DOI RECORD
Large-Scale Mechanisms During Multi-year Droughts in Southeast Australia
Abstract
Multi-year droughts have severe impacts across southeast Australia, yet the large-scale drivers of these events remain poorly understood. The relatively short instrumental record captures limited instances of multi-year droughts and accompanying climate mode combinations to robustly characterise these relationships. It also remains unclear how the behaviour of major climate modes changes across different stages of multi-year drought. In this study we evaluate ten single-model initial-condition large ensembles and select four to generate a large sample of multi-year droughts. We quantify, across drought development, persistence and termination stages, the phase frequency of individual and pairwise combinations of the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM), as well as the precipitation contribution of each individual climate mode.Across the drought stages, climate-mode phases shift consistently with their expected dry and wet associations. ENSO shows the largest overall and most asymmetric changes, with La Niña exhibiting stronger shifts than El Niño in both frequency and precipitation contribution across all three stages, highlighting projected La Niña frequency as a useful indicator of future drought behaviour. The change in IOD lags that of ENSO and SAM, peaking during drought persistence and showing comparatively modest frequency shifts. Changes to mode combinations during drought broadly mirror individual modes, with the largest shifts occurring when one mode is active and the other neutral. Wet climate mode combinations generally show larger frequency shifts than dry combinations across all drought stages, with stronger wet combinations showing greater shifts. Together, these results underscore the importance of asymmetries between wet and dry climate mode phases and inter-mode interactions in understanding the evolution of multi-year drought.