Holocene El Niño-Southern Oscillation variability reflected in subtropical Australian precipitation

Abstract

The La Niña and El Niño phases of the El Niño-Southern Oscillation (ENSO) have major impacts on regional rainfall patterns around the globe, with substantial environmental, societal and economic implications. Long-term perspectives on ENSO behaviour, under changing background conditions, are essential to anticipating how ENSO phases may respond under future climate scenarios. Here, we derive a 7700-year, quantitative precipitation record using carbon isotope ratios from a single species of leaf preserved in lake sediments from subtropical eastern Australia. We find a generally wet (more La Niña-like) mid-Holocene that shifted towards drier and more variable climates after 3200 cal. yr BP, primarily driven by increasing frequency and strength of the El Niño phase. Climate model simulations implicate a progressive orbitally-driven weakening of the Pacific Walker Circulation as contributing to this change. At centennial scales, high rainfall characterised the Little Ice Age (~1450-1850 CE) in subtropical eastern Australia, contrasting with oceanic proxies that suggest El Niño-like conditions prevail during this period. Our data provide a new western Pacific perspective on Holocene ENSO variability and highlight the need to address ENSO reconstruction with a geographically diverse network of sites to characterise how both ENSO, and its impacts, vary in a changing climate.

Figure 1

ENSO influence on surface precipitation. Spatial correlation between mean precipitation (Nov–Oct: the local hydrological year) across the greater ENSO region and mean sea surface temperature for the Nino3.4 region (box) for the period 1980–2016 CE. Location of the study site and other locations mentioned in the text are illustrated.

Figure 2

The Swallow Lagoon precipitation record. (a) The number of individual samples per non-overlapping century. (b) The Swallow Lagoon rainfall reconstruction with standard error (±88 mm; grey shading) and generalised additive location-scale model (GAM-LS: orange line with 95% confidence level shaded) illustrating significant trends in the data. Horizontal black dashed lines indicate ±2σ of the record, dotted line is the mean (1742 mm). (c) Standard deviation (σ) in mean annual rainfall with 95% confidence level shaded (see methods).

Figure 3

(a) Swallow Lagoon precipitation record (as per Fig. 2); (b) West Pacific warm pool SST; (c) lake sediment sand content from El Junco Lake, Galápagos Islands, and (d) sediment deposition at Laguna Pallcacocha, Ecuador, as proxies for El Niño event frequency; (e) simulated amplitude of ENSO variability as reflected by Nino3.4 SST variability with 95% confidence interval shaded and, (f) simulated strength of the Pacific Walker circulation in the Nino4 region with 95% confidence interval shaded, according to the CSIRO Mk3L climate system model (see methods); (g) Standard deviation in mean annual rainfall record from Swallow Lagoon (as per Fig. 2c); eastern tropical Pacific measures of ENSO variability derived from (h) variance of individual foraminifera (grey bars; original sample at 7 cal kyr BP not shown as it is considered spurious by the authors), (i) bivalves (blue boxes), and (j) Laguna Pallcacocha variance, in 100-year non-overlapping windows, derived from normal-transformed data (black solid line). Inverted triangles represent the location of radiocarbon ages in the Swallow Lagoon record. LIA: Little Ice Age.