South Australia

In South Australia, 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.

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

Sam herds cold fronts and weather events from the Southern Ocean, mainly affecting southern areas during the cooler months of the year.

Enso herds moisture from the Pacific Ocean. During El Nino he tends to send less moisture, during the cooler seasons. But in La Nina, he can affect the state by chasing greater amounts of moist tropical air across Australia.

Mojo can sometimes influence rainfall in SA, especially if one of his moisture waves feeds into a timely weather event.

The roundup	Enso	Indy	Ridgy	Sam	Mojo

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

 

Overview

 

South Australia's main climatic drivers are summarised in Table 1.

Table 1. Summary of South Australia’s main climatic drivers of weather

Climatic driver	Potential effect	When it is most active	Where in South Australia it has most effect
Sub-tropical ridge	cold fronts fine and dry	winter summer	statewide
Indian Ocean Dipole (positive)	less rain	June - November	statewide
Indian Ocean Dipole (negative)	more rain	June - November	statewide
Southern Annular Mode (positive phase)	less rain more rain	winter  spring/summer	mainly the coastal fringe of south-eastern South Australia
El Niño - Southern Oscillation	El Niño - less rainfall La Niña - more rainfall	May - November	mainly south-eastern parts of the state   most of the state
Southern Annular Mode (negative phase)	more rain	winter	mainly the coastal fringe of south-eastern South Australia

These climatic drivers can modify South Australia’s synoptic features as summarised in Table 2.

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Table 2. Summary of South Australia’s synoptic features

Synoptic feature	Potential effect	When it is most active	Where in South Australia it has most effect
Frontal systems	rainfall	all year, but more frequent in winter	statewide
Cut-off lows	rainfall with strong gusty winds	all year	statewide
Blocking highs	hot and dry conditions if the high is in the Tasman Sea increased chance of cut-off lows if the high is south of the Bight fog and frost if the high is centred near South Australia	all year     winter/spring	statewide
Cloud bands	rainfall	April - September	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.

The position of the ridge varies with the seasons, allowing cold fronts to pass over South Australia in the winter, but pushing them to the south in summer.

 

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Indian Ocean Dipole

 

 

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

The Indian Ocean Dipole (IOD) is the measure of changes in sea surface temperature patterns in the northern Indian Ocean.

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

A positive IOD is seen when waters are warmer than normal near Africa, and cooler than normal near Australia. This usually results in less rainfall than average in South Australia.

A negative IOD is associated with warmer waters off north-west Australia, and usually results in increased rainfall over South Australia.

These patterns vary over periods of weeks to months.

IOD events can be related to ENSO events. Positive IOD events sometimes occur during El Niño events – usually resulting in less rainfall over South Australia. Conversely, negative IOD events sometimes occur during La Niña events – usually resulting in increased rainfall over South Australia.

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.

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Southern Annular Mode

 

 

The Southern Annular Mode (SAM) can affect rainfall in southern Australia. The effect is strongest along the coastal fringe of south-eastern parts of South Australia.

The SAM describes a north-south movement in the belt of strong westerly winds across the south of the continent that varies over periods of weeks or months.

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

The mode can be in a positive or negative phase.

We can identify a SAM event 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 towards the South Pole. This causes weaker-than-normal westerly winds and higher pressure over southern Australia.

Winter rainfall may be reduced.

Spring/summer rainfall may be increased.

 

Negative phase

The negative phase is associated with a northward shift in the belt of strong westerly winds.

In winter, this can cause more storms and increase rainfall for southern Australia.

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El Niño - Southern Oscillation

 

 

Sea surface temperatures in the Pacific Ocean can affect rainfall right across Australia, but their most direct link is to rainfall in the eastern half of Australia.

The El Niño - Southern Oscillation (ENSO) is a major influence on Australia’s climate, though its effect is less marked over much of South Australia than for areas further east.

ENSO is the irregular oscillation between El Niño and La Niña conditions, interspersed with neutral periods.

These events are triggered by variations in sea surface temperature in the central and eastern tropical Pacific Ocean.

 

El Niño

El Niño is associated with extensive warming of sea surface temperatures in the central and eastern tropical Pacific, and, usually, cooling around northern Australia.

These changes are normally associated with lower than average winter/spring/summer rainfall over much of eastern Australia.

Daytime temperatures are normally warmer during El Niño events.

 

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 average winter/spring rainfall over much of eastern Australia.

Daytime temperatures are normally cooler in La Niña events, though there are some indications that hot spells in South Australia can last longer.

 

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.

 

Not all El Niño and La Niña years are the same

The start and finish times for ENSO events can vary, as can the exact patterns of sea surface temperatures around Australia.

This results in varying rainfall for South Australia, especially when combined with the influence of other weather and climate drivers.

In 2002, we saw a strong El Niño influence on rainfall across Australia, resulting in record low rainfall across much of South Australia’s pastoral areas (Figure 5).

This may have been moderated to some extent in South Australia’s agricultural area by a strongly negative Southern Annular Mode, the likely cause of the high rainfall in western Tasmania.

 

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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. Rainfall may also be heavy at times.

The intensity and track of cold fronts is affected by broader scale influences. A period with a stronger sub-tropical ridge or positive Southern Annular Mode can cause frontal systems to track further south and have less effect.

 

 

 

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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.

Each event may last several days, but the heavy rainfall can make or break a season.

Storm surges along the South Australian coastline are often associated with prolonged strong west-to-southwest winds associated with cut-off lows, as occurred in late October 2007.

Recent research for western Victoria indicates that cut-off lows produce, on average, 50% of growing-season rainfall. Since the early 1990s the number of cut-off lows has declined. These research results also apply to South Australia.

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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 and the systems around it.

A blocking high can produce hot and dry conditions for South Australia, such as the heatwave of March 2008 (Figures 10 and 11).

 

Blocking highs can also contribute to fog and frost occurrence, due to the lighter winds and stable conditions associated with the high-pressure system.

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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 bring sustained rainfall.

Cloud bands can form when:

• a trough of low pressure occurs in the upper levels of the atmosphere, or
• 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

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