Author: Neil Plummer, Blair Trewin
Source: The Conversation
Date: 18 January, 2013
Australia has started 2013 with a record-breaking heat wave that has lasted more than two weeks across many parts of the country. Temperatures have regularly gone above 48°C, with the highest recorded maximum of 49.6°C at Moomba in South Australia. The extreme conditions have been associated with a delayed onset of the Australian monsoon, and slow moving weather systems over the continent.
Australia has always experienced heat waves, and they are a normal part of most summers. However, the current event affecting much of inland Australia has definitely not been typical.
The most significant thing about the recent heat has been its coverage across the continent, and its persistence.
It is very unusual to have such widespread extreme temperatures — and have them persist for so long. On those two metrics alone, spatial extent and duration, the last two weeks surpasses the only previous analogue in the historical record (since 1910) – a two-week country-wide hot spell during the summer of 1972-1973.
A good measure of the spatial extent of the heat is the Australian-averaged maximum daily temperature. This is the average of the highest daily temperature of the air just above the surface of the Australian continent, including Tasmania. The national average is calculated using a three-dimensional interpolation (including topography) of over 700 observing sites each day.
On Monday and Tuesday last week (January 7 and 8) that temperature rose to over 40°C. Monday’s temperature of 40.33°C set a new record, beating the previous highest Australian daily maximum of 40.17°C set in 1972. Tuesday’s temperature came in as the 3rd highest on record at 40.11°C.
And it’s not like these sorts of days occur that often. The records set last week sit between two and three standard deviations above the long-term January mean of 35°C.
Perhaps more unusually, the Australian mean temperature (representing the average of the daytime maximum and night-time minimum) set record high values on both days at 32.22 (January 7) and 32.32°C (January 8), that were well above the previous high of 31.86°C, set in 1972.
However, it is really the duration of this extreme heat wave that makes it so unusual, and so significant in terms of impacts.
While some towns in Australia are famous for their extended runs of hot temperatures, the limited geographical nature of those events distinguish them from this January’s heat wave. Multiple days of extreme heat covering most of the continent are both rare, and isolated.
It is not that common for the Australian-average temperature to exceed 39°C for even two days in a row. A run of three days above 39°C has occurred on only three occasions, and a run of four days just once, in 1972.
The current heat wave has seen a sequence of Australian temperatures above 39°C of seven days, and above 38°C of 11 days straight.
The sequence of Australian mean temperature has been just as impressive. As things currently stand, the first two weeks of January 2013 now hold the records for the hottest Australian day on record, the hottest two-day period on record, the hottest three-day period, the hottest four-day period and, well, every sequential-days record stretching from one to 14 days for daily mean temperatures.
The number of records that have tumbled for individual sites are now too numerous to catalogue here, and the Bureau of Meteorology has prepared a Special Climate Statement with a detailed analysis the temperature records broken. The list of records is limited to just those stations with at least 30 years of records.