Victoria - weather and climate drivers

• Overview
• Sub-tropical ridge
• El Niño - Southern Oscillation
• Indian Ocean Dipole
• Southern Annular Mode
• Frontal systems
• Cut-off lows
• Cloud bands
• Blocking highs

Overview

 

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

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

Climatic driver	Potential effect	When it is most active	Where 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 feature	Potential effect	When it is most active	Where 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

 

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 4 animated sheepdogs 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

 

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 4 animated sheepdogs 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.

 

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.

 

 

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.

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Indian Ocean Dipole

 

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 4 animated sheepdogs 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

 

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.

 

Sam the Climatedog is one of 4 animated sheepdogs 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

 

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.

 

 

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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 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.

 

 

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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

 

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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