A Combined Climate Extremes Index for the Australian Region

Changes in the area of Australia experiencing concurrent temperature and rainfall extremes are investigated through the use of two combined indices The indices describe variations between the fraction of land are I experiencing extreme cold and dry or hot and wet conditions There is a high level of agreement between the variations and trends of the indices from 1957 to 2008 when computed using (i) a spatially complete gridded dataset without rigorous quality control checks and (ii) spatially incomplete high quality station datasets with rigorous quality control checks Australian extremes are examined starting from 1911 which is the first time a broad scale assessment of Australian temperature extremes has been performed prior to 1957 Over the whole country the results show an increase in the extent of hot and wet extremes and a decrease in the extent of cold and dry extremes annually and during all seasons from 1911 to 2008 at a rate of between 1% and 2% decade(-1) These trends mostly stem from changes in tropical regions during summer and spring There are relationships between the extent of extreme maximum temperatures precipitation and soil moisture on interannual and decadal time scales that are similar to the relationships exhibited by variations of the means However the trends from 1911 to 2008 and from 1957 to 2008 are not consistent with these relationships, providing evidence that the processes causing the interannual variations and those causing the longer term trends are different