern Oscillation (ENSO). But the magnitude of this effect is difficult to quantify, both because of intrinsic atmospheric variability and because of limitations in the record of ENSO variability during megadrought epochs. Here a new, isotope-enabled version of the Regional Ocean Modeling System (isoROMS) is used to develop improved 'forward model' translations between ENSO variability and the oxygen isotopic signal (delta-18O) recorded in tropical coral skeletons over the 1940-2009 period. These forward models are nonlinear, and additionally vary with spatial resolution: at Palmyra Atoll, the action of tropical instability waves is shown to significantly impact temperature and seawater delta-18O variability in the near-reef environment. The isoROMS forward models are applied to coral records spanning the past millennium, and the resulting ENSO estimates compared with a new suite of Last Millennium simulations performed with the NCAR Community Earth System Model. Implications for identifying megadroughts forced by the tropical Pacific are discussed.