Victoria

Victoria's DEPI also have specially bred, local Climatedogs you can check out.

In Victoria, Ridgy is great at blocking rain-bearing fronts. He lets cold fronts through much more in winter, and brings fine and dry weather in summer.

Enso herds moisture from the Pacific Ocean towards Victoria, especially in winter and spring. When there’s an El Nino, he causes less rainfall. During La Ninas, he chases greater amounts of moist tropical air across Australia.

Indy herds moisture from the Indian Ocean, bringing drier or wetter winter-spring seasons depending on whether he’s feeling positive or negative.

Sam herds cold fronts from the southern ocean, bringing key rainfall triggers to Victoria in the cooler months.

Eastie scampers along the south-east coast of Australia, can go into action overnight, and his favourite seasons are autumn and winter. He can cause strong winds, heavy rains and lots of rough weather.

Mojo can sometimes influence rainfall in Victoria, especially if one of his moisture waves feeds into a timely weather event. He’s most active from October to April.

The roundupEnsoIndyRidgySamEastieMojo
animated dogs rounding up weather on a map of Australia animated dog called Enso animated dog called Indy animated dog called Ridgy animated dog called Sam animated dog called Eastie animated dog called Mojo

Get regular updates on Victoria's climate drivers from Agriculture Victoria's free online newsletters - The Break, The Fast Break and The Very Fast Break.


Overview

Figure 1. The major weather and climate drivers across Australia (Bureau of Meteorology, 2010)

 

Victoria's main climatic drivers are summarised in Table 1.

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

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

cold fronts

fine and dry

winter

summer

statewide
El Niño - Southern Oscillation

El Niño - less rain

La Niña - more rain

May - November most of the state, particularly in spring

Indian Ocean Dipole (positive)

less rain June - November statewide

Indian Ocean Dipole (negative)

more rain June - November statewide

Southern Annular Mode (positive phase)

more rain

less rain

spring/summer

winter

southern and eastern Victoria

mainly southern parts

Southern Annular Mode (negative phase)

more rain winter southern parts

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

Table 2. Summary of Victoria’s synoptic features

Synoptic featurePotential effectWhen it is most activeWhere in Victoria it has most effect
Frontal systems rainfall all year; more frequent in winter statewide, particularly southern half
Cut-off lows rainfall with strong, gusty winds all year statewide
Cloud bands rainfall April - September statewide
Blocking highs

generally cold and wet if the high is in the Bight

generally hot and dry if the high is in the Tasman Sea, although moisture may be increased in the east during the summer months

increased chance of cut-off lows developing (bringing rain) if the high is in the Tasman Sea during the winter months

fog and/or frost if centred over Victoria

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 cold fronts to pass over Victoria in the winter, but pushing them to the south in summer.

Ridgy the Climatedog is one of 5 animated sheepdogs (The Climatedogs) developed by Victoria’s Department of Primary Industries to help farmers understand the climate processes that affect rainfall variability in Victoria.

At 103-seconds long, Ridgy is a quick, clear and fun way to learn about the sub-tropical ridge.

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El Niño - Southern Oscillation

Figure 2. ENSO – areas affected, timing and duration

 

Sea surface temperatures in the Pacific Ocean can affect rainfall across eastern and northern Australia, including Victoria.

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

ENSO is the irregular oscillation between El Niño and La Niña conditions, which describe the variations in sea surface temperatures in the central and eastern tropical Pacific Ocean.

Enso the Climatedog is one of 5 animated sheepdogs (The Climatedogs) developed by Victoria’s Department of Primary Industries to help farmers understand the climate processes that affect rainfall variability in Victoria.

At 79-seconds long, Enso is a quick, clear and fun way to learn about the El Niño - Southern Oscillation.

The Interdecadal Pacific Oscillation is another 'piece of the puzzle' of Pacific Ocean climatology you might have heard of. It is related to warm water moving around the Pacific Ocean on time scales of 10 to 30 years - much longer than ENSO but potentially linked.

 

El Niño

El Niño is associated with extensive warming of the sea surface in the central and eastern tropical Pacific.

It is often associated with below average winter/spring rainfall over much of eastern Australia.

The El Niño event of 2002–03 seriously affected rainfall over Victoria. Rainfall was well below average across the state (Figure 3), with many areas experiencing severe water shortages and high bushfire risk.

 

Figure 3. Below average rainfall in Victoria associated with the 2002-03 El Niño event

 

La Niña

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

It is often associated with above average winter/spring and summer rainfall over much of northern and eastern Australia.

 

El Niño Modoki

‘Modoki’ is Japanese for ‘similar, but different’ - as such, El Niño Modoki is a phenomenon under scrutiny. It is not yet clear whether El Niño Modoki is a different climate driver to conventional El Niño.

El Niño Modoki is associated with strong warming of sea surface temperatures in the central tropical Pacific and cooler sea surface temperatures in eastern and western tropical Pacific waters.

(Conventional El Niño conditions are usually associated with warm sea surface temperatures in the eastern tropical Pacific.)

In northern Australia, El Niño Modoki events are associated with:

  • a delayed, but shorter and more intense monsoon season
  • increased rainfall in January and February
  • decreased rainfall in December and March.

They also appear to cause a large-scale decrease in autumn rainfall over north-western and northern Australia.

The El Niño Modoki phenomenon is thought to have become more frequent than conventional El Niños since the late 1970s, occurring in 2002, 2004 and 2009.

Researchers need to observe more years of climate data to understand if El Niño Modoki is different and, if so, how it may affect Australian rainfall differently to ENSO.

Tropical cyclones are often less frequent during El Niño events.

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Indian Ocean Dipole

Map showing areas affected by IOD and when

 

Sea surface temperatures in the Indian Ocean have an impact on the rainfall patterns over much of Australia.

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 rain-producing systems.

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

When the IOD is positive, waters are warmer than normal near Africa and cooler than normal near Australia. Cloud near Australia reduces, resulting in less rainfall.

When the IOD is negative, waters are cooler than normal near Africa and warmer than normal near Australia. Warmer waters near Australia, particularly near Indonesia, can result in more rainfall.

IOD events can be related to ENSO events.

Positive IOD events sometimes occur during El Niño events, usually resulting in less rainfall over Victoria.

Negative IOD events sometimes occur during La Niña events, usually resulting in increased rainfall over Victoria.

The IOD effect was proposed in the late 1990s and is the subject of further research. As modelling of the ocean and atmosphere improves, the ability to forecast these patterns of sea surface temperature is also improving.

Indy the Climatedog is one of 5 animated sheepdogs (The Climatedogs) developed by Victoria’s Department of Primary Industries to help farmers understand the climate processes that affect rainfall variability in Victoria.

At 87-seconds long, Indy is a quick, clear and fun way to learn about the IOD.

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Southern Annular Mode

Areas in Australia affected by the Southern Annular Mode

 

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

It describes a north-south movement in the belt of strong westerly winds across the south of the continent.

This region of strong westerly winds is associated with cold fronts and storm activity, and influences weather in southern Australia.

The mode can be in a positive or negative phase.

We can identify all SAM events by observing the pattern of westerly wind flow and pressure to the south of Australia, which is monitored by the Antarctic Oscillation Index as produced by the US National Weather Service.

Positive phase

During a positive phase, the belt of strong westerly winds contracts toward the South Pole. This causes weaker-than-normal westerly winds and higher pressure over southern Australia.

In spring and summer, a positive phase can result in increased rainfall over parts of south-eastern Australia, by strengthening the moist easterly flow from the Tasman Sea.

In autumn and winter, a positive SAM phase results in fewer storm systems and less rainfall across the southern coastal regions of Australia.

Negative phase

The negative phase is associated with a northward shift in the belt of strong westerly winds. In autumn and winter, this can cause more storms and increase rainfall for southern Australia.

During July 2007, the SAM was in a negative phase. The belt of westerly winds was expanded towards the equator, resulting in slightly stronger westerly winds over southern Australia. These winds brought more cold fronts and increased rainfall to Victoria.

Figure 6. Increased rainfall in Victoria during a negative SAM phase in July 2007

 

Sam the Climatedog is one of 5 animated sheepdogs (The Climatedogs) developed by Victoria’s Department of Primary Industries to help farmers understand the climate processes that affect rainfall variability in Victoria.

At 91-seconds long, Sam is a quick, clear and fun way to learn about the Southern Annular Mode.

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Frontal systems

Areas in Australia affected by frontal systems

 

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

More intense systems are generally associated with heavier rainfall.

If frontal systems are slower moving, rainfall may occur for extended periods resulting in higher totals. It may also be heavy at times.

A cold front moved across Victoria on 17 July 2007 (Figure 8). Heavy rainfall was recorded in parts of Victoria (Figure 9) and there were reports of extensive storm damage. Many areas reported snowfalls, with road closures in Ballan, Daylesford, Trentham, Woodend and Mount Macedon due to snow and ice.

Figure 6. Increased rainfall in Victoria during a negative SAM phase in July 2007

 

Figure 9. Rainfall in Victoria, 18 July 2007

 

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Cut-off lows

Areas in Australia affected by 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 or near-stationary, rainfall may occur for extended periods and may be heavy at times.

Cut-off lows are the dominant rain-producing synoptic systems over much of inland Victoria, contributing a higher proportion of annual rainfall than fronts, although other processes can be more important in mountainous or coastal regions.

They often occur in conjunction with a blocking high.

Cut-off low, 24 October 2000

 Figure 12. Rainfall in Victoria, 23–25 October 2000: The Murray River around Albury-Wodonga experienced moderate flooding after the Hume Dam spilled.

 

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East-coast lows

East-coast lows are generally associated with strong and gusty winds, sustained heavy rainfall and high seas. They can cause widespread damage over a very short period of time.

East-coast lows are intense low-pressure systems which occur on average several times each year off the eastern coast of Australia.

They usually affect New South Wales, southern Queensland and eastern Victoria.

They are a type of cut-off low.

 map showing areas affected by east-coast lows

 

East-coast lows will often rapidly intensify overnight, making them one of the more dangerous weather systems to affect the south-east coast of Australia.

They can form at any time of year but are most common during autumn and winter, with a maximum frequency in June (Figure 18).

Individual east-coast lows generally only last for a few days.

Between 27-30 June 2007, a small but dangerous east-coast low formed off the coast of New South Wales (Figure 19). Over the four days, over 300mm were recorded in East Gippsland as a number of stations set records for the highest June daily rainfall. There was major flooding on several rivers with some flood peaks the highest on record. For details see the June climate summary for Victoria.

synoptic chart showing east-coast low, 8-9 June 2007

 

Eastie the Climatedog is one of 5 animated sheepdogs (The Climatedogs) developed by the Victoria Department of Primary Industries (in collaboration with the New South Wales Department of Primary Industries and the Bureau of Meteorology) to help farmers understand the climate processes that affect rainfall variability in Victoria.

At 101-seconds long, Eastie is a quick, clear and fun way to learn about the east-coast lows.

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Cloud bands

Areas in Australia affected by cloud bands

 

A cloud band is an extensive layer of cloud that can stretch across Australia, often from north-west to south-east.

Cloud bands can form when:

  • a trough of low pressure occurs in the upper atmosphere
  • 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

Cloud bands can be associated with widespread, often heavy, rainfall in Victoria. However, since 1997, there has been a marked reduction in the number of northwest cloud bands bringing rainfall to Victoria.

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Blocking highs

Areas in Australia affected by blocking highs

 

Blocking highs are strong high-pressure systems that form further south than the usual position of the sub-tropical ridge, and remain near-stationary for an extended period of time.

They block the west-to-east progression of weather systems across southern Australia, and are usually formed in the Great Australian Bight or the Tasman Sea.

A blocking high’s impact on the weather varies depending on its location, the time of year and the systems around it.

Generally, a blocking high in the Bight produces a cold spell and wet conditions in Victoria, while a blocking high in the Tasman often results in a hot spell and dry conditions, especially if this occurs in summer.

When blocking highs are associated with cut-off lows to their north, widespread rain frequently occurs.

Blocking highs can also contribute to fog and frost occurrence.

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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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