Trends in annual frequencies of extreme temperature events in Australia

Abstract

A number of indices have been developed to investigate recent changes in the annual frequencies of extreme temperature events in Australia. A high-quality daily temperature dataset including 88 station records is used to determine trends in these indices, generally over the period 1957 to 1996. Indices investigated include measures of the frequencies of daily maximum and minimum temperatures above and below fixed temperature thresholds, as well as frequencies above and below specified percentile levels. Trends in consecutive days and nights of extreme temperature are also considered. These trends indicate that occurrences of warm temperature extreme events have generally increased over the investigation period, whilst numbers of extremely cool temperature events have decreased. The trends are particularly strong for indices based on minimum temperature, with many statistically significant at the 95 per cent confidence level. Some of the trends display considerable regional variation. Examples of this are a downward trend in the frequency of warm extremes in parts of the far southeast, counter to the national trend, and an especially strong decrease in the frequency of relatively cool days along the east coast. A number of measures of daily temperature variability are also examined with many records showing significant declines for these indices. These trends may provide the first evidence of decreases in intraseasonal temperature variability consistent with those observed over large parts of the northern hemisphere landmass.