Climate of The Northern Territory

The Northern Territory's climate is distinctly different from that of southern Australia, and varies greatly between the Territory's northern part, known as the 'Top End' and the southern extremities.

Physical features

Four-fifths of the Territory lies north of the Tropic of Capricorn. The country within about 150 km of the coast is mainly flat or undulating up to about 200 m elevation, with extensive coastal swamps or wetlands in some parts. The interior of the 'Top End' is dominated by the rocky Arnhem Land plateau. To the southwest of the plateau lie the rugged hills of the southern Katherine region, while in the east the land generally rises more gently through the hilly country of the southern Roper-McArthur District to the grassy plains of the Barkly Tableland. These systems of hills divide the coastal river drainage systems from the broad but shallow inland basin, where streams are usually dry for most of the year. South again the land rises very gradually; western areas are dominated by sandy desert. Toward central Australia, the land rises more steeply into a higher plateau and rocky ranges, where a number of peaks exceed 1500 m elevation. The plateau declines steeply toward the sand dunes of the Simpson Desert in the southeast whilst the Lake Amadeus trough separates it from the lower ranges of the far southwest.

Major circulation systems

Two major atmospheric pressure systems affect Territory latitudes: the subtropical ridge of high pressure cells (highs or anticyclones), and a broad tropical low pressure region called the monsoon trough. The subtropical highs move from west to east across southern Australia in winter, and further south in summer, usually separated by low pressure troughs or cold fronts. The highs provide the driving force behind the southeast trade winds which dominate the Territory's weather in the winter months. The monsoon trough is a broad area of low atmospheric pressure running east-west through the tropics in the summer months. It follows the sun, shifting north and south between the hemispheres with the seasons. In the southern hemisphere it is the meeting place of the dry east to southeast winds generated by the subtropical highs, and the moisture-laden northwesterly monsoon winds. During the summer it lies for lengthy periods over north Australia, and is the source of much rainfall. The northern and, to a lesser extent, central parts of the Territory experience two distinct seasons: the 'wet' (October to April) and the 'dry' (May to September). The change between seasons is usually gradual, with transition months of October and November (often called the 'buildup') at the start of the wet, and April at the end. In central parts the contrast between wet and dry is not generally as marked as in the north. The Alice Springs district is dry for much of the year, and has an erratic rainfall pattern, with a slight summer maximum. While zero rainfall can be experienced in all calendar months, significant totals are also possible in all months, but are more likely in summer. Winters (June to August) are cool and summers (December to February) hot; the terms 'spring' and 'autumn' are not usually applied to the transition seasons in between. During the wet season, weather in the north is largely determined by the position of the monsoon trough, which can be in either an 'active' or an 'inactive' phase.

Fig. 1 Typical wind systems at different times of the year

The active phase is usually associated with broad areas of cloud and rain, with sustained moderate to fresh northwesterly winds on the north side of the trough. Widespread heavy rainfall can result if the trough is close to, or over land
(see Fig. l(a)).

An inactive or 'break' period occurs when the monsoon trough temporarily weakens or retreats north of Australia; it is characterised by light winds, isolated shower and thunderstorm activity, sometimes with gusty squall lines
(see Fig. 1(b)).

A typical wet season consists of a prolonged inactive period during the buildup, followed by two or three active/inactive cycles, each full cycle lasting from about four to eight weeks. Inactive periods are usually longer than active ones. Tropical cyclones can develop off the coast in the wet season, usually forming within an active monsoon trough. Heavy rain and high winds, sometimes of destructive strength, can be experienced along the coast within several hundred kilometres of the centre of a cyclone. In the southern and central parts, weather is more variable from October to April than in the north. Sometimes decaying tropical cyclones or the monsoon trough move well south into the central regions, bringing widespread rain and thunderstorms. In general, though, east to southeasterly winds and fine conditions predominate. Temperatures can be scorching, and dust devils, whirling dust pillars raised by columns of rising hot air, are frequently seen. During October to November bushfires are fairly common - usually ignited by lightning from dry, gusty thunderstorms. From May to September the prevailing southeasterlies bring predominantly fine conditions throughout the Territory
(see Fig. l(c)).

Rainfall in the north is low to nonexistent in most areas, although light showers are common about the northeast coast and occasionally develop elsewhere over the northern Top End. Controlled burnoff and uncontrolled human initiated bushfires are widespread in the north during this season, particularly in later months, fuelled by the abundant wet season growth that has been dried by the prevailing southeasterlies. Cold fronts between subtropical high cells frequently move across the Alice Springs district. This is particularly in the winter months, when they may also occasionally reach the Top End as a wind shift, separating the moist coastal air mass from the dry inland air mass. Winds before a front tend to be warm to hot and, in summer, humid. The front's passage may be marked by thunderstorms or, if rainfall has been very low for a prolonged period, a wall of dust. The cool southeasterlies which follow the front generally clear the sky of cloud and are often very dry. Large areas of cloud, known as 'northwest cloudbands' occasionally blow across Australia from the tropical Indian Ocean. These can bring widespread rain, and sometimes storms, to southern parts, especially if a cold front moves into the area and enhances the cloudband.