Western Australia

In Western Australia, Ridgy is great at blocking rain-bearing fronts. He lets cold fronts bearing rain through much more in winter in the south, and brings fine and dry weather in summer. When Ridgy heads north, he affects when the monsoon starts.

Indy herds moisture from the Indian Ocean to some parts of the state, bringing drier or wetter years depending on whether he’s feeling positive or negative.

Sam loves to play in the south-west – herding up cold fronts from the southern ocean, affecting rainfall in autumn and winter, and sometimes spring, bringing rainfall triggers to southern WA.

Enso herds moisture from the Pacific Ocean which can sometimes affect northern parts of WA.. When there’s an El Nino, he causes less rainfall and fewer tropical cyclones. During La Ninas, he chases greater amounts of moist tropical air across Australia and causes more tropical cyclones.

Mojo mainly affects northern Australia, and sends a wave of weather across the Indian Ocean which can create cyclones and bring widespread rain. He’s most active from October to April.

The roundup	Enso	Indy	Ridgy	Sam	Mojo

• Overview
• Sub-tropical ridge
• The monsoon
• Indian Ocean Dipole
• Southern Annular Mode
• El Niño – Southern Oscillation
• Tropical influences (cyclones and depressions)
• Cut-off lows
• West-coast trough
• Blocking highs
• Cloud bands
• Frontal systems

 

Overview

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

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

Climate driver	Potential effect	When it is most active	Where in Western Australia it has most effect
Sub-tropical ridge	frontal activity monsoon onset fine and dry	winter summer summer	southern parts northern parts southern parts
The monsoon	rainfall, often heavy	October - April	tropical areas
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	autumn/winter spring	south-west south-central
Southern Annular Mode (negative phase)	more rain	autumn/winter	south-west
El Niño - Southern Oscillation	El Niño - less rainfall, fewer tropical cyclones La Niña - more rainfall, more tropical cyclones	May - April	mainly northern parts

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

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Table 2. Summary of Western Australia’s synoptic features

Synoptic feature	Potential effect	When it is most active	Where in Western Australia it has most effect
Tropical influences (cyclones and depressions)	heavy rainfall, destructive winds, damaging storm surges (more severe for cyclones than depressions)	October - April (cyclone season is November - April)	northern and central parts of the state; occasionally systems may drift further south
Cut-off lows	rainfall with strong gusty winds	all year	the southern half of the state
West-coast trough	west of the trough - mild with sea breezes east of the trough - hot with thunderstorms	September - May	western parts
Blocking highs	hot and dry conditions if the high is in the Bight   drizzle and showers if the high is south of the Bight increased chance of a cut-off low over southern parts if the high is south of the Bight	winter   summer   all year	southern parts   south coastal regions   southern parts
Cloud bands	rainfall	April - September	statewide
Frontal systems	rainfall	all year, more frequent in winter	southern parts

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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 encircles the globe at the middle latitudes.

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

The movement of the ridge also allows the monsoon to develop in far northern Western Australia.

Conditions along the ridge tend to be stable and dry because of descending air in the high-pressure systems.

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The monsoon

The term ‘monsoon’ is used to describe the seasonal reversal of winds that occurs over parts of the tropics.

As the Australian summer approaches, the continent heats up. Low pressure is created, which effectively draws the monsoon trough—a zone of low pressure and rising air—over northern Australia.

This trough draws in moist air from the surrounding oceans and we refer to this influx of moist air as the monsoon. The monsoon can be in either an ‘active’ or an ‘inactive’ phase.

The onset, length and severity of the monsoon phases are influenced by the Madden-Julian Oscillation, which is one of the Northern Territory’s climatic drivers.

 

Active phase monsoon

The active phase is usually associated with broad areas of cloud and rain, with sustained moderate to fresh north-westerly winds on the north side of the trough.

Widespread heavy rainfall can occur during active phases.

 

Inactive phase monsoon

An inactive phase or ’break’ period occurs when the monsoon trough temporarily weakens or retreats north of Australia.

It is characterised by light winds and isolated shower and thunderstorm activity, sometimes with gusty squall lines.

A squall line is a long line of thunderstorm cells, sometimes several hundred kilometres in extent.

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Indian Ocean Dipole

 

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

The Indian Ocean Dipole is a 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. These changes in sea surface temperature can contribute to the formation of rain-producing systems.

A positive Indian Ocean Dipole is seen when waters are warmer than normal near Africa, and cooler than normal near Australia. Cloud near Australia reduces, resulting in less rainfall.

A negative Indian Ocean Dipole is seen when waters are cooler than normal near Africa, and warmer than normal near Australia. Warmer waters near Australia, particularly near Indonesia, enhance cloud formation and increase rainfall.

The Indian Ocean Dipole 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 SAM 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 heavily influences weather in southern Australia.

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

The mode can be in a positive or negative phase.

 

Positive phase

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

In spring, a positive phase can result in increased rainfall over parts of south-western Australia.

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.

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El Niño - Southern Oscillation

 

Sea surface temperatures in the Pacific Ocean can affect rainfall across most of Australia at certain times of the year.

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

 

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/summer rainfall over northern Western Australia.

Daytime temperatures are normally cooler during La Niña events.

Tropical cyclones are often more frequent during La Niña events.

 

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 have very little impact on Western Australia.

Daytime temperatures are normally warmer during El Niño events.

Tropical cyclones are often less frequent during El Niño events.

 

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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Tropical influences (cyclones and depressions)

 

The southern hemisphere’s wet season runs from October to April. The cyclone season runs from November to April.

During the wet season, tropical systems affect mainly northern and central parts of Western Australia but can occasionally also affect southern Western Australia.

Tropical systems include:

• tropical cyclones
• tropical depressions

Tropical cyclones are very intense low-pressure systems that produce heavy rainfall, destructive winds and damaging storm surges.

Tropical depressions are moderate strength low-pressure systems, often associated with the monsoon trough. They may develop into tropical cyclones if they are in a favourable location. They often produce significant rainfall.

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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, often heavy, rainfall, and can produce strong, gusty winds and high seas.

 

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West-coast trough

 

During the warmer months, the west-coast trough is a semi-permanent feature of the synoptic pressure pattern near the west coast of Australia. It is the dominant influence on west-coast weather at that time of the year.

Depending on the location of the trough, areas to the east of it can experience hot days, with temperatures above 40°C, and the possibility of thunderstorms, given sufficient atmospheric moisture.

To the west of the trough, milder conditions with sea breezes generally prevail.

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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.

Blocking highs have a wide range of impacts depending on their location and strength.

They can produce a hot spell, a cold spell, dry conditions or wet conditions depending on their location and the systems around them.

Blocking highs in the Great Australian Bight are often responsible for extended dry weather in south-west Western Australia, and can be associated with heatwaves.

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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.

They 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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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 slow-moving, rainfall may occur for extended periods and heavy falls are possible.

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