A Synoptic-Dynamic View of the Millennium Drought (2001–2009) in Southeastern Australia

Abstract

Australia has had several severe droughts in its recent history. Most studies have linked these droughts to large-scale modes of variability, whereas few studies have investigated droughts from the perspective of weather systems. The current study examines a wide range of weather systems focusing especially on heavy rainfall events, which are important to meteorological drought. Two distinct phases (development and recovery) are identified for the Millennium Drought based on the cumulative standardized precipitation index. Differences in precipitation from heavy rainfall events between the two drought phases are most pronounced in autumn and summer. The pronounced reduction in precipitation from autumn heavy rainfall events during the development phase is due to fewer, less intense, faster-moving warm conveyor belts. In contrast, increased precipitation from autumn heavy rainfall events in the recovery phase is explained by an interaction between warm conveyor belts and upper-level anticyclonic potential vorticity with a persistent anticyclonic circulation over the Tasman Sea acting to slow the eastward propagation of rainfall-producing weather systems. In summer, however, the difference in precipitation from heavy rainfall events in the two drought phases is due to changed moisture content within warm conveyor belts. In the recovery phase, the higher moisture content is associated with greater moisture transport over the Tasman Sea between Australia and New Zealand.