Extreme monthly rainfall archetypes for Australia

Abstract

The method of archetypal analysis is used to generate a set of monthly timescale rainfall archetypes for the Australian region. The patterns associated with the archetypes reflect continental and regional-scale wet and dry. The dominant pattern in terms of occurrence and persistence is one in which most of the continent is dry. This pattern is typically expressed over winter and spring. The next most frequent pattern is one where most of the continent is wet, mostly expressed during summer. It is rare to find periods where the whole continent is wet outside summer, though this does occur and is associated with very wet years for the continent. The archetype patterns have preferred seasonal expressions, and preferred transitions from one pattern to another. The continent-wide dry pattern is mostly followed by patterns in which both south-west and south-east Australia are wet during the autumn and winter. However, if the dry continental archetype persists through to spring, then it is usually followed by a pattern that is wet in the south-east but not the south-west. The analysis reveals pivotal months, such as April and November. These months mark the end of periods when only a few archetypes are expressed, allow expression of almost all the archetypes, and are then succeeded by periods when a smaller number of archetypes are expressed again. The archetype patterns successfully capture the large-scale spatial patterns of monthly rainfall in Australia, and provide a diagnostic tool to evaluate the onset, duration and transitions between wet and dry periods.