The distribution of climate zones across Australia: identifying and explaining changes during the past century
There are many reasons to classify climates aside from that expressed by Köppen (1931). 'People classify the climate in many different ways depending on who needs the information, how much they know about the climate system and what information they need to know' (WMO, 2012). One may undertake studies that examine climate change in terms of shifts in climate classification boundaries by using data from different historical periods. One may also view such schemes as a potential diagnostic tool for general circulation models, with the aim of developing future climatologies (Löhmann et al., 1993). This has become possible now (Institute for Veterinary Public Health, 2012) with the ready availability of model output data bases such as CliMond (Kriticos et al., 2012).
The present paper describes the application of the latest (updated June 2012) Australian Water Availability Project (AWAP) (Jones et al., 2009 and Raupach et al., 2009) analyses of monthly rainfall, maximum temperature and minimum temperature to the modified climate classification scheme. By this means, changes (both long-term trends and short-term fluctuations) in the distribution of climate zones across Australia during the past century may be identified and explained. The AWAP analyses represent an updated representation of the distribution of the individual weather elements that make up the nation's climate and incorporate the use of data that was hitherto unavailable or had not yet been subject to the Bureau's new quality climate data management system. The AWAP analyses are shown on the BoM's website at http://www.bom.gov.au/climate/maps/.
It is found that during the past century, many changes in the distribution of Australian climate zones have taken place, the most notable one being the contraction of the area covered by ‘Desert' climates and the corresponding increase in the area covered by 'Grassland' and 'Tropical' climates. This change is illustrated by the pair of small maps, which show the distribution of climate zones across the Northern Territory during the first (1911-1940) and most recent (1982-2011) 30-year periods of data available.
Other changes across Australia include the expansion, and southward advance, of the ‘Tropical' zone; the contraction of the ‘Temperate' area of southwest Western Australia; an increase in the area covered by ‘Hot Desert' just inland from the Head of the Bight; a decrease in the area of coastal Victoria classified as having a 'cool summer'; an increase, followed by a decrease, in the area covered by 'Temperate' climates; and the highland regions of Tasmania classified as ‘Polar' having all but disappeared. The large map shows the distribution of climate zones across Australia during the most recent 30-year period (1982-2011).
Key drivers of the Australian climate include the ENSO phenomenon, the Indian Ocean Dipole, the Madden Julian Oscillation, the Southern Annular Mode, the monsoons and the SE trades. Other drivers include synoptic features such as the position and intensity of long wave troughs, the jet streams, the subtropical ridge and the 'blocking' phenomenon. Using global reanalysis data from the NOAA/ESRL Physical Sciences Division (Boulder, Colorado), data sets from the BoM's Australian Data Archive for Meteorology, data sets reflecting the key drivers of the Australian climate, in addition to some very old data sets pre-dating the BoM (Hunt, 1911), possible explanations for some of the detected changes in the distribution of climate zones are given.
For example, the contraction of the area covered by ‘Desert' climates, and the corresponding increase in the area covered by ‘Grassland' and 'Tropical' climates, may be attributed to the apparent northward shift of the sub-tropical jet over the past century, and the associated increase in the frequency of the ‘blocking' phenomenon over the Tasman Sea, especially during the summer half of the year.
To summarise, the percentage area of Australia designated as Desert, has decreased over the past century from 51.1% to 37.9%; as Grassland, has increased from 26.3% to 36.1%; as Temperate, has decreased from 10.9% to 10.1%, as Tropical, has increased from 5.5% to 9.0%; as Subtropical, has increased from 5.5% to 6.5%; as Equatorial, has decreased from 0.7% to 0.3%; and, as Polar, has decreased from 0.004% to 0.001%. So, it may be seen how the AWAP analyses have enabled changes in the distribution of climate zones across Australia to be both identified and explained.