AFTER several dry years across northern Australia, wet season 2020-21 saw a welcome return to above-average rainfall, according to the Bureau of Meteorology.
In fact, overall, it was northern Australia's wettest and coolest October to April wet season since 2016-17.
BOM says the wet season was off to a flying start, with a wet October as La Nina became established in the tropical Pacific Ocean.
The northern rainfall onset is the first 50mm of rain accumulated after September 1. It occurred many weeks earlier than normal in central and north-western parts of the Northern Territory.
However, November was dry across the north. La Nina temporarily weakened and the Madden-Julian Oscillation stalled near Africa.
This meant weather patterns over Australia suppressed, rather than encouraged, rainfall. It was also the warmest November on record, with the overall mean temperature 2.37 degrees Celsius above the long-term (1961-90) average.
Wet conditions returned as monsoonal and tropical low activity contributed to the second-wettest December on record for northern Australia, behind 2000.
BOM said typical of a La Nina year, the monsoon at Darwin began earlier than usual. The official monsoon onset date of December 19 was about 10 days earlier than the long-term average.
It was more than a month earlier than in the previous three wet seasons.
Above-average rainfall continued into 2021, although not to the same extent as December.
Northern Western Australia was affected by several tropical lows. One brought heavy rains in the Kimberley and east Pilbara areas in January.
Another resulted in parts of western Gascoyne having their wettest February on record, along with extensive flooding.
During March, there was moderate to heavy rainfall across the Kimberley extending into western and southern Northern Territory. There was moderate flooding on the Fitzroy and Ord Rivers.
In response, water levels in Lake Argyle, Australia's largest water supply storage, reached 82.1 per cent of capacity by the end of March. In November 2020, following three below-average wet seasons, water levels there had fallen to 25pc, their lowest in 30 years.
BOM said Queensland rainfall in 2021 was largely restricted to northern areas and the Cape York Peninsula. Weather patterns were less favourable for rain further south.
A significant exception was from March 17 to 26, when tropical air interacted with a slow moving coastal low. This produced heavy rain and some flooding across central and southern Queensland.
Early March saw moderate rain along the Cairns coast and in northern Cape York Peninsula from tropical cyclone Niran.
On March 17, a deep surface trough off the Capricornia coast resulted in an isolated report of 550mm at Byfield, north of Yeppoon. This was the highest daily total of the wet season.
Far north Queensland had another major rain event when the coastal area around Cairns received between 600 and 1100mm between April 19 and 26.
Mt Sophia, south of Cairns, had the highest recorded total of 1104mm.
Such significant rainfall hasn't been seen during April in the Cairns area in over 100 years - since 1911.
Less cyclones
There were eight tropical cyclones this wet season. This is below the long-term average count of 11 and the average of nine since 2000.
However, there were around a dozen tropical lows. Several brought significant rain to northern Australia.
Of the tropical cyclones, four occurred off Western Australia and four off Queensland.
BOM said for the first time since 2015-16 there were no tropical cyclones within the Northern Territory.
Two cyclones made coastal crossings. Tropical cyclone Imogen made landfall on the south coast of the Gulf of Carpentaria in January as a category 1 system. Severe tropical cyclone Seroja had a significant impact on Western Australia's central west area.
Seroja made landfall in early April as a category 3 system. Winds of up to 170km/hour caused considerable damage to Kalbarri and nearby localities.
BOM said Seroja was likely the strongest cyclone to have affected the area since satellite observations began in the 1970s.
Cool temperatures
La Nina also contributed to relatively cool conditions across northern Australia during the wet season after several very warm years. The mean maximum and minimum temperatures were 0.45 deg C and 0.74 deg C above the long-term average respectively, the coolest since 2016-17.
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