Extending the instrumental climate record of southeastern Australia

Abstract

Southeastern Australia (SEA) is the most highly populated and agriculturally productive area of the Australian continent. The region also experiences one of the most variable climates in the world, particularly in terms of interannual rainfall. To understand the influence that anthropogenic climate change will have on future SEA climate, it is important to identify the full range of natural climate variability in the region. However, the current instrumental datasets used for SEA climate research only begin in the early 20th century, restricting efforts to identify long-term climate changes and low-frequency variability.

This thesis describes the first instrumental climate record for SEA to extend from European settlement in 1788 to the end of 2012. Historical instrumental observations of air temperature, atmospheric pressure and rainfall over SEA have been located and homogenised for the period 1788 to 1909. The historical observations have then been combined with existing high-quality SEA climate data for 1910 to 2012 to examine interannual SEA climate variability over the past 225 years. The extended climate record includes a number of previously unpublished observations during 1788–1859, providing new insight into the climate experienced by early European settlers in Australia.

The interannual climate variability identified using the extended record is in good agreement with SEA documentary records, palaeoclimate reconstructions and a historical reanalysis product, verifying the quality of the homogenised pre-1910 SEA climate data. Assessing rainfall variability during 1788–2012 identifies the most recent droughts in 1935–1942 and 1997–2009 as the longest periods of rainfall deficit in SEA since at least 1832, when continuous rainfall observations begin. Conversely, prolonged wet periods during the 1870s and 1890s appear to be more extreme than more recent wet conditions experienced in the 1950s and 1970s. Analysis of the extended SEA temperature record reveals that the current positive temperature trend seen in the region is the strongest and most significant since at least 1860.

Long-term stability of teleconnections associated with climate variations in the SEA region are also examined, with a focus on the influence of El Niño–Southern Oscillation (ENSO) on SEA rainfall. Breakdowns in the ENSO–SEA rainfall relationship are identified during 1835–1850 and 1920–1959, in agreement with previous observational and palaeoclimate studies. The decrease in ENSO–SEA rainfall correlations appear to be associated with changes in the Southern Hemisphere mid-latitude meridional pressure gradient, possibly linked to prolonged negative phases of the Southern Annular Mode.

The extended instrumental climate record developed in this thesis makes a significant contribution to the emerging field of historical climatology in Australia. It offers valuable new data for historians, climatologists and palaeoclimatologists exploring SEA’s past and present climate. As the regional impacts of anthropogenic climate change become an increasing reality, improved understanding of past climate variability is vital for future climate research.