New South Wales - weather and climate drivers

 

Overview

Map showing major weather and climate drivers across Australia

 

Positioned in the mid-latitudes of eastern Australia, New South Wales is affected by almost all the major weather and climate drivers of Australia at some stage through each year.

The Great Dividing Range is a major climatic barrier separating the state into distinct climate zones.

This divide runs almost north-south against the prevailing westerly winds in winter and the easterly winds in summer, resulting in more rainfall to the west of the divide in winter and to the east in summer.

The major oceanic climate drivers—ENSO, the Southern Annular Mode and the Indian Ocean Dipole—all interact over New South Wales producing a highly variable climate from year to year.

 

New South Wales’ main weather drivers are summarised in Table 1.

Table 1. Summary of New South Wales’ main climatic drivers of weather

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

frontal activity

fine and dry

winter

summer

statewide
El Niño - Southern Oscillation El Niño - less rain May - November west of the Great Dividing Range
El Niño - Southern Oscillation La Niña - more rain May - April west of the Great Dividing Range

Indian Ocean Dipole (positive)

less rain May- November west of the Great Dividing Range

Indian Ocean Dipole (negative)

more rain May- November west of the Great Dividing Range

Southern Annular Mode (positive phase)

less rain

more rain

winter

spring

southern NSW

statewide

Southern Annular Mode (negative phase)

more rain winter southern NSW

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These climatic drivers can modify synoptic features in New South Wales as summarised in Table 2.

Table 2. Summary of New South Wales’ synoptic features

Synoptic featurePotential effectWhen it is most activeWhere in NSW it has most effect
Trade winds rainfall October - March tropical and subtropical areas to the east of the Great Dividing Range
Easterly trough (or inland trough) rainfall September - May statewide
Upper-level trough rainfall all year statewide
Cut-off lows rainfall with strong, gusty winds March - October statewide
East-coast lows heavy rainfall with strong, gusty winds March - October east coast
Frontal systems heavy rainfall and strong, winds March - October southern New South Wales, particularly the south-west slopes and alpine areas
Cloud bands rainfall March - October statewide
Blocking highs variable conditions (temperature/rainfall) depending on the strength and position of the high pressure system all year statewide

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Sub-tropical ridge

The sub-tropical ridge, an extensive belt of high pressure, is a major feature of the general circulation of our atmosphere. It encircles the globe at the middle latitudes, directly affecting New South Wales.

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

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

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

At just under 2 minutes long, Ridgy is a quick, clear and fun way to learn about the sub-tropical ridge.

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El Niño – Southern Oscillation

2 maps showing areas affected by El Nino and La Nina and when

 

Sea surface temperatures in the Pacific Ocean can affect rainfall across the eastern half of Australia, including New South Wales.

The El Niño - Southern Oscillation (ENSO) is a major influence on eastern Australia’s climate including almost all of New South Wales.

ENSO is the irregular oscillation between El Niño and La Niña conditions, which describe the variations in atmospheric patterns across the Pacific Ocean, and 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 the New South Wales Department of Primary Industries (in collaboration with the Victoria Department of Primary Industries and the Bureau of Meteorology) to help farmers understand the climate processes that affect rainfall variability in New South Wales.

At 84-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.

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

Figure 3 shows the higher-than-usual sea surface temperatures in November 2002.

Figure 4 shows the below-average rainfall in New South Wales associated with the 2002–03 El Niño event.

Map showing the sea surface temperatures in November 2002

 

Map showing NSW rainfall deciles in March 2002 – January 2003

 

La Niña

La Niña is associated with extensive cooling of the sea surface in the central and eastern tropical Pacific, and is often associated with above average winter/spring rainfall over much of eastern Australia.

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

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Indian Ocean Dipole

map showing areas affected by IOD

 

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 contribute to the formation of rain-producing systems.

When the Indian Ocean Dipole 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 Indian Ocean Dipole is negative, waters are cooler than normal near Africa and warmer than normal near Australia. Warmer waters near Australia, particularly near Indonesia, bring more 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.

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

At 90-seconds long, Indy is a quick, clear and fun way to learn about the Indian Ocean Dipole.

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Southern Annular Mode

map showing areas affected by SAM

 

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

It describes a north-south shift 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.

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.

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

At 101-seconds long, Sam is a quick, clear and fun way to learn about the Southern Annular Mode.

 

Positive phase

During a positive SAM 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, particularly southern New South Wales, 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.

In winter, a positive SAM phase produces moist easterly winds on the New South Wales coast and is sometimes associated with the development of east-coast lows, producing abundant rainfall.

In June 2010 the Southern Annular Mode was in a positive phase, with the Antarctic Oscillation Index recording a value above +2 for the month. The resulting dominant easterly wind flow produced abundant rainfall on the NSW east coast with drier-than-average conditions to the west of the Great Divide (Figures 7 and 8).

 

Map showing NSW rainfall totals for 4 June 2010

 

Map showing NSW rainfall deciles for June 2010

 

Negative phase

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

In autumn and winter, this can result in stronger cold fronts and more storms passing over southern Australia, resulting in increased rainfall and strong winds.

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Trade winds

map showing areas affected by trade winds

 

Trade winds blow from an east-to-southeast direction across much of the southern hemisphere tropics.

Trade winds prevail along the north-east coast of New South Wales and extend south during summer and autumn as the sub-tropical ridge.

Collecting moisture as they move westward over the warm tropical Pacific Ocean towards the east coast of Australia, the trade winds bring rainfall to coastal tropical and sub-tropical areas.

Elevated areas on the eastern side of the Great Dividing Range in northern New South Wales and Queensland receive substantial rainfall from the forced ascent of the persistent trade winds. These areas, such as Dorrigo near Coffs Harbour, are among the wettest places in New South Wales.

The trade winds blow all year round because they are a part of the general circulation of the atmosphere. However, they are strongest in the winter months when they have the greatest affect on conditions (generally dry and stable) over inland northern Australia and the Top End.

Trade winds are at their strongest when a slow-moving high-pressure system is located in the southern Tasman Sea. These periods are most common in summer and autumn, and coastal New South Wales can be affected by a very moist south-easterly airflow with heavy rainfall.

map showing NSW average annual rainfall

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Easterly trough (or inland trough)

Inland troughs are a dominant feature of the synoptic pattern over Australia during the summer months.

map showing areas affected by easterly troughs

 

The easterly trough, sometimes known as the inland trough, is a major contributor to rainfall in eastern Australia, bringing rainfall to central and inland parts.

Rainfall can be particularly heavy when the trough interacts with other features, such as an upper-level trough approaching from the west.

The easterly trough is an area of low atmospheric pressure located on the lee side (inland side) of the Great Dividing Range. It forms a boundary between the moist air near the coast and dry air inland.

It extends through central Queensland and central New South Wales, and sometimes right down into northern Victoria.

The trough is partly formed by the intense heating of the land during the summer months, but the topography of the region also plays a role.

As the temperature rises during the day, the trough deepens and moves towards the coast, causing showers and thunderstorms to form in the unstable air.

The trough will also interact with any low-pressure troughs or cold fronts moving through southern Australia, increasing their impact.

On 3 February 2008, an easterly trough extending across central Queensland, central New South Wales and right down through Victoria (Figure 12) brought rainfall to most of New South Wales (Figure 13).

Synoptic chart showing easterly trough on 3 Feb 2008

 

Map showing rainfall totals on 4 Feb 2008

 

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Upper-level trough

An upper-level trough is a trough of low pressure which has formed in the upper levels of the atmosphere, and therefore cannot be seen on surface-level charts.

map showing areas affected by upper-level troughs

 

Upper-level troughs can occur at any time, in any region of Australia.

They generally last from between a few days to a couple of weeks and typically move from west to east.

They can result in the formation of cloud bands, which will often result in widespread rainfall near and to the east of the trough.

They can also help develop surface-level features, such as frontal systems, increasing their effect.

The presence of an upper-level trough may result in the development of a cut-off low, which in turn will increase rainfall over the affected region.

Upper-level troughs also increase rainfall from the forced ascent of trade winds in coastal New South Wales and on the eastern side of the Great Dividing Range.

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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.

map showing areas affected by cut-off lows

 

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.

A cut-off low (Figure 16) produced widespread and substantial rain across south-eastern Australia from 23 to 25 October 2000. Rainfall totals for south-west New South Wales for the week ending 25 October 2000 ranged from 50 to 100 mm (Figure 17).

 

synoptic chart showing cut-off low 24 October 2000

 

map showing rainfall totals 25 October 2000

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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.

On 7 June 2007, a small but dangerous east-coast low formed off the coast of New South Wales (Figure 19). Over the following few days it produced severe flooding in the Hunter region (Figure 20) and strong, damaging winds and high seas along the coast.

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

 

map showing rainfall totals NSW 11 June 2007

 

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

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

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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 and may be heavy at times.

Areas on the western slopes of the Great Dividing Range are most affected by frontal systems. The westerly winds associated with cold fronts are forced to rise over the ranges producing abundant rainfall, especially in winter and spring.

map showing areas affected by frontal systems

 

Often, especially in La Niña and negative Indian Ocean Dipole years, frontal systems can interact with tropical moisture to the north of Australia and produce northwest cloud bands. These moisture-laden systems can bring substantial steady rainfall to much of New South Wales over a 2 to 3-day period.

On 30 June 2009, two cold fronts moved across New South Wales (Figure 22) and resulted in significant falls at the start of July for areas southwest of Canberra, including the Snowy Mountains (Figure 23).

synoptic chart showing cold fronts in NSW 30 June 2009

 

map showing rainfall totals for NSW w/e 7 July 2009

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Cloud bands

Cloud bands bring widespread, often heavy, rainfall to New South Wales. However, in recent years, there has been a reduction in the number of cloud bands bringing rainfall to the state.

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

map showing areas affected by cloud bands

 

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

Between 16 and 20 June 2005, an extensive northwest cloud band stretched across Australia (Figure 25) bringing rainfall to New South Wales (Figure 26).

 

synoptic chart showing cloud band

 

map showing rainfall totals 29 June 2005

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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.

map showing areas affected by blocking highs

 

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

Generally, a blocking high in the Bight produces a cold spell and wet conditions in southern New South Wales, while a blocking high in the Tasman produces warmer conditions with coastal showers.

Blocking highs can also cause clear skies and cold overnight minimum temperatures, resulting in fog and frost in winter.

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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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