Heatwaves are periods of unusually hot weather that can have serious
impacts on human health and wellbeing, agriculture and the
environment. The Queensland Future Climate Science Program calculates
heatwaves using the Excess Heat Factor (EHF), which is the same method
used by the Bureau of Meteorology. However, for our latest (CMIP6) estimates instead of using temperature, we use the heat-index,
which incorporates temperature and humidity, which is particularly
important in tropical regions in Queensland. Using this method,
heatwaves are defined as periods of at least 3 days or more where the
heat index is high compared to the recent and long-term past.
How are heatwaves analysed in climate projections?
The EHF considers both acclimatisation (the ability to adjust to the
change in the heat index from the recent past), and how high the heat
index is relative to the 95th percentile of the heat index during the
reference period. We used 1981 - 2010 as the calibration period.
The EHF is obtained by combining two indices - the
EHIaccl and EHIsig. The heat acclimatisation index (EHIaccl) is calculated as the average heat index over the past 3 days
compared to the average heat index over the previous 30 days (equation
1 below). The heat significance index (EHIsig) is the average heat index over the past three days compared to the
95th percentile over the calibration period, 1981 - 2010 (equation 2).
The EHF is calculated as the product of these two indices (equation 3)
where Ti is the heat index on a specific day.
A heatwave occurs when the EHF is 1.0 or more for at least three
consecutive days. Based on this definition, we can calculate a time
series of the EHF in the climate simulation data that can then be used
to derive a number of heatwave metrics depicting its characteristics
over a particular season and for different time periods in the future.
As heatwaves are more frequent in warmer months (December, January and
February), it is important to preserve the continuity of the summer
season when representing its characteristics. Therefore, we provide
heatwave indices for the 1 July - 30 June hydrological year and the
November - April wet season.
The indices available on the Queensland Future Climate Dashboard and
Regional Explorer under the Heatwave 'theme' are shown below in Table
1
Table 1: Heatwave indices.
Heatwave Peak Temperature (CMIP5)
Maximum temperature (°C) of the hottest day of the season's
hottest heatwave
Heatwave Peak Heat Index (CMIP6)
Maximum heat index (°C) of the hottest day of the hottest
heatwave (also known as the heatwave amplitude)
Heatwave Frequency
Number of heatwave days in a season relative to the number of
days in the season i.e. (Heatwave days / Number of days in
season) * 100%
Heatwave Duration
Average duration of heatwaves in a season (days)
Maximum Heatwave Duration
Length of the longest heatwave in a season (days)
The figure below illustrates how heatwaves and their characteristics
are identified and extracted from a temperature time-series. For
additional information on heatwaves in Queensland, please visit the
Heatwaves case study.
An illustration of a heatwave and its characteristics over a
hypothetical temperature time-series (source: Queensland Future
Climate Heatwaves case study).
Heatwaves on Queensland Future Climate
The Queensland Future Climate Science Program has used
state-of-the-art high-resolution downscaled climate simulations from
both
CMIP5 and CMIP6 global
climate models to assess the impact of climate change on heatwaves.
The main tools for viewing climate projections data on the Queensland
Future Climate website are the
Queensland Future Climate Dashboard
and
Regional Explorer. Please refer to the
user guide
for detailed information on how to access and interpret information
available from these resources.
Examples of the heatwave information available on the Queensland
Future Climate Dashboard and Regional Explorer are shown below.
Sample map and plots from the Queensland Future Climate Dashboard
(CMIP6) showing the projected change in
the heatwave peak heat index for the Isaac
Regional Council area in 2090 under a high emissions scenario
(SSP3-7.0). The heatwave information can also be downloaded as
tables or shapefiles to enable further analyses.A sample timeseries chart from the Regional Explorer (CMIP6) showing projected changes in
heatwave frequency for the Isaac Regional Council
area under the 3 emissions scenarios - low (SSP1-2.6), moderate
(SSP2-4.5) and high (SSP3-7.0).
References
Nairn, J. R., and R. J. B. Fawcett. 2015. The Excess Heat Factor: A
Metric for Heatwave Intensity and Its Use in Classifying Heatwave
Severity. International Journal of Environmental Research and Public
Health 12:227-253.
Trancoso, R., J. Syktus, N. Toombs, D. Ahrens, K. K. H. Wong, and R.
Dalla Pozza. 2020. Heatwaves intensification in Australia: A
consistent trajectory across past, present and future. Science of the
Total Environment 742:140521.
