Analysis of ENSO\u2019s response to unforced variability and anthropogenic forcing using CESM

Benjamin Vega-Westhoff,Ryan L Sriver

doi:10.1038/s41598-017-18459-8

Benjamin Vega-Westhoff, Ryan L Sriver

Open Access

https://doi.org/10.1038/s41598-017-18459-8

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Journal: Scientific Reports	Publication Date: Dec 1, 2017
Citations: 26	License type: open-access

Affiliation: University of Illinois Urbana-Champaign

Abstract

Understanding how the El Niño-Southern Oscillation (ENSO) may change with climate is a major challenge, given the internal variability of the system and relatively short observational record. Here we analyze the effect of coupled internal variability on changes in ENSO under anthropogenic global warming using the Community Earth System Model (CESM). We present results from a ~5000 year control run with constant pre-industrial conditions and a 50-member climate change ensemble experiment, consisting of historical hindcasts (1850–2005) and future projections to 2100 following representative concentration pathway 8.5 (RCP8.5). Given this large single-model ensemble, we are able to use simple statistical analyses to compare the effects of anthropogenic climate change with the effects of natural modulations in ENSO sea surface temperature (SST) metrics, as well as how internal variability may change with global warming. Changes in eastern Pacific ENSO SST metrics due to climate change are secondary to the model’s natural modulations; however, central Pacific ENSO amplitude significantly decreases, to an extent comparable with natural modulations. We also assess the sensitivity of internal variability estimates to ensemble size. The primary role of natural modulations in this ensemble highlights the importance of careful assessment of ocean-atmosphere internal variability in ENSO projections.

Highlights

Natural variations within Earth’s climate system are an important source of uncertainty in projections of future climate, on inter-annual to decadal timescales[1]
El Niño (La Niña) events correspond to a positive Niño3.4 index, with the strength of the event tied to the magnitude of the temperature anomaly (e.g., Fig. 1a)
Several El Niño-Southern Oscillation (ENSO) metrics can be derived from the Niño3.4 index time series: the spectrum reveals the frequency distribution of events, skewness roughly measures the relative strength of El Niño and La Niña events, and the standard deviation is tied to ENSO sea surface temperature (SST) amplitude

Summary

Introduction

Natural (or internal) variations within Earth’s climate system are an important source of uncertainty in projections of future climate, on inter-annual to decadal timescales[1]. In climate model simulations with constant pre-industrial conditions, ENSO frequency, event location diversity, and SST amplitude undergo strong decadal modulations. This single-model approach, including both control and forced data, allows for comparisons of ENSO’s internal variability and response to climate change.

Results

Conclusion