Extreme ENSO variability during MIS5e evidenced by fossil mollusk isotopic records from Peru

Abstract:

Assessing the full range of natural ENSO variability requires paleo-ENSO reconstructions in a wide range of past climates. During the last interglacial period, 130 to 116 thousand years ago, the Earth experienced warmer climate in response to a strong orbital forcing. This period offers an opportunity to explore ENSO behaviour under different mean climate conditions. We present here a quantitative estimate of ENSO variability during MIS5e in coastal Peru, a region where ENSO influence is among the highest in the world. Seasonal to interannual variability of sea surface temperature (SST) was reconstructed from monthly-resolved oxygen isotope records (δ¹⁸O) in 10 well preserved fossil mollusk shells (cumulating ~30 years) from an uplifted marine terrace. Shell δ¹⁸O variations are only determined by SST changes in this hyperarid region. Results are compared to modern and Holocene reconstructions using shells of the same species from the same area. We find that the amplitude of the annual cycle of SST was ~30% and 66% larger compared to today and the Holocene, respectively. ENSO variability, estimated as the standard deviation of seasonal range anomalies, was 110% and 140% larger in the MIS5e sample compared to the modern and the Holocene sample respectively. The skewness of ENSO anomalies in the MIS5e was larger than today due to very strong El Niño events, suggesting a prevalence of the Eastern Pacific ENSO mode in that period. The result supports a positive relationship between the amplitude of the annual cycle and the amplitude of ENSO variability. This paleoclimate record shows that ENSO variability can potentially be twice as strong as today, a level of activity that was unknown so far in the Eastern Tropical Pacific.