Recorded temperatures occurred across the state of Victoria on February 7 this year, contributing to the deadliest bushfires in Australian history, which ultimately killed 173 people and destroyed 1800 homes. Of the 35 climate stations in Victoria that have records going back 30 years or more, 24 recorded the hottest temperature ever, while five more recorded the highest temperature for the month of February. Melbourne reached 46.4ºC, the highest temperature since records began 154 years ago.
It was also very dry. Many parts of Victoria had no rain at all during January. Melbourne had the driest start to the year on record with no rain for 35 days before the fires. And on February 7, record low relative humidity occurred, falling to five per cent in places, which dramatically increased the fire hazard. There could not have been more perfect conditions for bushfires. And when they eventually broke out, strong winds intensified the fires and spread the flames rapidly across the countryside, catching many victims on the roads as they tried to flee. A fatal change of wind direction in the late afternoon drove one fire into the town of Kinglake, where many died. Survivors described trees igniting instantly as a wall of flame 15 m high roared uphill, giving people only minutes to react.
The extreme temperatures were caused by a stationary anticyclone over the Tasman Sea which produced persistent northerly winds over Victoria, assisted by an active monsoon trough bringing flooding rains to north Queensland. As water vapour condensed to form rain, large amounts of latent heat were released into the atmosphere. (Three quarters of the heating of the tropical atmosphere comes from this process.) As the air crossed the Great Dividing Range and flowed downhill it was further warmed by compression, before beginning the long journey south over the arid interior where it was warmed again by contact with hot land.
Normally the air would be cooled a little by evaporating water from the soil or vegetation. However, the inland area was experiencing severe drought following the driest 12 years on record. By the time the air flow reached Victoria it was dry as dust, sucking any remaining moisture from the forest and creating a tinderbox.
Recent studies have linked the persistent drought to patterns of the sea surface temperature across the tropical Indian Ocean. When the surface temperature is warmer than average at the western end of the tropical Indian Ocean, near Kenya, it tends to be colder than average at the eastern end, between northeast Australia and Indonesia.
This phenomenon has been christened the Indian Ocean Dipole (IOD) and shares some characteristics with the El Niño Southern Oscillation (ENSO) phenomenon that affects the tropical Pacific Ocean. Like ENSO, the IOD has a marked effect on rainfall. In the current situation, known as the positive phase of the IOD, warmer waters near Africa enhance the rainfall there, while cooler waters between northwest Australia and Java suppress rainfall in that area, as well as downwind over southeast Australia. In the negative phase of the IOD, the pattern of sea temperature and rainfall is reversed.
The IOD has now been in its positive phase for the last three years. Moreover, the IOD has either been positive or neutral every year since 1992, the longest such period since records began over 100 years ago.
The extreme temperatures on the day of the fires came hard on the heels of a record-breaking heatwave in the last week of January. Melbourne’s maximum temperature exceeded 43°C three days in a row, from January 28 to 30, while inland, at Mildura, the maximum temperature rose above 40°C for 12 days in a row. In Tasmania, the all time record was broken two days in a row and nearly half the state had its hottest day on record.
Breaking records by wide margins over such large areas has been acknowledged by scientists and politicians as a sign of global warming, caused by the large increase in greenhouse gases from human activities.
Ironically, some of the conditions that caused such an inferno in the south brought a deluge of rain and explosion of life to other areas of Australia. The heat in Victoria came partly from heavy rain in Queensland, some of which crossed the Great Dividing Range into the headwaters of the Diamantina and Georgina rivers. After weeks flowing 1000 km south through the Channel Country, water reached Lake Eyre for the first time in five years. This giant salt lake 700 km north of Adelaide covers an area of 6000 sq km and lies 15 m below sea-level.
Usually dry, the lake is home to a variety of lizards which feed on ants, which in turn feed on algae growing between the salt crystals. One species of lizard expels excess salt by sneezing it out its nostrils. A flood brings an explosion of life. Shrimp and fish eggs lying dormant under the lake bed hatch, and frogs buried in suspended animation awake and emerge. Nomadic birds such as pelicans, seagulls, cormorants, ibises, egrets and budgies arrive and breed in the thousands.
Rapid plant growth in the Channel Country also triggers a boom in the long-haired rat population. After a gestation of only three weeks, females produce litters of five to ten, which, in turn, are ready to breed after only 70 days. The abundance of rats is a boon for predators such as desert pythons and owls, as well as the letter-winged kite, which usually waits for the rats to swarm before breeding. Then the trees become so crowded with nests that the birds sometimes share them.
How much water will reach Lake Eyre this time is yet to be seen, though a deep flood is not expected. A four-metre-deep flood occurs about every 10 years, while a full flood happens about four times a century. For that you need something like an ex-tropical cyclone from the northwest meeting a cold front from the southwest, and for that you need the negative phase of the IOD—when that’s coming, nobody knows.