Southeast Australian Heatwave and Victorian Firestorm January-February 2009
My view, having followed the meteorological developments of the heat wave, is that the meteorological conditions were not unusual but were rare in intensity.
Heavy monsoonal rains, cyclonic activity and some flooding have been a persisting feature over the tropics across and to the north of Australia for the past month. As a consequence, there has been widespread ascending air motion associated with the clouds and rainfall. With all of the ascending air there had to be compensating mass subsidence and this was largely across central and southern Australia, helping to explain the long dry spell.
During the last week of January the tropical rainfall was very heavy with flood rain. These tropical rain conditions helped to maintain the subsiding air during the southern heat wave. During this latter period a cooler airmass stretched across southwestern Australia from the Indian Ocean.
Thus, in the low and middle atmosphere, there was a northwesterly airflow from the Indian Ocean across southern Australia to the Tasman Sea. In this airflow there was a relatively strong temperature gradient between the cooler air to the southwest and the warmer subsiding air over the continent; the boundary of the air masses extended from north of Perth crossing the southern coast to the west of Adelaide and to Bass Strait. It is of interest that there was a persisting long high-level cloud line marking the edge of the air mass boundary; for a few days there was even light rainfall recorded on the national radar along the south coast east of Esperance in the cooler moist air to the west of the air mass boundary. On the eastern side of the boundary the air was hot and very dry with dew point values as low as 0C.
It is my interpretation that there was strong subsidence in the dry sector of the northwesterly airflow; subsiding air from the middle atmosphere is the only explanation for such hot and dry conditions at the surface (solar heating alone does not reduce the dew point temperature). During the daytime, the surface heating combined with the subsidence to support mixing of air from the middle troposphere to the surface. The air mass boundary acted as a barrier to the air from the tropics; to the northeast of the air mass boundary the middle tropospheric air was channelled downward in the northwesterly airflow and heated by compression. It is also likely that the thermodynamics of conservation of absolute vorticity at the air mass boundary contributed to the subsidence of the air over southeastern Australia.
Cooler surface air actually extended inland over southern Victoria during early February bringing some respite across the far south but the air mass boundary of the middle atmosphere seemed to maintain its position.
On 7 February there was a re-establishment of the air mass boundary across Bass Strait, with subsiding air in the northwesterly airflow over southern Australia (over western Tasmania and offshore there was actually rain in the cooler air). During the morning the dry air reached the surface and temperatures rose rapidly across southeastern Australia reaching record values over a wide area. Two fires that became established to the north of Melbourne late in the morning quickly developed in intensity within the hot dry airflow. The plumes of the fires, with their smoke, debris and burning embers, were so intense that they could be followed on the Bureau of Meteorology radar image available on the Internet. The plumes quickly extended southeastward, merged and reached out to sea off the Gippsland coast. Once the initial fires had become established in the hot dry airflow it was much like putting a roman candle fire cracker into a wind tunnel - the embers were carried kilometres ahead to spot and start secondary fires, which themselves developed and became the source of new embers. Within hours there were fires extending a great distance downwind and amalgamating into a northwest-southeast firefront.
Horrendous as the conditions were during the day, they were compounded late in the afternoon as the air mass boundary moved inland as a cool change. All of a sudden the long lateral northeastern boundary of the firefront became the leading edge as the strong southwesterly winds of the cool change advanced. The air ahead of the firefront was still dry, temperatures were high, and the fires were fanned by strong winds. Even after the cool change has extended inland there are remaining fires being fanned by strong winds that are a continuing danger.
It should be emphasised that the firestorm conditions have resulted largely from the conditioning of the vegetation over a prolonged period and the unusually intense interaction the tropical and middle latitude weather systems that brought about the heatwave. The latter interactions themselves have not been unusual but the intensity of the 2009 event has been rare; there are heat waves almost every summer but the 2009 event broke temperature records previously reached in 1939, although temperatures across southeastern Australia had previously approached similar intensity in 1983.
From the perspective of statistics, the record 2009 heat wave and accompanying bushfires is a rare event and should be treated as such. In itself, the fact that it exceeded the previous records set in 1939 does not establish a trend. Additionally, it is not possible to say that the statistical distribution, of which the event is a member, has changed its characteristics, or that the underlying physics and thermodynamics regulating the intensity of the event have altered. It would be most wrong, without further evidence, to suggest on the basis of this event that the record intensity was either a signal for or an outcome of climate change (human caused or otherwise).
William Kininmonth is a former head of Australia's National Climate Centre (1986-1998), Australian delegate to the World Meteorological Organization's Commission for Climatology (1982-1998, including two terms on its management
board) and author of Climate Change: A Natural Hazard (Multi-Science Publishing Co., UK 2004).