LAST year was the second-warmest on record for Western Australia as a whole and the fourth-warmest on record for Australia, with the annual national mean temperature 1.15 degrees above average.
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The warmth was persistent throughout the year, with six of 12 months placing in the 10 warmest on record for each of mean, maximum, and minimum temperatures for their respective months.
Annual mean temperatures were above or very much above average for most of Australia and were in the highest 10 per cent of historical observations for most of WA.
According to the Bureau of Meteorology, maximum temperatures for the year were also well above average across most of Australia and were also in the highest 10pc of historical observations for most of WA.
"Extreme heat affected northern Australia and the Top End of the Northern Territory during August," BoM stated.
"There was significant early-season heat in northern Western Australia late in August with some stations breaking their previous August record multiple times.
"The heat across northern Australia continued into spring, with September mean maximum temperatures the warmest on record for much of the northwest of Australia and further early-season records set in the northwest during the first half of the month."
Annual mean minimum temperatures were also in the highest 10pc of historical observations for much of WA, except the Kimberley and parts of the south and west coast.
According to BoM, rainfall in 2020 was very much below average for the west and southwest of WA, with annual rainfall totals in the lowest 10pc of historical observations for small parts of the west coast.
However, rainfall was above average for the north and east of the State.
"January rainfall was above average for much of WA away from the northeast and west coast, owing to tropical systems, including tropical cyclones Blake and Claudia," BoM stated.
"February rainfall was above average for most of the western half of WA and along the track of tropical cyclone Esther across the base of the Top End and the northern half of the Kimberley.
"The Southern Annular Mode (SAM) was positive for much of May and June - during winter, a positive SAM typically means less rainfall for southwest Western Australia as the rain-bearing westerly winds contract towards the south pole and fewer storm systems and cold fronts encounter Australia's southern coasts."
During November the La Nia weakened temporarily, with rainfall for the month above average for much of the north, west and southwest of WA, but below or very much below average for much of the eastern two thirds of Australia.
On top of that, a large number of cold fronts brought rain to the South West Land Division during November in what had otherwise been a sequence of generally below average rainfall months for that part of the State since April.
BoM stated that while short-term rainfall deficiencies for periods less than one year in length diminished across the first half of the year and were largely removed by late winter, the multi-year deficiencies persisted with much less significant change.
"Large areas of WA saw very little rain during April to July and short-term rainfall deficiencies emerged across much of southwest WA starting from April," the Bureau said.
"This was in addition to the existing multi-year rainfall deficiencies in the region for the period commencing in early 2018."
There has been a significant decline in April to October rainfall observed over southwest and southeast Australia, including in higher rainfall parts of the Murray-Darling Basin in recent decades.
"There has been a decline of about 16pc in April-October rainfall over the southwest of Australia since the last 1990s," BoM stated.
"The drying trend is particularly strong for May-July across southwest Western Australia; this region has seen May-July rainfall decrease by about 20pc since 1970."
Tropical lows affected the west of the country around mid-December, bringing torrential rain there too and for the country as a whole, it was the fourth-wettest December on record.
