Guest post by Blair Trewin. Blair is a senior climatologist at the Bureau of Meteorology’s National Climate Centre. He recently took the lead in writing the Special Climate Statement, Exceptional winter heat over large parts of Australia, issued 26th August 2009, updated 1st September 2009.
Australia has just experienced an exceptionally warm August. Almost the entire country experienced above-average temperatures during the month, but the warmth was most extraordinary in the subtropics. Over most of the southern Northern Territory and the southern half of Queensland (away from the coast), maximum temperatures for August were more than 5°C above the long-term average. Maximum temperatures were the highest on record for August over 49% of Australia.
Averaged over Australia as a whole, maximum temperatures were 3.20°C above the long-term August average, and daily mean temperatures (day and night combined) were 2.47°C above average. Both values are the highest on record for August by close to a degree. In terms of how far the month was above normal, the maximum temperatures in August 2009 are also the highest on record for any month, breaking the record of +3.11°C set in April 2005; the daily mean temperatures rank second behind April 2005.
The month was marked by some individual days which were exceptionally hot for August, especially in northern NSW and Queensland. State records were set for August in both states (37.8°C at Mungindi and 38.5°C at Bedourie respectively). Perhaps more exceptional were the margins by which some records were broken, and the number of days on which previous records were exceeded. Collarenebri broke its pre-2009 August record by 5.4°C, and numerous other locations, including Murwillumbah, Moree, Gatton, Miles and Taroom, broke August records by 4°C or more. Such margins are not unheard of at exposed coastal sites – where everything has to go right to achieve an extreme high temperature (not only having a very hot air mass, but having the wind in the right direction to prevent conditions being moderated by sea breezes) – but are virtually unknown at inland locations.
Many locations exceeded pre-2009 August records on five or more days. An especially striking example was Windorah in western Queensland, which prior to 2009 had never reached 35°C in August. In 2009 it happened seven times, and their August record was lifted six times, eventually peaking at 38.0°C on the 29th.
The extremely warm August combined with generally above-average (but not record-breaking) temperatures in June and July to give record or near-record winter temperatures in many areas. Australian daily mean temperatures for winter (1.33°C above average) fell just 0.01°C short of the 1996 record, and maximum temperatures surpassed the record set in 2002. NSW, Victoria and South Australia all had their warmest winters on record, which may come as a surprise to residents of the latter two states, in a season which was distinguished more by an almost complete absence of significant cold than by any major warm extremes.
In terms of weather systems, the month was marked by a persistent high-pressure ridge over the subtropics, preventing cooler air from penetrating from the south into central and northern Australia (until the last two days of the month, by which time it was too late to make much difference). Pressures were also well below normal south of Australia, resulting in very strong and persistent westerlies south of Australia (which made it an extremely wet month in Tasmania). An interesting comparison exists with October 1988, which had very similar pressure patterns, and was also dry over the mainland and very wet in Tasmania. In October 1988 Australian mean temperatures were 2.16°C above average, which was a record at the time (it now ranks fourth). The difference of 0.31°C between the two months is close to the size of the warming trend over Australia in that 21-year period, and suggests that the long-term background warming trend is playing a role in increasing the frequency of high temperature extremes of the type seen in August 2009.