Put simply, rain tends to cluster around the warmest parts of the ocean because the warmth produces rising air, and this in turn generates cloud and rain. La Ninas are usually associated with enhanced rainfall over eastern Australia with moist tropical air often extending down from Queensland well into the southern states, even as far as Tasmania, where flooding occurred during the second week in January 2011.
The present La Nina weather pattern across Australia kicked in rapidly around May of 2010, flipping across from the opposite El Nino phase that is usually associated with dry weather for eastern Australia.
The onset of the La Nina resulted in a rising crescendo of major rainfall events that rolled across Queensland from September onwards. Catchments right across the state became increasingly saturated as one rainfall event followed another.
The Barron Falls inland from Cairns, in raging flood during January 2011. Photo by Barbara Menz.At the end of 2010 most of the catchments across south-eastern Queensland were totally saturated and any additional rainfall would then just run off into the adjacent river systems – a critical situation to be in during only the early stages of the Australian tropical wet season.
Disastrous flash flooding erupted through Toowoomba on Monday 10th January, when a wall of water swept through the town, killing an unknown number of people and causing immense property damage as houses and cars were literally swept away by the deluge. This occurred as high intensity rainfall, around 100mm in just one hour, fell on the already saturated catchment of the Lockyer Creek.
Major flooding then burst across much of southeast Queensland on Tuesday 11th and Wednesday 12th January with Brisbane itself inundated as the Brisbane River burst its banks and rose to levels just short of the 1974 flood. The degree of flooding in the area places the disaster in the “top three” of floods, alongside 1893 and 1974.
Major flooding around Boonah in Queensland in January 2011. Photograph by Leona Taylor.The La Nina phenomenon marches to an irregular beat but has always been a big part of Australia’s climate, as has the opposite El Nino phase. Dorothea Mackellar summed up their impact poetically when she described our climate as one of “droughts and flooding rains”. Indeed it has been said, somewhat tongue in cheek, that average rainfall in many parts of Australia equals drought plus flood divided by two.
Following the massive flooding of early 2011, many people are asking if climate change is somehow behind the tremendous rainfall intensity we have just seen. Here expert opinion is somewhat divided.
Total inundation of the surrounding countryside near Moorang, Queensland during January 2011. Photo by Angela GraySome climatologists believe that rising ocean temperatures around northern Australia, that are about 1.5C above pre 1970’s levels, will produce more intense La Ninas, even if their frequency remains the same. Warmer oceans produce warmer air above and an atmosphere of this type can hold far more moisture that then becomes available for rain.
The unusually strong monsoon that produced devastating floods across India and China around mid 2010, followed by the flooding disaster in Pakistan in August has been linked to this pattern.
However other experts have remarked that although it is almost certain that rising ocean temperatures will be affecting the La Nina/El Nino see-saw it is too early to say how. They point out that the period of recorded history is too short to reach any firm conclusion.
However there is little doubt that the extraordinary weather events that occurred right around the world during 2010, together with our own Australian experience of extreme weather, has brought the topic of climate change back on the front page after a notable absence during much of 2010.
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