Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Heatwaves in the 21^st century – More frequent, Longer and Hotter

Since the mid-21^st century extreme heat waves in summer and warm spells in winter have had a significant impact of the Australian climate, where there has been an increased frequency and duration of these events. This study investigated projected changes in heat waves and warm spells across Australia for 2 future emission scenarios with the use of Coupled Model Intercomparison Project phase 5 (CMIP5) climate models. For the historical period that encompasses the late 20^th century (1950-2005) an ensemble mean of 15 models is capable of capturing the spatial distribution that is observed in the frequency and duration of summer heat waves, in spite of overestimating these metrics along coastal regions. A better comparison to observations is achieved by the models in their simulation of temperature anomaly of the hottest heat waves. The largest increase in frequency and duration to occur across northern tropical regions of summer heat waves is projected by model ensemble mean projects by the end of the 21^st century, while predicting an increase of ⁓3^oC in the maximum temperature of the hottest southern Australian heat waves. It is suggested by model consensus that winter warm spell in winter will increase in frequency and duration at a greater rate than summer heat waves, and that the hottest events will be increasingly for both seasons by the end of the century. Temperature increases of the hottest events are projected for southern Australia. It is also suggested by results also suggest that following a strong mitigation pathway in the future is more effective in reducing the heat wave and warm spells frequency and duration in the southern regions compared to the northern tropical regions.

In many heavily populated regions of the world, which includes southern and central Europe (e.g. Fischer & Schar, 2010; Kyselý, 2010; Carril et al., 2008; Della-Marta et al., 2007; Trigo et al., 2005, North America (e.g. Bumbaco et al., 2013; Wu et al., 2012b; Mastrandrea et al. 2011), China (Wu et al., 2012a; Ding et al., 2007), Russia (Trenberth & Fasullo, 2012; Barriopedro et al., 2011), central and southern Africa (Hao et al., 2013; Lyon, 2009), and southern Australia (Bureau of Meteorology 2013; Lewis & Karoly, 2013; Nairn & Fawcett, 2013; Tryhorn & Risbey, 2006). Consecutive days above a threshold maximum temperature is often used to characterise these types of events; they also incorporate a distinct lack of night-time temperature relief with consecutive nights above a minimum threshold (Pezza et al., 2012; Nairn & Fawcett, 2013).

Heat waves occur in all seasons in Australia and in the austral winter they are often referred to as warm spells. Across the central interior and tropical regions where temperatures are high, often exceeding 35^oC, though the daily variability if low, based on the maximum temperature definition of heat waves from (Perkins & Alexander, 2013), there are about 2 heat wave events per extended summer, though such events tend to be only ⁓2^oC–4^oC higher than the climatically maximum temperature. In southern Australia, which is a region of great agricultural importance, which is also where about ⅓ of the population live, there are on annual average 1 or 2 heat waves per summer; such events can, however, be up to 15^oC warmer than the climatological maximum (Perkins & Alexander, 2013). This results from a high frequency of synoptic prefrontal weather systems (Tryhorn & Risbey, 2006) in the southern latitudes, which cause heat waves that are more severe to develop due to the greater variability of summer temperatures (Nairn & Fawcett, 2013). In the midlatitude regions of Australia processes such as breaking Rossby waves and persistent anticyclones (Pezza et al., 2012; Perkins & Alexander, 2013; Marshall et al., 2014; Parker et al., 2013) often trigger heat waves, which allow advection of warm air over the region that is affected. A low pressure trough or tropical depression over the northwest of Australia often accompany anticyclones (Hudson et al., 2011; Pezza et al., 2012), which may reinforce directly the anticyclone that is downstream (Parker et al., 2013).

It is suggested by recent observations that the frequency, duration and intensity of heat wave events are increasing over land regions around the world (Coumou & Rahmstorf, 2012, and references therein), which includes many regions of Australia (Pezza et al., 2012; Perkins et al., 2010; Perkins and Alexander, 2013). In spite of various limitations in the simulation or regional changes (Hao et al., 2013), climate models are projecting that these trends will continue with increasing emissions of greenhouse gases (Coumou & Robinson, 2013; Meehl & Tebaldi, 2004). Also, there is considerable evidence suggesting that anthropogenic forcings have contributed to recent extreme heat wave events across the Northern Hemisphere (Hansen et al., 2012) and to record warm summer temperatures across Australia (Lewis &Karoly, 2103). The likelihood of a contribution by humans to ⁓2.5-5 times greater than that without greenhouse warming, according to Coupled Model Intercomparison Project phase 5 (CMIP5) climate models. It is also projected by CMIP5 models that there will be a 7-fold increase in global monthly heat extremes by the middle of the 21^st century (Coumou & Robinson, 2013). According to Cowan et al. this follows on from large increases (decreases) in the warm/dry (wet/cold) extremes across many tropical and subtropical regions throughout the late 20^th century, which are captured well by the CMIP5 models (Hao et al., 2013). A comprehensive, detailed assessment of the way in which warming in the future will impact heat waves across Australia by the use of CMIP5 climate model simulations, however, has not yet been undertaken.

Therefore, Cowan et al. investigated the changes in heat waves across Australia in the 21^st century that have been projected, comparable to the analysis of (Perkins & Alexander, 2013), which examined trends that were observable. The importance of understanding of how the trends in heat wave metrics will change in a world that is warming is underscored by the impacts of these extreme events on human health and mortality. Many studies have detailed the increased risk of heat-related deaths that result from extreme heat events in populated regions of Europe (Le Tertre et al., 2006), North America (Zanobetti & Schwartz, 2008), and Australia (e.g. Loughnan et al., 2010; Tong et al., 2010). According to Cowan et al. this risk is likely to increase severalfold by the late 21^st century, based on little or no acclimatisation (Gosling et al., 2009). Therefore, communities require greater certainty in understanding the risk of increased heat waves in the future and a better understanding of whether the likelihood of extreme heat conditions will be reduced by mitigation (as in Coumou & Robinson, 2013).

For thus study, Cowan et al. define Australian heat waves as events that only occur in summer, as this is when they have the greatest impact on human health (e.g. Pezza et al., 2012) as a result of their intensity, as well as having an influence on other extreme events that only happen in summer such as bushfires (Karoly, 2009). Winter warm spells, however, will also be examined to investigate the seasonality of the projected changes. The way in which heat wave and warm spell frequency and hottest event anomalies across Australia will change under 2 emission scenarios over the 21^st century, by use of CMIP5 data.

Discussion and conclusions

It is indicated by CMIP5 projections for Australia that by the end of the 21^st century hotter, more frequent and longer heat waves, and warm spells in winter will occur, with more extreme conditions under RCP8.5 compared to RCP4.5. The largest changes in the heat wave frequency and duration are projected to occur across the northern tropical regions of Australia, which is consistent with other studies into heat extremes around the globe (Diffenbaugh & Scherer, 2011; Stillmann et al., 2013), while in southern Australia the maximum of heat waves increase more than those in the north. In general, the CMIP5 models simulate adequately the heat wave climatologies that are observed over the late 20^th century and the early 21^st century, though they tend to simulate an overly zonal climatologically spatial pattern for frequency and duration, which means that there is a degree of uncertainty on the changes that they project in the 21^st century. There is greater confidence in the simulated increases in the future, as the MMM performs better when simulating the amplitude of heat waves that are observed. The models simulate little increase in frequency for cities in southern Australia over 1950-2005, which is consistent with heat waves along southern Australia; however, they simulate an increase in frequency of 6- to 20-fold in summer and a noticeable increase in summer duration  by 2100 for RCP8.5.

It is suggested by the seasonality of the heat wave changes that are projected that a larger increase in the frequency and duration of summer heat waves and winter warm spells in the austral winter will occur across the Australian populated regions, which includes the southern Murray-Darling basin, when compared to any other season. Models project the largest changes for the central and northern interior in spring and summer heat waves. Conversely, changes in hottest seasonal heat waves are largest in winter, with the exception of across southern regions, where in spring the largest changes occur (South Australia) and summer (southwest and southern Australia in RCP4.5). This seasonality mirrors to a large extent the spatial variations in the climatology, which exhibit heat waves of higher frequency and duration in summer across northern Australia, though higher amplitude heat waves across southern Australia. As such, it is suggested by the results that, as well as the substantial increase in frequency, duration and maximum temperature of heat waves during the 21^st century, there will be enhanced seasonality and spatial variation of heat waves. The greatest increase in hottest heat waves occurs in spring for populated regions such as southeast and southwest Australia, which is consistent with the trend towards more events of the Indian Ocean Dipole that often lead to severe bushfires in summer (Cai et al., 2009).

The main cause of these heat wave trends in the future is the robust global warming that results from anthropogenic greenhouse gas emissions (e.g. Lewis & Karoly, 2013). The amplification of atmospheric circulation patterns may also be responsible for forcing additional changes in the intensity and frequency of heat waves, as has been simulated for regions in Europe and North America (Meehl & Tebaldi, 2004). It is suggested by studies that used CMP3 models that a result of global warming is that the intensity of the subtropical ridge over Australia will increase over the 21^st century (Timbal & Drosdowsky, 2013), and the position of the ridge will shift polewards (Kent at al., 2013), thereby reducing rainfall in southeastern Australia. This model also projects drier winters across southwest Australia in the 21^st century (Cai et al., 2011a), with below average rainfall that is often associated with higher maximum temperatures (Nicholls, 2004). Cowan et al. suggest that this may account for the largest increase in warm spells that are simulated occurring during the austral winter, as well as the maximum temperatures in winter having a smaller range than they do in summer, i.e., a narrower probability distribution function. Atmospheric blocking is expected, in spite of model biases, to increase in the summer months in the Tasman Sea (Grose et al., 2012) to the east of South Australia (Sadler et al., 2012). However, future warming of the Tasman Sea and a trend of positive southern annular mode are also likely to promote an increase in summer rainfall over southeast Australia (Shi et al., 2008), which may account for the weakening in the summer HWF in Melbourne, compared to the case in Perth and Sydney. The summer domain in the northern tropics is not expected to change significantly in the future, in spite of a general predicted increase in summer rainfall (Wang et al., 2013), it is expected that there will be delays and shorter durations in onset of the monsoon (Zhang et al., 2012), which Cowan et al. suggest may partially explain the large increases in frequency and duration in this region.

A robust result of this study is that there will be an increase in frequency, duration and amplitude of winter warm spells and summer heat waves across Australia, and this increase is strongly proportional to the emission scenario, as well as the latitude of a particular region, i.e., tropical versus extratropical. Also, the hot events will gradually become hotter by the end of the 21^st century over southern Australia, especially during winter and for the RCP8.5 high emission scenario, even if the heat waves are referenced to a mean state that is warming. Humans can benefit from warm spells in winter, such as by the reduced incidence of disease and fatalities (McMichael et al., 2006), though there are also likely to be adverse effects to agriculture that is temperature dependent (e.g. Turner et al., 2011). Also, there will be severe adverse effects on human health related to increases in summer heat waves (Kyselý, 2010; Sherwood & Huber, 2010), as well as ecosystems and agriculture (Coumou & Robinson, 2013, and references therein). While it has been established that temporal evolution of heat waves occurs, it has not yet been understood if a change in heat waves across is manifested by a men state temperature change alone, or is in part due to changes in the circulation of the atmosphere and ocean. A companion study will investigate patterns of circulation and sea surface temperature prior to and during summer heat waves, in order to gain a better insight into how dynamics during summer heat waves across southern Australia will change in a warming world.

Sources & Further reading

Cowan, T., et al. (2014). "More Frequent, Longer, and Hotter Heat Waves for Australia in the Twenty-First Century." Journal of Climate 27(15): 5851-5871.