An analysis of the meteorological variables leading to apparent temperature in Australia: Present climate, trends, and global warming simulations

Highlights

• Trends of air temperature and apparent temperature are calculated for Australia.

• Three data-sets are used for trend analysis: observational, reanalysis and CMIP3.

• The majority of Australia had warming temperatures and/or apparent temperatures.

• ENSO, SAM and their influence on thermal comfort are also explored for Australia.

Abstract

This study is a comprehensive analysis of thermal comfort and apparent temperature around Australia. It includes a long-term historical trend analysis using observational weather station data, in which it was found that eight out of the ten chosen urban locations experienced warming trends in temperature and/or the apparent temperature over the second half of the twentieth century. Annual trends in temperature and apparent temperature were studied spatially across Australia using high resolution ERA Interim reanalysis data over the period 1979 to 2010. The reanalysis revealed that generally the apparent temperature is warming faster than the air temperature, amplifying the expected exposure to discomfort due to global warming in the subtropical region.

Future apparent temperature trends were explored using high resolution Coupled Model Intercomparison Project 3 model data to assess the impacts of global warming on human comfort. A best practice model for the Australian climate was used as well as best case and worst case scenario models selected using the Commonwealth Scientific and Industrial Research Organisation Representative Climate Futures framework. It was found that at 2070 using the A1B emissions scenario the temperature is projected to warm faster than the apparent temperature by up to 1 °C in central Australia, suggesting that the cooling power of the wind can partially offset the impacts of global warming. This occurs in conjunction with the accelerated drying predicted to occur in many areas of Australia in future climates.

Finally, the impact of the El Niño Southern Oscillation and the Southern Annular Mode on the spatial characteristics of the temperature and apparent temperature around Australia was studied. This revealed that the inherent atmospheric humidity variability of these large-scale processes resulted in milder thermal comfort conditions across Australia regardless of whether the temperature was anomalously warm or cool. Using an apparent temperature framework, this study looks into many facets of the Australian climate uncovering knowledge that is useful for risk assessments as well as future urban planning.

Introduction

Thermal comfort is a way of exploring the impacts of weather and climate on human populations and health. Apparent temperature (hereafter AT) is a metric that quantifies thermal comfort i.e. how an average person would ‘feel’, based on environmental conditions such as temperature, wind and humidity. As an example, this is done by measuring the effects of high wind speed in cold conditions (wind chill), and high humidity in warm conditions (heat stress) to understand the thermal comfort, or discomfort, of an average person. This concept can be equally applied outdoors as well as indoors.

AT and thermal comfort effects have been studied worldwide including North America (e.g. Gaffen and Ross, 1999, Grundstein and Dowd, 2011), South America (e.g. Coronato, 1993, Monteiro and Alucci, 2005), South Korea (Kim et al., 2009), Australia (Steadman, 1994), the Mediterranean (Pantavou et al., 2008, Segnalini et al., 2011), China (Wang and Gaffen, 2001) and Europe (e.g. Di Cristo et al., 2007). Thermal comfort is not only relevant to assess safe exposure times to extreme temperatures; it is also relevant for building planning (Yang and Zhang, 2007, Haase and Amato, 2009) and agricultural planning (Segnalini et al., 2011).

Long-term historical thermal comfort trend analysis has been conducted in the United States (Gaffen and Ross, 1999, Grundstein and Dowd, 2011) and China (Wang and Gaffen, 2001) over the second half of the twentieth century. This was performed using a thermal comfort equation that accounted for heat and humidity, but not wind speed. Both countries exhibited a rise in dew point temperature of several tenths of a degree Celsius per decade and this, coupled with positive temperature trends, resulted in a rise in thermal discomfort. Steadman (1994) conducted the first analysis of thermal comfort for Australia, finding unique conditions such as the cooling effect of the wind having a greater influence during summer than winter. Jacobs et al. (2013) created unified bio-comfort thresholds for Australia's second largest city, Melbourne. In their study, human comfort was assessed using known thresholds of heat stress, air pollution and grass pollen, showing that robust synoptic weather patterns were associated with each type of discomfort. However, trends in thermal comfort have yet to be explored around Australia.

Delworth et al. (1999) studied heat stress and 21st century projected thermal comfort in relation to climate change for several Intergovernmental Panel on Climate Change (IPCC) emission scenarios. They determined that increased atmospheric moisture levels in a warming world were found to have an amplifying effect on the AT, driving it to possibly unbearable levels in regions prone to moisture such as northern Australia, southeast Asia, southeastern United States, India and the tropics as a whole. Furthermore, we also note that very humid heat waves can also affect continental midlatitudes such as in the United States, Europe and Asia. Conversely, Delworth et al. (1999) found that in regions like Australia, the AT did not change greatly in the midlatitudes because it is a dry continent.

Australia is a country of very significant temperature and precipitation variability. Located mostly in a subtropical area with a large desert inland, its climate is subject to severe heat stress varying from extremely dry and hot conditions to hot and humid conditions. The Australian climate is also highly affected by the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). The SAM measures the strength of the westerlies and the north–south pressure gradient (Gong and Wang, 1999). A positive SAM phase is normally associated with dry conditions across southwestern and southeastern Australia, while the reverse occurs with a negative SAM phase (Sen Gupta and England, 2006, Hendon et al., 2007). Widespread drought typically occurs during El Niño years (e.g. Cai et al., 2001, Taschetto and England, 2009) and heavy rains tend to accompany a La Niña phase (e.g. Cai et al., 2001). The seasonal properties of ENSO and SAM have been studied spatially across Australia with respect to temperature (Jones and Trewin, 2000, Hendon et al., 2007), yet how these modes of variability impact the AT around Australia is unknown.

This paper presents an assessment of long term historical AT trends around Australia using urban observational weather station data and reanalysis data. Moreover, high resolution AT trends from Coupled Model Intercomparison Project 3 (CMIP3) model output are also studied for Australia using a best practice model for the Australian climate and using the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Representative Climate Futures framework best case and worst case scenario climate models. While the Representative Climate Future concept will be discussed in Section 2.2, the principle is that the best case scenario would present the least projected increases in AT, while the worst case scenario would maximise the increase. Analysis into the influence of SAM and ENSO on the AT around Australia illustrates the importance of the thermal indicator on a local and regional scale as it can impact tourism and the local economy. As the population increasingly grows into new areas of Australia, whether for mining or agricultural reasons, this research will enable better decision making for urban development planning.