Our ability to predict the impacts of climate variability and climate change, and to separate the roles that each play in present and future climate, can be increased through a better understanding of the processes that drive present-day climate.
There are two distinct ways of conducting such research. One way is to examine details of the large-scale atmospheric processes that are known to influence climate such as the El Nino Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), the Hadley circulation, and the baroclinic circulation of the mid-latitudes. These processes are often called the climate drivers, even though it is a semantic question as to whether they drive the climate or whether it is the climate that drives them. True climate drivers, such as greenhouse gas forcing, aerosols and, to some extent, ozone are those that arise from external forcing to the climate system.
The other way to conduct such research is to evaluate the performance of global climate models, such as those archived as part of CMIP5 [2] so as to determine
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The interactions that determine the regional change in rainfall and how climate change may affect them in the future.
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Weather and climatic extremes, persistent and extensive anomalous circulations associated with long-lasting droughts and floods and their relationship to climate change.
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The roles of greenhouse gas forcing, aerosols, ozone and natural variability.
CMIP5, the fifth climate model inter-comparison project, consists of stored results of climate change models output from over 20 modelling groups from around the worlda.
Regional processes
Sea surface temperature (SST) in the Pacific Ocean is a dominant influence on the climate of both Oceania and Australia. To understand better the Pacific, and the small island developing countries situated there, the Australian Bureau of Meteorology and the CSIRO worked with 15 partner countries to help generate scientific insight into the state of climate change in the Pacific now and in the future, under the Pacific-Australia Climate Change Science Adaptation Planning program (PACCSAP) [3].
Brown et al. [4] examined the behaviour of the West Pacific Monsoon for the 2080–2099 period using the IPCC high emission scenario known as RCP8.5. They found that during December, January and February there are increases in rainfall across the Southern Hemispheric equatorial regions whereas during June, July and August there are rainfall increases across the Northern Hemispheric equatorial regions – in other words increased rainfall during the respective summer period. Brown et al. [4] also examined the changes to summer rainfall variability for the West Pacific Monsoon by examining the distribution of the rainfall standard deviation during the summer rainfall season for the (1986–2005) period and the future (2080–2099, using RCP8.5 scenario) period. While the year-to-year rainfall variability does not change greatly, it does seem to change for tropical Australia, Solomon Islands and Palau such that the direction of change is similar for these three regions: the tail of the distribution expands (more variability), i.e. wetter summer rainfall years become even wetter and drier summer rainfall years become drier. There seems to be a particularly strong signal of this expansion for the Solomon Islands.
Brown et al. [5] examined the South Pacific Convergence Zone (SPCZ), which is a band of cloud and rainfall in the south-west Pacific. The observed SPCZ is shown in Figure 1. Most CMIP3 and CMIP5 models are able to simulate this rainfall band, but it tends, when averaged over all the model results, to be too zonal rather than diagonal as observed. When examining climate change simulations, the SPCZ (in DJF) does not change position, but the area and intensity increase. This is consistent with increased tropical moisture convergence in the warmer climate.
National
The Australian Climate Change Science Program (ACCSP) [6] encourages collaboration amongst scientists within Australia. In most cases the collaboration is facilitated through co-investment with regionally based, externally funded, projects such as SEACI [7] – the South Eastern Australian Climate Initiative that dealt with the climate, weather and hydrology of the Murray-Darling Basin [8]. In Western Australia a similar research activity, known as the Indian Ocean Climate Initiative – IOCI [9] started in 1998 and continued through three stages until 2012.
Because of the long-lasting Millennium Drought that lasted from 1995 to 2009, Australian research on climate processes concentrated on examining the mechanisms responsible for such rainfall declines. Beer [10] notes that there are numerous definitions of drought, none completely satisfactory. The reason is that the term drought is used with at least three different meanings. A meteorological drought (normally measured in terms of rainfall deciles) occurs when the rainfall, over a period of time, is substantially below normal levels. An agricultural drought (measured, for example, by the moisture deficit in the top 200 mm of soil) occurs when the soil moisture, over a period of time, is substantially below normal levels. A hydrological drought (measured, for example, by water availability) occurs when drinking water impoundments, over a period of time, are substantially below normal levels.
Droughts in eastern Australia are associated with El Nino, with prolonged El Nino events leading to prolonged droughts. The two most extreme drought periods in Australia are the Federation drought [11] of 1896–1902, and the recent Millennium Drought [12]. There is some controversy over the start and end dates of these droughts with some authors extending the Federation drought from 1895 to 1903 and some starting the Millennium drought as early as 1991 with others claiming a start date of 1997. In the case of the Millennium drought there is agreement that it ended with the two back-to-back La Nina years of 2010–2011 and 2011–2012 [8].
During the 1970s and 1980s the work of oceanographers (e.g. [13]) was influential in establishing the El Nino as an oceanic phenomenon with profound, large-scale climatic consequences. Climatologists [14, 15] had found that the El Nino could be quantified using the Southern Oscillation Index, and that it had a profound effect on south-eastern Australian rainfall. Consequently the phenomenon is frequently referred to as ENSO, which stands for El-Nino Southern Oscillation. Basically when ENSO is in an El Nino phase the eastern equatorial Pacific is warmer than usual, pressures are higher across much of Australia, and rainfall is reduced across much of south-eastern Australia. When ENSO is in a La Nina phase the equatorial Pacific is colder than usual and rainfall is increased across much of south-eastern Australia.
The sub-tropical ridge, STR, is the descending branch of the Hadley circulation. The location of the STR has long been known to affect climate. More recently it was found that sea surface temperature in the Indian Ocean, as measured by the Indian Ocean Dipole (IOD), and winds in the Southern Ocean, as measured by the Southern Annular Mode (SAM) [16] also play a role in determining the climate and rainfall of south-eastern Australia [17]. The Indian Ocean Dipole (IOD) refers to an out of phase variation in sea-surface temperatures between the western and eastern tropical Indian Ocean. It is quantified using the Dipole Mode Index (DMI) that is based on the measured difference between sea-surface temperature in the western (50° E to 70° E and 10° S to 10° N) and eastern (90° E to 110° E and 10° S to 0° S) equatorial Indian Ocean. The IOD typically develops in early winter, peaks in October - November and disappears by January. A negative IOD is associated with wetter conditions across south-eastern Australia, while a positive IOD is associated with drier conditions. The IOD is the most dominant large-scale influence on climate in south-eastern Australia in winter and spring, when it explains up to 40 percent of the rainfall variance in parts of south-eastern Australia.
Feng et al. [18] provide an alternate explanation for the decline in early winter rainfall over the southwest Western Australia. They note that the climate of southwest Australia exhibits a monsoon-like atmospheric circulation that is coupled to the rainfall in southwest Western Australia, but seems to be largely independent of the large-scale atmospheric circulation processes.
Because the dominant effect of prolonged drought is the reduction of water supplies (i.e. hydrological drought), the management of drought in Australia is primarily a role for water authorities, which are entities that are controlled by State governments, rather than the Federal Government. In this respect the Western Australian water authorities have accepted that even though Western Australia has not had the high-profile droughts of eastern Australia, there has been a steady decline in rainfall and available water resources [19] over the past 40 years.