Teleconnections and relationship between the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) in reconstructions and models over the past millennium

Christoph Dätwyler,Raphael Neukom,Nathan J Steiger,Martin Grosjean

doi:10.5194/cp-16-743-2020

Abstract

Abstract. The climate of the Southern Hemisphere (SH) is strongly influenced by variations in the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Because of the limited length of instrumental records in most parts of the SH, very little is known about the relationship between these two key modes of variability over time. Using proxy-based reconstructions and last-millennium climate model simulations, we find that ENSO and SAM indices are mostly negatively correlated over the past millennium. Pseudo-proxy experiments indicate that currently available proxy records are able to reliably capture ENSO–SAM relationships back to at least 1600 CE. Palaeoclimate reconstructions show mostly negative correlations back to about 1400 CE. An ensemble of last-millennium climate model simulations confirms this negative correlation, showing a stable correlation of approximately −0.3. Despite this generally negative relationship we do find intermittent periods of positive ENSO–SAM correlations in individual model simulations and in the palaeoclimate reconstructions. We do not find evidence that these relationship fluctuations are caused by exogenous forcing nor by a consistent climate pattern. However, we do find evidence that strong negative correlations are associated with strong positive (negative) anomalies in the Interdecadal Pacific Oscillation and the Amundsen Sea Low during periods when SAM and ENSO indices are of opposite (equal) sign.

Highlights

The El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are the Earth’s and Southern Hemisphere’s (SH) leading modes of interannual climate variability (Marshall, 2003; McPhaden et al, 2006), respectively
Since the relationship between ENSO and SAM using perfect pseudo-proxy records resembles the relationship seen in the model truth more than when using noisy pseudo-proxies, we argue that proxy-based reconstructions can reproduce the ENSO–SAM correlation pattern in a realistic way, if there are enough proxy records contributing to the reconstructions
If a significant amount of noise is added to the pseudo-proxy records, the negative correlations weaken pre-1800 and even further pre-1600, when the number of available proxy records drops below a critical threshold (∼ 30 proxy records), indicating reduced reliability of the reconstructed ENSO–SAM correlations during this period

Summary

Introduction

The El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are the Earth’s and Southern Hemisphere’s (SH) leading modes of interannual climate variability (Marshall, 2003; McPhaden et al, 2006), respectively. Despite the prominent role of these two modes in driving regional SH climate, little is known about the interplay between them (Ribera and Mann, 2003; Silvestri and Vera, 2003; Carvalho et al, 2005; Fogt and Bromwich, 2006; L’Heureux and Thompson, 2006; Cai et al, 2010; Gong et al, 2010; Pohl et al, 2010; Fogt et al, 2011; Clem and Fogt, 2013; Lim et al, 2013; Yu et al, 2015; Kim et al, 2017) This interplay and its stability over time are, key factors for understanding tropical–extratropical teleconnections in the SH. Understanding SH climate dynamics and identifying the key drivers of interannual to multi-decadal variability

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Conclusion