Tasmania

Date: 28 Apr 2022

Seasonal climate risk information for Tasmania

Volume 5 | Issue 4 | 28 April 2022

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Rainfall has been useful east and west of Launceston with 25 plus mm, but only 10-25mm has fallen in the rest of the Midlands. Up until 27th April this was rated at 20-40 per cent of normal. Soil moisture is also ranked drier at decile one to three in the Midlands with pasture paddocks predicted to be 0-25 per cent full of plant available moisture.

The eastern Pacific Ocean remains cooler to depth and slightly increased in extent, capable of further La Niña activity, but the surface temperatures have become closer to normal. Trade winds are a bit stronger in the western Pacific but could be more convincing. They are still holding warmer water to our north particularly south of the Equator. Cloud patterns at the Dateline and a strengthening SOI indicate that the atmosphere is not quite ready to give up on La Niña like behaviour. Seas are very warm to Australia’s north and evaporating more moisture, lowering the pressure. Models are generally leaning towards the neutral Pacific continuing, but 4/12 models pick it back up again for a third La Niña in a row. The Indian Ocean is completely normal with basin-wide warming. Some stronger westerly wind towards Java is likely to increase warming in this region, something a number of models are predicting. All models predict a negative Indian Ocean Dipole to form in coming months, but this is an unreliable time of the year for IOD prediction. It’s also not completely obvious where the model signal is coming from. Just slightly warmer water to depth and the stronger westerly wind activity is all that can be seen at the moment.

The Southern Annular Mode has stayed positive for another month, and this this can have erratic effects in autumn. The stronger easterly wind over Bass Strait has possible been keeping the far north in business but has really kept the west drier than normal. The +SAM has also been conspiring with the higher pressure to be sending rainfall triggers further south.

My assessment of 12 climate models for Tasmania shows neutral rainfall predictions and likely warmer temperatures for the next three months.

People are reminded that in autumn, the resetting nature of the tropical oceans makes it an uncertain time for predictions for both autumn and winter. For more information about the autumn predictability barrier check out our eLearn: https://rise.articulate.com/share/laU0Fh8td92Wodvmg6M6kQZLovAJCNgx

Soil moisture

Map of Tasmania showing very little moisture in midlands.
The BoM Australian Water Outlook (AWO) modelled plant available soil moisture (10-100cm) for actively growing pasture has increased the area of drier than normal this month. Large areas of the west are ranked in the driest one per cent of years, the Northeast is closer to normal. Rainfall of 10-25mm in April has just kept up with evaporation. Soils have stayed similar in moisture content to March and are now fairly dry and ranked decile two to three in the Midlands.

Model distribution summary for the next three months

Graphs showing two neutral/wetter and nine neutral rainfall forecasts and 12 warmer temperature forecasts for May to July 2022 in Tasmania.
Graphs showing the distribution of 12 global model forecasts for May to July 2022 with neutral rainfall and likely warmer temperatures forecasted.

Model distribution summary for the next four to six months

Graphs showing one wetter, seven neutral and one drier rainfall forecast and one neutral, and eight warmer temperature forecasts for August-October 2022 in Tasmania.
Graphs showing the distribution of nine global model forecasts for August-October 2022, with neutral rainfall forecasts and likely warmer temperatures.

Model consensus forecast for the next six months

Current outlook (28 April)

Previous outlook (28 March)

May-Jul

Aug-Oct

Apr-Jun

Jul-Sep

Pacific Ocean

Slightly cool / cool
(weak La Niña)

Normal

Cool (La Niña)

Slightly cool /
normal

Indian Ocean

Warm (-IOD)

Warm (-IOD)

Slightly warmer

Warm (-IOD)

Rainfall

Neutral

Warmer

Neutral

Warmer

Temperature

Neutral

Warmer

Neutral

Warmer

Sea surface temperature anomalies

Map of the world showing sea surface temperature anomalies, a coolish Pacific Ocean with a warm Coral Sea and Indian Ocean.
The Equatorial Pacific Ocean varied over March, with NINO3 slightly cooling and NINO3.4 values slightly warming at -0.49oC and -0.62oC respectively (as of 26 April). Both these values are neutral, but the overarching horseshoe pattern of the Pacific is that of a La Niña. The ocean is warm around most of Australia and especially to our north. The whole southern Indian Ocean is warmer. The Dipole Mode Index (DMI) is currently at +0.01oC, completely normal and showing no IOD. Sea surface temperatures are the key to the world’s rainfall. For more information on how they are measured, maps created and how to read them, check out our eLearn: https://rise.articulate.com/share/kJceF16KasOxoeypJi1UWsAz0IzKKUF1

Equatorial Pacific sub-sea temperature anomalies

Cross section of the Equatorial Pacific Ocean chart showing a larger area of cool with less intensity in the east at depth.
The cool Pacific Ocean Equatorial sub-surface temperatures have increased in area to depth in April but are not as intensely cold as for March. There is still a possibility this could hang around long enough for a third La Niña in a row.

Southern Oscillation Index

Graph of the SOI shows the value is currently at +17.4
The SOI has remained strongly positive throughout April currently +17.4 (at 26 April 2022). Pressure patterns around the equator are behaving very much like La Niña with lower pressure at Darwin and higher pressure at Tahiti.

Dipole Mode Index (DMI)

Graph of the DMI shows the value is currently at +0.01C
The DMI is the difference between the west and east boxes of ocean monitored for the IOD. The IOD is currently neutral (currently +0.01oC as of 26 April 2022), normal for this time of the year. Water in both the eastern and western boxes of the Indian Ocean is normal to warmer.

Pacific Ocean surface wind anomalies

Map showing stronger equatorial Pacific Ocean trade winds and a burst of stronger westerly Indian Ocean winds into Java.
Easterly trade winds in the Pacific have remained stronger in the western Pacific but are strongly southerly in the eastern Pacific. Continued stronger easterlies will hold the pool of warm water to Australia’s north. A region of stronger westerly winds in the Indian Ocean is further south than for a classic -IOD, but will be holding warmer water in against Java. There has been stronger easterly wind over Bass Strait, still in keeping with a positive SAM and not conducive for rain in the west.

World cloudiness anomalies

Map of the world showing decreased cloud at the equatorial junction with the dateline and decreased cloud over northern Australia.
A lack of cloud over the international dateline junction with the equator continues and is classic La Niña-like behaviour. A large area of less cloud over the eastern equatorial Indian Ocean is due to reduced Madden Julian Oscillation (MJO) activity again this month, this has persisted over northern Australia as well. Cloud has been slightly less over the south of the island.

Southern Annular Mode

Graph of the SAM showing weak positive values for April.
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) has spent April mainly positive, with a few dips to neutral and just one current dip into significant negativity. A positive SAM has historically led to erratic climate effects in autumn. Clearly the dragging south of frontal systems has been having a more winter like effect of the +SAM. The NOAA and BoM models agree that the SAM will dip into moderate negativity for a short while and then bound back into weak or moderate positivity.

Air pressure

Map of the world showing the STR of high pressure at a slightly more southerly position than is usual for Autumn.
In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) continues to be further south than is normal for autumn, a hangover of both the La Niña and the positive SAM. A high positioned with Tasmania at the centre, is not a good place to be if you are wanting rainfall triggers. A normal position in autumn would be centred around Adelaide, so expect to see the ridge make a move north at some stage.

Air pressure anomalies

Map of the world showing lower pressure over most of mainland Australia but higher pressure over Tasmania.
The sub-tropical ridge of high pressure over Australia has been lower making it easier for moisture transport pathways and low pressure troughing to occur from the north. In a counter to this, pressure has been higher over Tasmania pushing moisture triggers south. Pressure is higher at Tahiti and lower over Darwin which is why the SOI is positive, indicative of La Niña like pressure conditions. The pressure differential in the equatorial Indian Ocean is the opposite of that for a negative IOD.

Definitions

For a list of climate acronyms and explanations

Modelled Climate and Ocean Predictions for Tasmania from April 2022 run models

Twelve climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Tasmania.
Twelve climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Tasmania.
Click here to download this table in MS Word format
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