Northern Territory

The Northern Territory experiences tropical climates in its north, through to a semi-arid or desert climate in the south.

Ridgy patrols our pressure pattern. In winter in the far south, he lets through cold, wet fronts, and in the north lets trade winds keep things fine and dry. In summer in the north, Ridgy helps bring on the monsoon, and leaves the south fine and dry.

Enso herds moisture from the Pacific Ocean towards the NT. 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 loves to mess around in tropical areas – he can influence when the monsoon starts and how long it’s active, and tropical cyclones – and is most active from October to April.

Indy herds moisture across from the Indian Ocean to parts of the NT, bringing more or less rain depending on whether he’s feeling positive or negative.

The roundupEnsoIndyRidgyMojo
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 Mojo

 

Overview

 The major weather and climate drivers across Australia

 

The climate of the Northern Territory (NT) is distinctly different from that of southern Australia.

The NT is so large that its climate varies greatly between its northern and southern extremities, ranging from a tropical climate in the north through to a semi-arid or desert climate in the south.

The northern and, to a lesser extent, central parts of the NT experience two distinct seasons:

  • the 'wet' (October to April)
  • the 'dry' (May to September)

The southern part of the NT experiences all four conventional seasons;

  • summer
  • autumn
  • winter
  • spring

The two main atmospheric pressure systems that drive the NT’s climate are the sub-tropical ridge and the monsoon trough.

The NT’s main climatic drivers are summarised in Table 1.

Table 1. Summary of the Northern Territory’s main climatic drivers of weather

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

frontal activity

fine and dry, trade winds

fine and dry

monsoon onset

winter

winter

summer

summer

far southern parts

northern parts

southern parts

northern parts

The monsoon / monsoon trough rainfall, often heavy November - March tropical areas
El Niño – Southern Oscillation

El Niño - less rain, fewer tropical cyclones 

La Niña - more rain, more tropical cyclones

May - April territory-wide
Madden-Julian Oscillation

influences the timing of the onset of the monsoon

monsoon transition from active to inactive phase

cyclones

October - April tropical areas

Indian Ocean Dipole (positive)

less rain May - November southern parts of the NT and the Top End
Indian Ocean Dipole (negative) more rain May - November southern parts of the NT and the Top End

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

Table 2. Summary of the Northern Territory’s synoptic features

Synoptic featurePotential effectWhen it is activeWhere in the NT it has most effect
Tropical systems (cyclones and depressions) heavy rainfall, destructive winds, damaging storm surges (more severe for cyclones than depressions) November - April tropical areas
Trade winds less rainfall and drier conditions all year, but more so in April - September territory-wide
Cloud bands rainfall April - September central and southern parts of the NT
Frontal changes rainfall, cool southerly changes all year, but more so in winter southern parts of the NT

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Sub-tropical ridge

The sub-tropical ridge is an extensive area of high pressure that lies across southern Australia in winter, and further south in summer.

It is the driving force behind the southeast trade winds which dominate much of the NT’s weather, especially in the winter months, bringing drier conditions.

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The monsoon / monsoon trough

The monsoon affects northern Australia from November to March, bringing cloud and heavy rainfall to the north of the NT.

map showing areas affected by the monsoon

 

The term monsoon is derived from the Arabic word ‘mausam’, meaning season. It is used to describe the seasonal reversal of winds that occurs over parts of the tropics.

In northern Australia, the dry season is characterised by southeast trade winds, while the wet season is characterised by northwest monsoonal winds.

The monsoon trough is where the dry east-to-southeast trade winds meet the moisture-laden northwest monsoon winds.

The trough is a broad area of low atmospheric pressure which follows the sun, shifting north and south between the hemispheres with the seasons.

The movement of this trough is relatively small over ocean areas but, with the sun overhead during summer, the Australian landmass heats up, creating a broad area of low pressure over Australia.

This area of low pressure helps to draw the monsoon trough, a zone of low pressure and rising air, further south, over the north of the country.

The inflow of moist west-to-northwest winds into the monsoon trough is referred to as the monsoon.

The monsoon can be in either an active or an inactive phase. The transition from active to inactive may be associated with the Madden-Julian Oscillation.

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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 monsoon trough.

Widespread heavy rainfall can result if the trough is close to, or over, land.

map showing monsoon trough across the north of Australia

 

satellite image showing cloud over the north of Australia 

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Inactive phase monsoon

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

map showing monsoon trough well north of Australia

 

It is characterised by light southeast winds and a return to more 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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Monsoon depressions

The monsoon trough frequently spawns individual low-pressure systems, sometimes called monsoon depressions.

These low-pressure systems often move from east to west across northern Australia, producing heavy rain and flooding. Sometimes they develop into tropical cyclones when they are over water.

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El Niño - Southern Oscillation

The El Niño - Southern Oscillation (ENSO) is a major influence on the climate of Australia, though its effect is less marked over the NT than for eastern Australia.

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

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

 

El Niño

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

This warming causes changes to the atmospheric circulation pattern, which affects rainfall patterns in Australia.

El Niño is often associated with below average winter/spring rainfall over much of northern and eastern Australia.

Of most significance for the Northern Territory, an El Niño is often associated with a late onset and shortened duration of the monsoon in the tropics, and a slower start to the cyclone season.

Tropical cyclones are often less frequent during El Niño events.

El Niño events usually start to develop in autumn and early winter, they peak towards the end of the year, and then start to decay during summer.

Autumn is usually a transition period when the Pacific switches to neutral or to La Niña conditions.

In 2002 a strong El Niño influenced rainfall across Australia. Vast areas of the NT received below-average rainfall.

 Map showing the sea surface temperatures in November 2002

map showing NT rainfall deciles in March 2002 – January 2003

 

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La Niña

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

This cooling causes changes to the atmospheric circulation pattern, which affects rainfall patterns in Australia.

Higher-than-average winter/spring/summer rainfall is often recorded over much of northern and eastern Australia during a La Niña.

Tropical cyclones are often more frequent during La Niña events.

A series of successive La Niña events affected Australia for 35 months from May 1998 to March 2001. During that time, 99% of the NT received rainfall in the highest decile (very much above average).

Successive La Niña events are uncommon.

Usually, a La Niña event, like an El Niño, starts to develop in autumn and early winter, peaks towards the end of the year and then decays during the summer and autumn of the following year.

 

 Map showing NT rainfall deciles in 1998-2001

 

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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Madden-Julian Oscillation

The Madden-Julian Oscillation (MJO) is a large, slow-moving area of rainfall and cloudiness that travels eastwards around the globe along the equator.

It generally takes about 30–60 days to travel around the globe.

map showing areas affected by the MJO

 

The MJO influences the timing of the onset of the monsoon and can also influence the transitions from active to inactive monsoon phases.

Tropical cyclones are more likely to develop when the MJO is active in Australian regions.

The MJO can be hard to detect because its signal is not always present in the tropical atmosphere.

MOJO 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.

At just under 2 minutes long, MOJO is a quick, clear and fun way to learn about the Madden-Julian Oscillation.

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Indian Ocean Dipole

map showing areas affected by the IOD

 

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 cloud bands.

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

The IOD is positive when waters are warmer than normal near Africa, and cooler than normal near Australia. Cloud near Australia reduces, resulting in less rainfall than usual over parts of central and southern Australia.

The IOD is negative when waters are cooler than normal near Africa, and warmer than normal near Australia. Warmer waters near Australia, particularly near Indonesia, result in more rainfall over much of Australia.

The IOD is a winter and spring phenomenon. It is inactive during summer and autumn when the monsoon trough is in the southern hemisphere.

It is related to the El Niño - Southern Oscillation in that positive IOD events often occur with El Niño events and negative IOD events often occur with La Niña events.

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Tropical systems (cyclones and depressions)

The wet season runs from October to April, and the cyclone season from November to April.

During this period, tropical systems affect the tropical areas of the NT.

Tropical systems include tropical cyclones and tropical depressions.

 

map showing areas affected by the tropical cyclones and other tropical influences

 

Tropical cyclones

Tropical cyclones are very intense low-pressure systems with mean winds greater than 34 knots (62 km/h).

They produce heavy rainfall, destructive winds and damaging storm surges.

On average, 2–3 cyclones form in the waters surrounding the NT each season. However, cyclone numbers have ranged from zero to as many as five in a season.

In April 2006, Severe Tropical Cyclone Monica moved into the NT after crossing the Queensland coast south of Lockhart River as a Category 3 cyclone.

Monica affected the small islands north of the Arnhem Land coast as a Category 5 cyclone, before finally making landfall on the northwest Arnhem Land coast, just 35 km west of Maningrida.

Widespread tree damage and moderate damage to infrastructure was reported along the Arnhem Land coast.

Maningrida community received substantial damage as the cyclone passed just to the north of the township, with several houses damaged by fallen trees.

The uninhabited coastal crossing point, just 35 km west of Maningrida, suffered severe vegetation damage, with 50–70% of all trees felled.

There was evidence of a 5–6 metre storm surge in Junction Bay.

Some of the heaviest rainfall occurred long after Monica made landfall.

Ex-Tropical Cyclone Monica moved southwards through the Top End and Victoria River District, producing heavy rainfall and causing major flooding in the Adelaide River catchment, and moderate flooding in the Daly, Katherine and Victoria River catchments.

 

map showing tropical cyclone Monica’s track

 

satellite image of tropical cyclone Monica

 

Tropical depressions

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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Trade winds

map showing direction of trade winds

 

The trade winds are the broad streams of air flowing toward the equator from high-pressure systems within the sub-tropical ridge.

In the southern hemisphere, they are south-easterly winds.

map showing areas affected by trade winds

 

Trade winds tend to be strongest in winter when high-pressure systems are more intense (April to September).

When these high-pressure systems are located over southern parts of the continent in the dry season, they direct a cool south-easterly wind towards northern Australia.

By the time this air has reached northern Australia it has been warmed and dried by the long overland trip.

If the high is intense, the winds may be quite strong.

synoptic chart showing flow of trade winds 

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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.

showing areas affected by cloud bands

 

Cloud bands can form when:

  • a trough of low pressure occurs in the lower and 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

Cloud bands can bring widespread rain, and sometimes storms, to southern and central parts of the NT, especially if a cold front moves into the area and makes the cloud band more active.

Sometimes, however, the atmosphere over the NT is too dry and the rain evaporates before it reaches the ground.

A cloud band crossed central Australia (Figure 19) in early July 2005, contributing to well-above-average rainfall in the southern NT for that month (Figure 20).

 satellite image of cloudband crossing Australia, 7 July 2005

 

map showing rainfall totals for July 2005

 

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Frontal changes

Frontal systems, such as cold fronts, generally move from west to east across the Southern Ocean and vary in their intensity and speed. They are often referred to as troughs or wind changes.

Frontal changes affect the southern NT more often in winter.

A frontal change is the term used to describe the change in the weather associated with the passing of a frontal system. Often this is the arrival of a band of rain, but it can also just be a change in the direction of the wind.

The passage of the front may be marked by thunderstorms or, if rainfall has been very low for a prolonged period, a wall of dust.

Cool south-easterly winds usually follow the front.

More intense systems are generally associated with heavy rainfall.

map showing areas affected by frontal changes

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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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