Central Tropical Pacific Corals Reveal Reduced ENSO Variability 3-5kyBP

Future projections of the strength of the El Niño-Southern Oscillation (ENSO), the largest source of year-to-year global climate extremes, are highly uncertain. Potential shifts towards stronger and more frequent ENSO extremes (Cai et al., 2014; Kim et al., 2015) would have a profound effect on climate globally. However, the instrumental record of ENSO activity is too short in time to resolve potential anthropogenic trends in ENSO properties, and limits our understanding of the ENSO phenomenon. Thus, we must rely on high-resolution paleoclimate reconstructions of ENSO that extend through the last centuries to millennia to provide a comprehensive view of ENSO variability. Coral δ18O records from the heart of the ENSO region, in the central tropical Pacific, provide monthly-resolved reconstructions of ENSO activity over the last 7000 years. Here, we quantify ENSO variability in 10 new monthly-resolved fossil coral δ18O records from Kiritimati Island (2°N, 157°W) that are U/Th-dated to the 2-6kyBP interval. When combined with previously published coral δ18O records from Cobb et al., 2013, the new coral δ18O records support a prolonged reduction of ~60% in ENSO variability during the 3-5kyBP interval, as compared to the late 20th century. In comparison, ENSO variability during the last millennium was ~30% reduced compared to the late 20th century. These results are consistent with foraminifera and mollusk records from the eastern tropical Pacific (Koutavas and Joanides, 2012; Carre et al., 2014), implying that the observed 3-5kyBP reduction in ENSO variability was not confined to the central Pacific. Taken together, these new records represent a new target - both in terms of amplitude and timing - for modeling efforts designed to uncover the mechanisms governing past ENSO variability. Such data-model comparisons are critical to refining the simulation of ENSO in simulations of future climate change.