Queensland's flood

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  • Queensland's flood
    13.01.2011 08:45

     

    At any time there may be three reasons for torrential rains and floods in Australia: cyclones, persistent heavy rains for days or even weeks and individual severe thunderstorms or lines of slow-moving thunderstorms. Queensland has recently had all three.

    While the extent of the devastation is hard to comprehend, a very similar flood disaster occurred across New South Wales and Queensland in 1974, and Brisbane was badly affected.

    There were three weeks of floods, partly sparked by Tropical Cyclone Wanda, which gave torrential rain itself but then drew the monsoon trough southwards across eastern Australia leading to further frequent and lengthy downpours. One-third of Brisbane’s city centre flooded. There were 14 deaths and 300 injuries.

    The position in 2010/2011 is similar. There were sporadic heavy rains from October onwards, then Tropical Cyclone Tasha moved across Queensland from the northeast in December, when isolated flooding started. Around Christmas-time the monsoon trough moved southwards from the Coral Sea across eastern Australia, causing rain to persist heavily into January.

    Moreover, both in 1974 and during 2010 there was a strong La Niña event (the anomalous warming of the equatorial Pacific), and this latest one is only just started to wane. La Niña is well known for producing higher than average rainfall across eastern Australia, and the latest event is the strongest since 1973 when it was partly to blame for the floods early the following year.  The 2010 La Niña is probably the strongest since 1973’s.

    The current situation is likely to have been exacerbated by very warm seas off the coast – sea surfaces temperatures off Queensland are near record high levels. A warmer sea provides more energy and fuel for the development of severe storms such as that which has just hit Toowoomba, causing greater evaporation from the sea-surface and therefore more moisture to subsequently re-condense into clouds and teeming rainfall.

    2010 was the third wettest on record for Australia, and for Queensland (and eastern Australia in general) it was the wettest ever December, with 400-1200mm of rain in places. Brisbane’s average rainfall in December, for comparison, is about 130mm (160mm in January), and its yearly average 1400mm.

    Through Monday and Tuesday parts of southeast Queensland had 48-hour rainfall totals of 400 to 500mm, with much if it falling in rather short order.

    The extent, impact and severity of this flooding has been the most significant in Australia since at least the 1970s, and locally undoubtedly the worst ever known.

    By: Stephen Davenport