An Investigation of Mechanisms Driving Prolonged Droughts in Southeast Australia

Abstract

This study investigates the mechanisms driving prolonged droughts in Southeast Australia (SEA), an important agricultural region that is highly vulnerable to water scarcity due to significant precipitation variability. Employing a three-stage analytical framework consisting of development (12-month pre-drought persistence period), persistence (sustained period with SPI 12 � -1.0 for at least 12 consecutive months), and termination (recovery period from when SPI 12 rises above -1.0 until reaching +1.0), this research uses 30-member GFDL_SPEAR_MED large ensemble climate model simulations (1921�2100) and 12-month Standardized Precipitation Index (SPI-12) to address five key questions examining: (1) the characteristics of prolonged SEA droughts across these temporal stages; (2-3) the behaviour of individual climate drivers including the El Ni�o�Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM), and their combinations during drought temporal evolution; and (4-5) the frequency, intensity, and spatial patterns of synoptic-scale weather systems throughout the drought lifecycle. SPI-12 analysis shows that droughts develop either directly, occurring in 40�60% of events with a rapid onset from normal or above-normal conditions, or gradually, emerging from below normal conditions or from an incomplete recovery after prior droughts. Persistence phases substantially exceed termination phases in duration (approximately 30 vs. 10 months), with extreme events extending beyond 200 months, whereas drought termination proceeds rapidly and consistently once initiated. Large-scale climate driver analysis shows ENSO as the dominant driver of droughts, remaining influential across all drought stages. The suppression of rainfall associated with La Ni�a events tends to be more critical than the drought enhancement linked to El Ni�o during dry periods. Notably, a return to neutral or weak La Ni�a conditions does not necessarily end droughts in SEA; relief occurs mainly during strong to extreme La Ni�a events, often alongside -IOD and +SAM phases. The IOD amplifies drought conditions during the mid-development and persistence stages, rather than serving as the initial trigger. SAM shows notable shift during development, ranking second to ENSO in overall changes in phase frequency. When considering the co-occurrence of different phases climate drivers: standalone climate driver events show the largest deviations overall, neutral-neutral combinations increase substantially during development, whereas more extreme phase pairings generally produced greater shifts. At the synoptic scale, anticyclone frequency rises and cyclone frequency declines over SEA during drought development and persistence, with opposite trends during termination. Spatial analysis identifies the Tasman Sea as a hotspot for both cyclones and anticyclones, with their intensity anomalies increasing toward higher southern latitudes. ENSO, IOD, and SAM collectively modulate prolonged droughts by altering large-scale circulation and moisture availability, creating conditions that either favour or suppress precipitation. Whilst synoptic-scale systems such as cyclones and anticyclones determine the immediate precipitation patterns at regional scales. By clarifying the roles of climate drivers and synoptic processes in prolonged drought stages, this research improves our understanding of prolonged drought mechanisms and offers insights to enhance prolonged drought prediction and support long-term water resource management under a changing climate.