El Nino–Southern Oscillation Reconstructions Research Articles

Paleo-ENSO revisited: Ecuadorian Lake Pallcacocha does not reveal a conclusive El Niño signal

Information about decadal to millennial variability of El Nino Southern Oscillation (ENSO) is fundamental for the assessment of ENSO responses to natural and anthropogenic forcings. Despite a growing number of ENSO reconstructions, the overall picture of Holocene ENSO variability is inconsistent. Here, we revisit the iconic Holocene ENSO sediment record of Lake Pallcacocha, Ecuador (Rodbell et al., 1999). We asked: (i) How coherent are the records of clastic layers (flood layers) in the sediments of Lake Pallcacocha and adjacent Lake Fondococha? (ii) What are the synoptic-scale atmospheric conditions that lead to intense precipitation and, potentially, to alluvial activity promoting the deposition of clastic layers in these lakes? (iii) Is intense precipitation in this area associated with El Nino, or not? We analyzed clastic layers in Late-Holocene sediments from multiple cores in Lakes Pallcacocha and Fondococha from Cajas National Park, southern Ecuadorian Andes. Additionally, we investigated precipitation data from 13 nearby meteorological stations to test if intense precipitation (percentiles P0.95, P0.99, P0.995) is predominantly related to El Nino conditions or not (based on 15 different ENSO indices). Our results show that the absolute flood frequencies (clastic layers per 100 years) differ substantially from lake to lake. This indicates that the frequency of clastic layers reflects different sensitivities (thresholds of precipitation) of the catchments to alluvial activity. 210Pb ages suggest that neither the 1982/83 nor the 1997/98 very strong El Ninos produced clastic layers comparable to those found in the late Holocene. Daily precipitation records from meteorological stations close to Lake Pallcacocha including a high-altitude station from the western slope of the Andes did not show unusually high precipitation during the super El Nino 2015/16. We further find that intense precipitation in this area occurs at roughly equal probability under El Nino, La Nina and neutral conditions. Although the spectral properties of the late Holocene clastic layers in Lake Pallcacocha fall into the typical ENSO frequency band, we do not find evidence in the recent sediments and the meteorological data that would support a diagnostic link between alluvial activity in Lake Pallcacocha and strong El Nino events. Our data do not support the idea that the (late) Holocene flood record of Lake Pallcacocha is a conclusive paleo-El Nino record

Divergent El Niño responses to volcanic eruptions at different latitudes over the past millennium

Detection and attribution of El Nino-Southern Oscillation (ENSO) responses to radiative forcing perturbation are critical for predicting the future change of ENSO under global warming. One of such forcing perturbation is the volcanic eruption. Our understanding of the responses of ENSO system to explosive tropical volcanic eruptions remains controversial, and we know little about the responses to high-latitude eruptions. Here, we synthesize proxy-based ENSO reconstructions, to show that there exist an El Nino-like response to the Northern Hemisphere (NH) and tropical eruptions and a La Nina-like response to the Southern Hemisphere (SH) eruptions over the past millennium. Our climate model simulation results show good agreement with the proxy records. The simulation reveals that due to different meridional thermal contrasts, the westerly wind anomalies can be excited over the tropical Pacific to the south of, at, or to the north of the equator in the first boreal winter after the NH, tropical, or SH eruptions, respectively. Thus, the eastern-Pacific El Nino can develop and peak in the second winter after the NH and tropical eruptions via the Bjerknes feedback. The model simulation only shows a central-Pacific El Nino-like response to the SH eruptions. The reason is that the anticyclonic wind anomaly associated with the SH eruption-induced southeast Pacific cooling will excite westward current anomalies and prevent the development of eastern-Pacific El Nino-like anomaly. These divergent responses to eruptions at different latitudes and in different hemispheres underline the sensitivity of the ENSO system to the spatial structure of radiative disturbances in the atmosphere.

Time-varying spectral characteristics of ENSO over the Last Millennium

The characteristics of El Nino–Southern Oscillation (ENSO) spectra over the Last Millennium are examined to characterise variability over past centuries. Seven published palaeo-ENSO reconstructions and Nino3.4 from six Coupled Model Intercomparison Project-Phase 5 and Paleoclimate Modelling Intercomparison Project-Phase 3 (CMIP5–PMIP3) Last Millennium simulations were analysed. The corresponding Historical and pre-industrial Control CMIP5–PMIP3 simulations were also considered. The post-1850 spectrum of each modelled or reconstructed ENSO series captures aspects of the observed spectrum to varying degrees. We note that no single model or ENSO reconstruction completely reproduces the instrumental spectral characteristics. The spectral power across the 2–3 years (near biennial), 3–8 years (classical ENSO) and 8–25 years (decadal) periodicity bands was calculated in a sliding 50 year window, revealing temporal variability in the spectra. There was strong temporal variability in the spectral power of each periodicity band in observed Nino3.4 and SOI and for all reconstructions and simulations of ENSO. Significant peaks in spectral power such as observed in recent decades also occur in some of the reconstructed palaeo-ENSO (around 1600, the early 1700s and 1900) and modelled series (around the major volcanic eruptions of 1258 and 1452). While the recent increase in spectral power might be in response to enhanced greenhouse gas levels, the increase lies within the range of variability across the suite of ENSO reconstructions and simulations examined here. This study demonstrates that the analysis of a suite of ENSO reconstructions and model simulations can build a broader understanding of the time-varying nature of ENSO spectra, and how the nature of the past spectra of ENSO is to some extent dependant on the climate model or palaeo-ENSO reconstruction chosen.