Extreme El Nino Events Over the Last Millennium: A Model/Proxy Synthesis Approach

The El Nino/Southern Oscillation (ENSO) is the strongest source of interannual climate variability, yet its response to climate change remains uncertain. Paleoclimate evidence from tropical coral reefs, in the form of oxygen isotopic ratios (δ¹⁸O) is often used to provide a longer observational baseline with which to evaluate climate model projections. However, the interpretation of δ¹⁸O records can be difficult, as they are sensitive both to changes in sea surface temperature and the δ¹⁸O of the surrounding seawater, which tracks hydrological processes. Here we present results from a synthesis of data sources, illustrating the dynamics of changes to coral δ¹⁸O over the last millennium: these include coral δ¹⁸O records, observational data from a central Pacific field campaign and a reanalysis product with the isotope-enabled Regional Ocean Modeling System (isoROMS) covering the 2014-present period, and new simulations with the isotope-enabled Community Earth System Model (iCESM) spanning 850-2005. The temperature and hydrological impacts of the 2014-15 and 2015-16 El Nino events are explored in detail, and used to develop metrics for 'extreme' El Nino events further back in time. The ability of the iCESM to simulate observed changes to extremes is discussed, as is the potential role for externally forced trends in their behavior.