Reconciling simulated and coral-inferred changes in El Niño-Southern Oscillation variability during the Holocene

Abstract:

Uncertainties surrounding the El Niño-Southern Oscillation’s (ENSO) response to anthropogenic warming motivate the study of past tropical climate variability, which are time periods when changes in background conditions forced climate changes different than those today. The Holocene Epoch (defined as 11.65 thousand years ago to present) provides an opportunity to test how ENSO responds to slowly varying changes in orbital, namely precessional forcing that cause changes in the seasonal insolation. Although several climate model simulations for the mid-Holocene (6 thousand years ago, ka) show a reduction in ENSO variance, proxy observations paint a more nuanced picture. Here we present a model-data comparison using coral geochemical observations from the central equatorial Pacific in conjunction with model output for several time slices throughout the Holocene (12 ka, 9 ka, 6 ka, 3 ka, and 0 ka) from the Community Earth System Model (CESM version 1). By processing the climate model output with a coral proxy system model of intermediate complexity, we assess the simulated changes in paleo-ENSO variance through the lens of a coral, and demonstrate a hypothesis with which to test using coral proxy data. The CESM time slices coupled to the coral proxy system model, simulate lower ENSO variability during the early and mid-Holocene and increased ENSO variability during the late Holocene, a result supported by a synthesis of published coral geochemical data from the Kiritimati and Fanning Atolls. Taken together, we provide a new perspective on long-term changes in ENSO variability, and advance efforts to constrain changes in interannual variability during the Holocene.