Cross-continent comparison of high-resolution Holocene climate records from southern Australia — Deciphering the impacts of far-field teleconnections

Abstract

Several well-dated and high-resolution, multi-proxy Holocene palaeoclimatic records have recently emerged that permit a cross-continent comparison of climatic changes in Australia. This synthesis seeks to compare these records with recently published records from southern Australia and investigate the role of far-field teleconnections that have played in the mid to late Holocene climatic history of the region. The modern climate of southeastern Australia (SEA) is influenced by the Indian Ocean sector of the Southern Annular Mode (IOS-SAM), the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) and contrasts with southwestern Australia (SWA), which is predominantly influenced by variations in the IOS-SAM. Both regions have undergone a period of increasing aridity through the second half of the Holocene in response to the southerly migration of the mean position of the Inter-Tropical Convergence Zone (ITCZ) and the southerly migration of the IOS-SAM.Comparison of the palaeorecords stemming from both SEA and SWA suggests that the SWA record differs from the climate recorded from SEA from approximately 5.6ka. This difference is ascribed to an increased influence of large-scale climate drivers including the IOD and ENSO variability in SEA, which have negligible impact on the climate of SWA.Throughout the Holocene, the southerly migration of the ITCZ results in a weaker Asian summer monsoon. However, the rate of Asian summer monsoon weakening slowed during the middle Holocene and this coincides with a larger sea-surface temperature gradient across the equatorial Indian Ocean that resulted in a prolonged mode of positive IOD and a period of significantly increased evaporation in SEA. Those phenomena engendered a general decline in the lake levels in SEA, which is not seen in SWA as this region was not significantly affected by the IOD throughout the Holocene. This positive IOD period lasted until ~4.3ka, before a return to the strengthening (weakening) of the Australian (Asian) summer monsoon conditions that resulted in a southward shift of the ITCZ during the austral winter. The southerly shift in the ITCZ was associated with a more southerly mean position of the IOS-SAM and drier conditions for SEA and SWA. The climate affecting SEA after 4.3ka coincides with an increasing incidence and strengthening of the El Niño (non-El Niño or La Niña) mode coupled with positive (negative) modes of the IOD that resulted in a net deficit (excess) of water. The driest phase in SEA occurs at ca. 1.8ka, when far-field palaeoclimate records indicate a peak in El Niño intensity, although the impact of the IOD cannot be discounted. Conversely, both the IOD and ENSO phenomena do not affect the climate of SWA, thus leading to significant differences in the climate across the southern margin of the Australian continent.