Tasmania - weather and climate drivers

 

Overview

Map showing major weather and climate drivers across Australia

 

Tasmania’s main climatic drivers are summarised in Table 1.

Table 1. Summary of Tasmania’s main climatic drivers of weather

Climatic driverPotential effectWhen it is most activeWhere in Tasmania it has most effect
Sub-tropical ridge

stronger and more frequent cold fronts

weaker and less frequent cold fronts

winter

 

summer

 statewide
Southern Annular Mode (positive phase) less rain in the west all year, but with the most effect during winter and spring west (and the north in winter) 
Southern Annular Mode (negative phase)  more rain in the west all year, but with the most effect during winter and spring west (and the north in winter) 
Indian Ocean Dipole (positive)  less rain 

May - November 

northern half of the state
Indian Ocean Dipole (negative)  more rain  May - November  statewide

El Niño – Southern Oscillation    

El Niño - less rain

La Niña - more rain 

can be all year, but especially May - November  north and east

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These climatic drivers can modify synoptic features in Tasmania as summarised in Table 2.

Table 2. Summary of Tasmania’s synoptic features

Synoptic featurePotential effectWhen it is most activeWhere in Tasmania it has most effect
Frontal systems  rain, temperature changes, strong winds all year, but more so in winter west

High pressure systems and blocking highs   

little rainfall

fog more likely

cool when the high is to the west, warm when the high is to the east

all year

statewide

Cut-off lows 

 

heavy rain

strong winds

large swells  

all year, but particularly from March to October east 

Upper-level troughs and cloud bands

 

more rainfall 

warm nights and cool days

all year   statewide

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Sub-tropical ridge

The sub-tropical ridge, an extensive area of high pressure, is a major feature of the general circulation of our atmosphere. It is a major influence on the climate of southern Australia.

Its position varies with the seasons, allowing stronger, more frequent cold fronts to pass over Tasmania in the winter, but weakening them and pushing them to the south in summer.

 

Southern Annular Mode

The Southern Annular Mode (SAM) can affect rainfall in southern Australia.

The SAM describes a north-south movement of the “roaring forties”, the belt of strong westerly winds that blow around the southern hemisphere between 40° and 50° south. It varies over periods of weeks or months.

Because these winds regularly blow over Tasmania and markedly influence rainfall patterns over the state, especially in the west during winter, changes in the SAM often lead to changes in rainfall patterns over Tasmania.

 

map showing areas affected by SAM and when

 

One measure of SAM is the Antarctic Oscillation Index, produced by the US National Weather Service. Typically, this index varies between -1 and +1, with more extreme values indicating a particularly abnormal event.

The SAM can be in a positive or negative phase.

 

Positive SAM phase

When the SAM is positive, the belt of strong westerly winds contracts towards the South Pole.

Westerly winds over Tasmania weaken, the pressure increases and fewer fronts cross the state.

This means less rain falls in the western half of Tasmania, particularly during winter and spring.

It is not clear how rainfall in the west is affected during autumn, with some studies suggesting it may be increased during a positive SAM event.

A positive SAM also leads to drier-than-usual conditions in the north during winter.

There is some evidence to suggest a tendency for increased rainfall on the east coast during spring and summer (as easterly weather patterns are more likely to develop).

 

Negative SAM phase

When the SAM is negative, the belt of strong westerly winds moves towards the equator.

More fronts cross Tasmania, usually leading to more rain in the west during winter and spring.

In July 2009, the SAM was in a negative phase. The average Antarctic Oscillation Index for the month was -1.2, and at one stage fell below -3.

The westerly winds over Tasmania increased and cold fronts crossed the state regularly (Figure 3). Only a few high pressure systems moved nearby.

It was wetter than usual through the west and north, but rainfall was near or slightly below normal in the east (Figure 4).

chart showing Tasmania 27 July 2009

 

map showing Tasmania’s rainfall totals, July 2009

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Indian Ocean Dipole

The Indian Ocean Dipole (IOD) is a measure of changes in sea surface temperature patterns in the northern Indian Ocean. These changes contribute to the formation of cloud bands.

The IOD is derived from the difference in sea temperature between the western Indian Ocean, near Africa, and the eastern Indian Ocean near northern Australia.

An IOD event usually starts around May or June, peaks between August and October, and then rapidly decays.

map showing areas affected by IOD and when

 

The Dipole Mode Index is a measure of the Indian Ocean Dipole. It is calculated from the difference in sea surface temperature anomalies in the western and eastern equatorial Indian Ocean. When the index is greater than 0.4 or less than -0.4, the IOD is active. It exceeds 1.0 (or is less than -1.0) only occasionally.

The impact of the IOD on weather and ways to predict its development are areas of active research.

 

Positive dipole

The IOD is positive when waters are warmer than normal near Africa and cooler than normal near Australia.

This leads to an increase in the easterly winds across the Indian Ocean and reduced cloudiness (and, therefore, rainfall) near Australia.

In Tasmania, the result is often below-average winter and spring rainfall in the northern half of the state.

In many years when the IOD is positive, the Pacific Ocean is in an El Niño state, further increasing the likelihood of below-average rainfall.

 

Negative dipole

The IOD is negative when waters are cooler than normal near Africa and warmer than normal near Australia.

Warmer waters near Australia, particularly near Indonesia, contribute to cloud formation and increase rainfall.

So, a negative IOD usually leads to a wetter-than-usual winter and spring in most of Tasmania.

 

1983 and 1992 – positive and negative IOD years

The IOD was positive is 1983. Winter-to-spring rainfall in Tasmania was below average in most of the north, but near or even slightly above average in the south (Figure 6). This rainfall pattern is often, but not always, seen in positive IOD years.

In contrast, the IOD was negative in 1992. Winter-to-spring rainfall was above average across much of Tasmania (Figure 7).

Both 1983 and 1992 were neutral ENSO (El Niño - Southern Oscillation) years, so they more clearly show the effect of the IOD without the effect of ENSO.

 

 map showing rainfall totals for Tas June-Nov 1983

 

map showing rainfall totals for Tas June-Nov 1992

 

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El Niño – Southern Oscillation

The El Niño - Southern Oscillation (ENSO) is a major influence on Australia’s climate.

ENSO is the irregular oscillation of pressure, sea surface temperature and weather patterns across the tropical Pacific Ocean.

The two extremes of this oscillation, El Niño and La Niña, affect rainfall over much of eastern Australia, including the east and north of Tasmania.

The influence is most pronounced during winter and spring, and negligible in autumn when ENSO is usually neutral or in transition.

 

El Niño

An El Niño event is characterised by extensive warming of sea surface temperatures in the central and eastern tropical Pacific, and, usually, cooling around northern Australia.

Often this causes drier-than-normal conditions in eastern and northern Tasmania during winter and spring and in north-western Tasmania during summer, but there is much variation between each event.

Occasionally, Tasmania has been wetter than usual in an El Niño year (as in 2009).

The relationship between El Niño and rainfall in southwest Tasmania is not clear.

 

1994, 2002 and 2006 – not all El Niño years are the same

2006 was an El Niño year. Winter-to-spring rainfall across almost all of Australia was well below average.

Winter-to-spring rainfall for Tasmania (Figure 8) was well below average in the north and east – the classical El Niño response pattern.

Even in the southwest, where the classical El Niño response is for near-average winter-to-spring rainfall, 2006 was drier than usual.

 

map showing rainfall totals for Tas June-Nov 2006

 

2002 was another El Niño year with dry conditions over much of Australia.

However, winter-to-spring rainfall in Tasmania was near normal in the east and north and above average in the west (Figure 9), illustrating that not all El Niño years are the same.

El Niño does not explain all of Tasmania’s rainfall variability. However, it is more likely to be dry in the north and east in El Niño years.

 

 map showing rainfall totals for Tas June-Nov 2002

 

In 1994 both El Niño and a positive Indian Ocean Dipole were in effect.

Winter-to-spring rainfall across almost all of Australia was well below average.

Tasmania had the classical El Niño response pattern with very dry conditions in the north and east, and near-to-slightly-above-average rainfall in the west (Figure 10).

 

map showing rainfall totals for Tas June-Nov 1994

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La Niña

La Niña is associated with extensive cooling of sea surface temperatures in the central and eastern tropical Pacific.

We usually see a warming of the waters to the north of Australia and higher than usual winter and spring rainfall in the east and north of Tasmania.

It can also be wetter than usual in the north of Tasmania during summer.

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Frontal systems

map showing areas affected by frontal systems and when

 

Frontal systems, such as cold fronts, generally move from west to east across the Southern Ocean and vary in their intensity and speed.

As Tasmania is located around 42 °S in the “roaring forties”, cold fronts cross the state regularly.

These cold fronts are usually connected to a low pressure system over the Southern Ocean, which can pass very close to Tasmania or well to the south.

Fronts are more common in winter, and sometimes cross the state every day for more than a week.

A front almost always brings rain or showers to the west of the state, but the strength of the front determines how much rain there is and if the rain will reach the east coast.

More intense systems are generally associated with heavier rainfall; however, a slow-moving front can also bring heavy falls if the rain persists over an extended period.

Thunderstorms sometimes also accompany a frontal system.

Cold fronts generally bring a drop in temperature, which may be barely noticeable but can be very marked.

Summer-time cold fronts can cause temperatures to drop more than 15 °C in a day.

Winter-time fronts can bring snowfalls at sea level.

The hottest days usually occur when there are northerly winds ahead of a cold front.

The coldest nights occur when a high pressure system moves over the state, immediately behind a cold front.

Vigorous wind changes can accompany frontal changes. Winds may gust to over 100 km/h as a front crosses. Strong and damaging winds are almost certain when the associated low is very deep or close to Tasmania.

Very large swells can develop in the west and south if these lows are deep and move fast across the Southern Ocean.

A strong sub-tropical ridge or a positive Southern Annular Mode can cause frontal systems to track further south than usual and have less effect on Tasmania.

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December 2009 – cold front brings 20-degree temperature swing

A slow-moving cold front crossed Tasmania on 16–17 December 2009. Temperatures climbed above 30 °C on the 16th as strong northerly winds blew across the state.

In stark contrast, temperatures the next day struggled to reach 15 °C in many areas and rain was widespread, with some snow on the higher peaks.

Several sites were 20 °C cooler after the change, including Hobart which reached 34 °C on the 16th but just 12.4 °C on the 17th.

 

chart showing cold front crossing Tasmania 11pm 16 Dec 2009

 

map showing rainfall in Tasmania for the 24 hours to 9 am 18 Dec 2009

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August 2009 – a spate of cold fronts brings floods and damaging winds

A cold front crossed Tasmania almost every 24 hours between 19 and 31 August 2009.

Each front brought moderate-to-heavy rainfall to the west and north, which led to widespread and persistent river flooding.

Many of the fronts caused damaging winds to blow across the state. On the 31st, winds gusted to over 80 km/h across the state and over 100 km/h in several areas, reaching 174 km/h at Maatsuyker Island in the southwest.

Waves reached 15 metres in the west, there were thunderstorms in the southwest and snow fell to around 300 metres in the south.

chart showing cold front crossing Tasmania 31 Aug 2009

 

map showing rainfall in Tasmania for week-ending 31 Aug 2009

 

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High pressure systems and blocking highs

map showing areas affected by high pressure systems and blocking highs and when

 

High pressure systems are a major influence on Tasmania’s weather.

They prefer to sit over mainland Australia during winter but south of the continent during summer.

Their effect varies according to their position, intensity, movement and other weather systems nearby.

A high centred right over Tasmania will generally bring fine weather, light winds, and very cold nights (during winter). Fog will often form if there is enough moisture in the air.

Highs over the Great Australian Bight often cause showers or drizzle in the west and south, and cooler-than-normal temperatures.

As a high moves to the south of Tasmania, easterly winds develop over the state, often resulting in drizzle about the east, which can last all day.

When a summer-time high moves over the Tasman Sea, high temperatures and mostly fine conditions are the usual outcome, though there may be some light precipitation in the north and northeast.

Blocking highs are strong high-pressure systems that remain near stationary for an extended period of time.

They block the west-to-east progression of weather systems across southern Australia, and often form over the Tasman Sea.

In such a situation, north-to-northeast winds blow over Tasmania for several days and temperatures can climb steadily. Moisture will also increase, making fogs more likely and sea fog may form in the north and east. Days are mostly fine, though there may be some drizzle in the north and northeast or some isolated showers.

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May 2009

A blocking high persisted over the Tasman Sea between 19 and 24 May 2009 (Figures 17 and 18).

The weather was mostly fine – any cold fronts that approached Tasmania were forced to move south or weaken before reaching the state.

By the 22nd, air moisture had increased enough for some patchy drizzle to fall in the north. North-to-northeast winds blew over the state and temperatures slowly warmed, especially overnight.

When the high was close to Tasmania on the morning of the 19th, minimum temperatures were very cold and severe frosts formed in the midlands.

Cressy, which fell to almost -3 °C on the 19th, had a minimum of almost 9 °C on the 24th. Maximum temperatures of around 13 °C rose to near 20 °C during this period.

There was extensive fog almost every morning which, in some areas, failed to clear during the day.

 

chart showing high over Tasmania 19 May 2009

 

chart showing high crossing Tasmania 24 May 2009

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Cut-off lows

Cut-off lows are low pressure systems that break away from the main belt of low pressure that lies across the Southern Ocean.

They are associated with sustained rainfall and can produce strong, gusty winds and high seas.

If a cut-off low is slow-moving, rain may fall for extended periods and be heavy at times. Rain can fall over several days.

map showing areas affected by cut-off lows and when

 

Cut-off lows can develop over Australia, to the east of Australia or over the Great Australia Bight.

East-coast lows are a type of cut-off low that forms over the Tasman Sea to the east of Australia. They can direct warm, moist air over Tasmania, sometimes bringing over 100 mm of rain to the east coast.

The strong and gusty southeast winds and large easterly swells often associated with east-coast lows can cause widespread damage.

 

December 2008

A cut-off low passed over Victoria early on 13 December 2008 (Figure 20), then intensified near Flinders Island, bringing heavy rain and strong and gusty southeast winds to the northeast of Tasmania.

Winds gusted over 100 km/h.

Rainfall of over 100 mm was common in the northeast, but little rain fell in the west (Figure 21).

Rivers in the northeast flooded and the rain and wind caused significant damage on Flinders Island.

Strong winds and low pressure combined with high tides to blow water over some coastal roads in the east.

 

chart showing cut-off low over Tasmania 13 Dec 2008

 

map showing Tasmania’s rainfall totals for the 48 hours to 9 am on 14 Dec 2008

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Upper-level troughs and cloud bands

Though not as common as cold fronts or high pressure systems, upper-level troughs and cloud bands sometimes move over Tasmania and influence the weather.

map showing areas affected by upper-level troughs and cloud bands and when

 

The two phenomena are linked; a trough of low pressure is often (but not always) accompanied by cloud, and cloud bands can form when a trough of low pressure occurs in the upper atmosphere.

Cloud bands can also form when warm, moist tropical air originating over the Indian Ocean moves towards the pole (generally south-eastward), and is forced to rise over colder air in southern Australia. Such a cloud band is called a northwest cloud band, and these can affect Tasmania.

Upper-level troughs and cloud bands often bring rain to Tasmania. Rain may be statewide or confined to a particular area, depending on the circumstances.

When cloud covers Tasmania, sunlight is blocked during the day, leading to cooler than normal temperatures. But when the sun sets, heat is trapped near the surface so overnight temperatures are warm.

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Sources

Australian climate influences - The Bureau of Meteorology

Antarctic Oscillation Index (Southern Annular Mode) – US National Weather Service

Coughlan, M 1983, ‘A comparative climatology of blocking action in the two hemispheres’, Aust. Met. Mag., vol. 31, pp. 3–13.

Hendon, H, Thompson, D & Wheeler, M 2007 ’Australian rainfall and surface temperature variations associated with the southern annular mode’, J. Climate, vol. 20, pp. 2452–67.

McBride, J & Nicholls, N 1983, ‘Seasonal relationships between Australian rainfall and the Southern Oscillation’. Monthly Weather Review, vol. 111, pp. 1998–2004.

Saji NH, Goswami BN, Vinayachandran PN & Yamagata T 1999, ‘A dipole mode in the tropical Indian Ocean’, Nature , vol. 401, pp. 360–63.

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