Tropical cyclone activity in the 2010–11 season was, overall, near normal, with 10 tropical cyclones in the Australian region, while the 2011–12 season was below average, with 7 tropical cyclones. Usually, during La Niña events, tropical cyclone numbers around Australia's north are higher than the long-term average (11) over the November to April tropical cyclone season.
However, five of the tropical cyclones during 2010–11 were in the severe category, which is above average. At least 29 systems developed into tropical depressions, which is one level below a tropical cyclone. This is well above the number of tropical depressions observed in any tropical cyclone season in the Australian region since at least the mid-1990s (20 tropical depressions developed in 2011–12, also above average). In addition, three tropical cyclones (Tasha, Yasi and Anthony) crossed the Queensland coast in the 2010–11 season, and two ex-tropical cyclones (Grant and Jasmine) in 2011–12. Historically, multiple landfalls of severe tropical cyclone between Port Douglas and Ballina on the Queensland coast in a single season have only occurred during La Niña years. It remains unclear why both seasons saw numerous tropical depressions but an unusually small number of these eventually formed into tropical cyclones.
Tropical cyclone Yasi as it approached the Queensland coast just south of Cairns, captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite, 2 February 2011
Severe tropical cyclone Yasi, possibly the most powerful cyclone to make landfall in Queensland since 1918 (when two powerful tropical cyclones hit Innisfail and Mackay, also at the tail end of a La Niña), crossed the coast between Cairns and Townsville on 3 February 2011.
Tropical cyclone Carlos brought very heavy rainfall to the greater Darwin area between 15 and 17 February 2011, breaking numerous records, including record high daily rainfall at Darwin Airport with 367.6 mm on 16 February and a record three-day total of 684.8 mm. The average February monthly rainfall at Darwin Airport is 376.1 mm.
Tropical cyclone Grant brought very heavy falls to the Top End over 25 and 26 December 2011, resulting in flooding north of Katherine, which caused significant infrastructure damage, cutting roads and rail links, as well as derailing a freight train.
Tropical cyclone Lua made landfall on the Pilbara coast as a severe tropical cyclone on 17 March 2012. It was the strongest cyclone to cross the Western Australian coast since Laurence in 2009, and brought significant rainfall to a broad area of western and central Western Australia.
A tropical depression made landfall in the Gascoyne region of Western Australia, and while it never reached official tropical cyclone status it resulted in 207.8 mm of rainfall at Carnarvon on 17 December 2010 and 255 mm for the event. Carnarvon's average December rainfall is 5.6 mm and its annual average total rainfall is 231 mm.
Cloud cover
During the peak of the 2010–11 La Niña event, cloudiness over Australia increased. This increase in cloud can be correlated with the increased rainfall (as seen on page 10), reduced daytime temperatures and increased night-time temperatures (pages 12–13) observed during the La Niña event.
Outgoing long-wave radiation (OLR) anomalies over the greater Australasian region for summer 2010–11. Negative anomalies (blue and purple colours) indicate increased cloudiness and less heat lost to space, while positive anomalies (yellow and red colours) indicate reduced cloudiness. The widespread cloud over the Australia–Indonesia region during the summer is clearly shown.
