El Niño–Southern Oscillation variability, teleconnection changes and responses to large volcanic eruptions since AD 1000

Abstract

The El Niño–Southern Oscillation (ENSO) is the earth's dominant mode of inter‐annual climate variability. It alternates between warm (El Niño) and cold (La Niña) states, with global impacts on climate and society. This study provides new ENSO reconstructions based on a large, updated collection of proxy records. We use a novel reconstruction approach that employs running principal components, which allows us to take covariance changes between proxy records into account and thereby identify periods of likely teleconnection changes. Using different implementations of the principal component analysis enables us to identify periods within the last millennium when quantifications of ENSO are most robust. These periods range from 1580 to the end of the 17th century and from 1825 to present. We incorporate an assessment of consistency among our new and existing ENSO reconstructions leading to five short phases of low agreement among the reconstructions between 1700 and 1786. We find a consistent spatial pattern of proxy covariance during these four phases, differing from the structure seen over the instrumental period. This pattern points towards changes in teleconnections in the west Pacific/Australasian region, compared to the present state. Using our new reconstructions, we find a significant response of ENSO towards more La Niña‐like conditions 3–5 years after major volcanic events. We further show that our new reconstructions and existing reconstructions largely agree on the state of ENSO during volcanic eruptions in the years 1695 and 1784, which helps put into perspective the climatic response to these events. During all other large volcanic eruptions of the last 1000 years, there is no reconstruction coherency with regard to the state of ENSO.

Teleconnections and relationship between ENSO and SAM in reconstructions and models over the past millennium

“…While there is some literature about the stability of the individual ENSO and SAM indices over the past centuries (e.g. Wilson et al, 2010;Villalba et al, 2012;Dätwyler et al, 2018;Dätwyler et al, 2019) and their driving factors (e.g., Dong et al, 2018), hardly any studies exist that use palaeoclimate evidence to investigate the relationship between these modes of climate variability back in time. A study based on palaeoclimate records by Gomez et al (2011) found a waxing and waning relationship between ENSO and SAM on century to millennial long time scales.…”

“…In the case of ENSO, the palaeoclimate reconstruction is taken from Dätwyler et al (2019), but adapted to the austral summer season December to February (DJF, Fig. S1).…”

“…The proxy data combine the PAGES 2k global temperature database (PAGES 2k Consortium, 2017), the SH proxy records from Neukom et al (2014), the Antarctic ice core isotopes from Stenni et al (2017), and also ENSOrelated hydroclimate records from the northern hemisphere (Emile-Geay et al, 2013;Henke et al, 2017). In brief, the reconstruction method is based on PCA: ENSO is quantified as the first PC of the available proxy records in each overlapping 80-year period over the last millennium (Dätwyler et al, 2019). The key advantage of this approach is that it does not require formal calibration with instrumental data and is, therefore, not prone to the assumption that the relationships within the proxyinstrumental data matrix are stable over time.…”

“…Such changes may happen when the relationship between the proxy and target variable changes. For details we refer to the original paper (Dätwyler et al, 2019).…”

“…We consider the 13 strongest volcanic events in the last millennium with a forcing stronger than 2.5 W/m 2 (1177, 1214, 1259, 1276, 1285, 1342, 1453, 1601, 1642, 1763, 1810, 1816 and 1836). For methodological details on the SEA see Dätwyler et al (2019).…”