After a mild winter with higher-than-average maximum and minimum temperatures, spring in Tasmania is set to follow the same trend, the Bureau of Meteorology says.
Following typical winter rainfall for the state, there is a high chance of above-average rainfall for the east, but it is likely to be much drier than usual in western Tasmania.
While there were still some very cold mornings, and even some snow to low levels, temperatures were mostly higher than average for winter.
Maximum temperatures were higher than average for most of the state, tending to near average on the east coast. Minimum temperatures were also higher than average for most parts, becoming closer to average in the centre of Tasmania.
August temperatures were relatively warm; it was the seventh-warmest August on record for Tasmania overall, and the warmest since 2011.
Some sites in western Tasmania even observed their highest winter temperature on record, as strong north-easterly winds brought unusually warm air to this part of the state on August 29.
Despite warmer than usual temperatures for the season, July's minimum temperatures were below average due to clear skies and dry conditions.
Near-average winter rainfall
Winter rainfall was close to average for most of Tasmania; only 1 per cent lower than the long-term average for the state, above average rainfall in the south-west during June, and above average rainfall across the rest of the state in August, offset the fifth-driest July on record since 1900.
November 2021 to April 2022 was the driest November to April since 1900 for most of the west and parts of central Tasmania.
A wet spring is expected for most of the eastern half of Australia where there is a high chance of above-average rainfall. But for Tasmania, the spring rainfall outlook is more complex.
Spring is likely to be wetter than average across eastern Tasmania, the Bass Strait Islands, and the far north-west.
There is more than double the likelihood of unusually high rainfall across most of the east, and the Bass Strait islands.
In contrast, it is likely to be much drier than usual in western Tasmania.
Hobart has a 64 per cent chance of above median rainfall.
Average spring rainfall totals range from over 800 mm near Queenstown in the western district to less than 200 mm across the Midlands and East Coast districts.
The spring outlook shows a high chance of warmer temperatures across all of Tasmania; both maximum and minimum temperatures are likely to be above the median.
Maximum temperatures from January to the end of August were in the warmest 10 per cent of records, across most of the state; minimum temperatures were in the warmest 10 per cent of records for almost the entire state.
June was the only month of the year so far with below-average maximum temperatures, and July was the only month with below-average minimum temperatures.
Competing drivers
There are several major climate drivers influencing Tasmania's weather this spring.
The Bureau has declared a La Niña event is underway in the Pacific Ocean and communities in eastern Australia, including Tasmania should be prepared for above-average rainfall over spring and early summer.
La Niña refers to changes in the patterns for both the ocean and atmosphere across the Pacific Ocean.
During La Niña events, waters in the eastern tropical Pacific are cooler than normal, and waters in the western tropical Pacific Ocean warmer than normal.
This causes changes in wind, cloud and pressure patterns over the Pacific Ocean.
When this change in the atmosphere combines with changes in ocean temperature, it can influence global weather patterns and climate, including increasing rainfall over large parts of Australia.
La Niña and negative IOD combine for above-average rainfall
However, La Niña is not the only driver influencing this wet outlook.
To our west, a significant negative Indian Ocean Dipole (IOD) event looks set to continue through to late spring or early summer, increasing the chance of above-average rainfall for north-western Tasmania in the coming months.
The IOD represents sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean.
A negative IOD is typically associated with above-average spring rainfall for much of Australia as the warmer waters off north-west Australia provide more available moisture to weather systems crossing the country.
When a La Niña and negative IOD combine, it further increases the likelihood of above-average rainfall over Australia, particularly in the eastern half of the continent.
Meanwhile, the Southern Annular Mode (SAM) has been positive and is expected to remain positive throughout spring.
The SAM refers to the (non-seasonal) north-south movement of the strong westerly winds that blow almost continuously in the mid to high latitudes of the southern hemisphere.
A positive SAM has a drying influence for western Tasmania at this time of year but increases the likelihood of rainfall in eastern New South Wales, far eastern Victoria, and parts of southern Queensland.
There has been an increase in the frequency and intensity of heavy rainfall events in Australia in recent decades. The intensity of short-duration (hourly) extreme rainfall events has increased by 10 per cent or more in some regions over recent decades, with larger increases observed in the north of the country.
Warmest 20 years on record occurred in last 32 years
Globally, the warmest 20 years on record have occurred since 1990.
A warmer atmosphere can hold more water vapour than a cooler atmosphere.
One degree of global warming can increase moisture in the atmosphere by 7 per cent. This can increase the likelihood of heavy rainfall events.
Saturated catchments continue the risk of flooding in spring
August was much wetter than normal for parts of the north-east.
Where soils and catchments are wet, and streamflows are high, the outlook for above-normal rainfall in the east of the state this spring means the risk of flooding remains.
This is because the ground is like a sponge and its ability to absorb water depends on how dry it is: the wetter the soil, the less it can absorb water.
When the soil can hold no more water, it is '"saturated", so water will flow freely across the ground and join streams, creeks and eventually, rivers.
A river has a maximum capacity to carry water. If this capacity is exceeded, the river will rise higher than its banks and eventually flow out into areas next to the river.
