Heatwaves: started early and ended late
A series of significant heatwaves were recorded across the country from November 2012 through to March 2013.
The first of these occurred in late spring, in the final week of November. It was documented in Special Climate Statement 41.
Temperatures reached record highs for this time of year in many areas, particularly northern Victoria and southern inland New South Wales. A new record was also set for the highest spring temperature in Victoria.
The heatwave contributed to an unusually warm three months, with spring 2012 Australia’s third-warmest spring average maximum temperature.
The heat continued into the summer period, with an extended nationwide heatwave that began in the west just after Christmas and peaked during the first two weeks of January 2013.
The start of autumn was marked by a third major heatwave event. This was especially pronounced in the southeastern states where numerous cities and towns including Melbourne, Mt Gambier and Launceston set records for consecutive hot days and warm nights (see Special climate statement 45).
Articles about the recent heat
Bureau climate scientists prepared two articles on the recent heat for the academic news site, The Conversation. Extracts, together with links to the articles, are provided below.
Australia started 2013 with a record-breaking heatwave that lasted more than two weeks across many parts of the country. Temperatures were regularly above 48°C, with the highest recorded maximum of 49.6°C at Moomba in South Australia. The extreme conditions were associated with a delayed onset of the Australian monsoon, and slow moving weather systems over the continent.
The Bureau’s Special climate statement 43 details some of the records that fell during the first two weeks of January 2013.
Highest daily maximum temperature during the first two weeks of January 2013.
The record summer temperatures were determined using the Australian Climate Observations Reference Network – Surface Air Temperature (ACORN-SAT) dataset.
The record heatwave in January was analysed using both the Australian Water Availability Project (AWAP) and ACORN-SAT datasets. This means that two datasets were used to verify the record independently. Because the datasets are prepared in entirely different ways and use different underlying networks, their comparison provides an important validation of daily records.
The ranks for record-breaking daily-averaged temperature for the January heatwave are consistent across the AWAP and ACORN-SAT datasets. Ranks for the hottest three days on record are from the same events in both analyses. That is, the first and third hottest days were set in January 2013, while the second hottest day was set during the December 1972–January 1973 heatwave event.
As noted in the material above, perhaps the most notable feature of January 2013 event was the duration for which continent-wide extreme temperatures persisted. In this case, consistency in the duration records was again found across both operational datasets maintained by the Bureau.
This fact greatly increased our confidence in reporting on the exceptional nature of the January 2013 event.
Further details regarding the methodologies and supporting research papers prepared by the Bureau are included in the document Temperature data methodologies at the Bureau of Meteorology.
Summer mean temperature anomaly, 1910-2012.
The Bureau of Meteorology’s temperature data
The Bureau of Meteorology maintains two operational temperature datasets, as well as ‘raw’ station data.
- Real-time monitoring temperature data: analysis of some 700 thermometers located across Australia. At 1 pm AEST each day, temperatures for the previous day are analysed onto a regular grid (including topography) across mainland Australia and Tasmania. These data provide important real-time intelligence of how weather has unfolded across the country on a day-to-day basis. This data is also known as AWAP temperatures.
- The Australian Climate Observations Reference Network – Surface Air Temperature (ACORN-SAT ) data: Sourced from a network of specially chosen sites with long records, the ACORN-SAT dataset has been analysed to account for various changes in temperature measurement over the years. ACORN-SAT is the nation’s long-term temperature dataset used to gauge climate variability and change. It is perhaps the world’s only continent-wide, long-term daily temperature dataset that has been prepared specifically for climate change analysis and research.
The methodologies used in the preparation and analysis of all datasets prepared by the Bureau are published in peer-reviewed journals. Once they are published, the Bureau’s datasets are available to the public.
A detailed comparison of ACORN-SAT and AWAP data can be found in the research paper linked to from the ACORN-SAT website.
The fact sheet Improving Australia’s climate recordprovides further information about the ACORN-SAT dataset.
Methods used to calculate temperature records
The record summer temperatures were determined using the ACORN-SAT data.
The record heatwave in January was analysed using both the AWAP and ACORN-SAT data. This means that both datasets were used to independently verify the record. Because the datasets are prepared in entirely different ways, and use different underlying networks, comparison of the two datasets provides an important validation of daily records.
The ranks for record-breaking daily-averaged temperature for the January heatwave are consistent across AWAP and ACORN-SAT datasets. Ranks for the hottest three days on record are from the same events in both analyses.
That is, 1st and 3rd were set in January 2013, while 2nd was set in the December 1972 – January 1973 event.
As noted in the material above, perhaps the most notable feature of this event was the duration for which continent-wide extreme temperatures persisted.
In this case, consistency was again found across both operational datasets maintained by the Bureau.
This fact greatly increased our confidence in reporting on the exceptional nature of the January 2013 event.
Further details regarding the methodologies and supporting research papers prepared by the Bureau are included in the document Temperature data methodologies at the Bureau of Meteorology.
The satellite record versus surface thermometers
The satellite based Microwave Sounding Unit (MSU) temperature record provides recent estimates of temperatures over Australia, with records starting in the late-1970s. Satellite data has one advantage over surface-based observations in that it has total coverage over the Australian continent.
Satellites also have several disadvantages as well, when it comes to climate monitoring.
- They are of short duration – only dating back to 1979.
- They do not measure surface temperature – rather they measure an “average” temperature through a depth of several kilometres in the atmosphere.
- They are not global – for example high elevations and polar regions require interpolation or extrapolation.
- Individual satellites and satellite sensors tend to have a short lifetime and so temperatures records require the piecing together of numerous satellite missions.
A major source of potential inconsistency in the satellite record comes from the splicing together of data from multiple satellite missions over time. These satellite missions have different instrumentation. All types of thermometers that ever existed, for example, could be said to belong to a 'family of instruments'. Missions may have different orbital characteristics, and slight changes in the orbits of satellites over time have been shown to introduce inconsistencies in the data. The satellite record is complimentary to all other temperature data, and the Bureau and climate scientists compare these records routinely. The Bureau's surface temperature measurements for Australia compare well with the remotely sensed satellite record in terms of area-averaged variability and warming trends. These have been published in the papers found on the ACORN-SAT website.
Globally, the satellite data are warming at the same rate as the surface observations and show remarkable agreement. This is also the case over Australia, where warming trends since 1979 are very similar for satellite and surface data.
Differences between satellite observations and ground based thermometers during hot summers
Satellites measure the temperature of the air well above the surface. They measure the average temperature through the lowest few kilometres of the atmosphere. Surface temperatures and temperatures in the lower atmosphere are similar – but not the same. Further, they are not similar all of the time.
Some differences between the satellite record and the surface thermometers are understood and to be expected. This is particularly true over Australia during El Niño events or particularly dry and hot periods, such as the recent summer.
Research over the past 20 years has shown that dry summers are hot at the surface and wet summers are cool at the surface. The 'reverse' holds aloft (higher in the atmosphere) due to differing lapse rates – the rate at which temperatures cool as you go higher in elevation. The upper levels of air tend to cool off considerably, compared to the surface, when it is hot and dry over Australia.
This means that record summer temperatures in Australia are less likely to be matched by records higher in the atmosphere.
These differences were documented many years ago. For example see Drosdowsky, W., and M. Williams, 1991: The Southern Oscillation in the Australian region. I: Anomalies at the extremes of the oscillation. J. Climate, 4, 619-638.
Climate change tracker
Climate change analysis requires datasets that not only cover a long span of time, but are also homogeneous through time. A homogeneous climate record is one in which all observed climate variations are due to the behaviour of the atmosphere, not other influences, such as changes in location, exposure of the observation site, instrumentation type or measuring procedure.
