Climate of the tropical savannas

Townsville during a flash flood in January 1998 Photo: Greg Calvert

Australia features a wide range of climatic zones, from the tropical regions of the north, through the arid expanses of the interior, to the temperate regions of the south. Australia is the world's second-driest continent (after Antarctica). Eighty per cent of Australia receives less than 600 mm of rainfall per year, while 50 per cent receives fewer than 300 mm.

The tropical savannas region lies in a tropical climatic zone (Bureau of Meteorology 1989) where temperatures are warm throughout the year and there are two distinct seasons: the 'wet' and the 'dry'. The 'wet' season lasts from approximately November through to April. Almost all of the year’s rain falls during this period and generally arrives in heavy bursts from thunderstorms, widespread monsoon depressions or from the passage of associated tropical cyclones (Gentilli 1972). This time of the year is hot and humid.

The dry season, from May through to October exhibits dry, cool weather with little rain, low humidity and wider-ranging temperatures.

Across the savannas the more stable dry season is characterised by prevailing dry south-east winds (moister on the east coast), cooler temperatures, greater temperature variation during the day, clear skies and low humidity (Gentilli 1972). Light rains may fall, particularly in the south-east portions, during the early winter months but can occur in any month. Occasionally cold air from the south penetrates well into the tropics (Gentilli 1972) and night-time temperatures may drop enough for frosts to form in some inland regions in the east and west. As the dry progresses through August and September the temperatures begin to rise as the sun moves more directly overhead and the south-easterly winds begin to subside (Gentilli 1972).

The period before the wet season begins is known as the 'build-up'. This time of the year, usually between August and November can be quite testing for everyone living in the tropical savannas! The humidity increases as the skies become overcast with rain clouds and often spectacular thunderstorms with plenty of lightening, which may go on for months with little or no rain. Everyone anticipates the relief that the first rains bring at this time of the year.

Aboriginal seasons

Many Aboriginal people living in the tropical savannas region refer to six seasons, not just the 'wet' and the 'dry'. In the Kakadu region Bininj/Mungguy (local Aboriginal people) referred to these seasons as:

• Gunumeleng: Pre-Monsoon Storm Season
• Gudjewg: Monsoon Season
• Banggerreng:  Knock 'em down storm Season
• Yegge: Cooler but still humid Season
• Wurrgeng: Cold Weather Season
• Gurrung: Hot Dry Weather

These six seasons are very similar to the Western Arnhem Land seasonal calendar.

Climatic variability

Due of their extent, the tropical savannas of northern Australia cover around 1900 km ² across the top of the continent, there is climatic variability within the region. Different areas are wetter and drier at different times of the year. The latitude (distance from the equator), topography (whether mountainous or flat) and distance from the coast can play major roles in determining the extent of this variability.

During summer a band of low pressure systems called the Intertropical Convergence Zone (ITCZ)stretches across northern Australia (Tapper & Hurry 1993) determining the direction and source of prevailing winds blowing across the tropical savannas. The majority of the savannas, but particularly the northern sectors, are influenced by warm, moist, monsoonal north-westerlies. These winds, originating in equatorial regions, bring prolific rains to northern coastal areas. The annual average rainfall gradually diminishes, from more than 2400 mm to less than 800 mm (Bureau of Meteorology 1989), with distance inland and to the south.

The tropical savannas region of northern Australia experiences many of nature’s more extreme phenomena; including droughts, floods, tropical cyclones, severe storms and bushfires.

Tropical cyclones

Tropical cyclones are intense low pressure systems which generate strong winds and heavy downpours. Cyclones are relatively common in the tropical savannas region, and are the cause of much of the regions annual rainfall. The most susceptible areas of the tropical savannas are north of Carnarvon on the west coast and north of Rockhampton on the east.

Cyclones can cause great destruction to homes and other infrastructure as well as natural ecosystems including forests and wetlands. Darwin was almost completely destroyed by Tropical Cyclone Tracey on 25 December 1974.

Tropical cyclones begin forming close to the Equator, over water that is warmer than approximately 26°C. They can vary in size and strength significantly and once formed are classified as category 1 (weakest) to 5 (strongest). Category 4 and 5 cyclones have wind gusts exceeding 225 km/hr. The central core of the cyclone, where the most destructive winds are, is normally quite narrow, only about 50 km wide in the case of Tracey, and rarely more than 300 km.

Warm water acts as the cyclone’s energy source. As a result, tropical cyclones rapidly lose their strength as they move over land, but can regain strength after moving back over water. Rainfall continues to fall well after the cyclone has moved on, occasionally bringing heavy rains deep into the inland and causing widespread flooding.

Floods

Flooding in the tropical savannas occurs mainly in the in the north and eastern coastal area. There are different types of flooding that can occur including;

• flash floods, which are generally localised and often emanate from severe thunderstorms,
• short-lived floods lasting a few days that occur in shorter coastal streams, and spill out over the natural or modified floodplain. These are the most economically damaging floods, that affect the major river valleys of the tropics.
• long-lived floods of the major inland basins. These floods can occur following heavy summer rains in inland Queensland. Floods of this type can take several months to move from the upper catchments to the lower Darling or to Lake Eyre.

Droughts

While the tropical savannas region receives some of the highest annual rainfall in Australia, small areas are also prone to drought. The term 'drought' refers to a severe shortage of water to meet a specified demand. Demands for water are very diverse, and droughts therefore can be considered on a variety of timescales. Rainfall in a single year is important for the growth of pastures and crops that aren't irrigated, while for dams the amount of rain that falls over multiple years is of importance. A number of low rainfall years in a row can create severe water storages when added up over an extended period.

While droughts can occur in all parts of Australia, in the tropical savannas they are most severe in eastern Queensland. This region has been affected by drought since 1999-2000.

The period since 2001 has been the driest on record over parts of eastern Australia , meaning that many large dams have not recovered from the 2002-03 drought. For eastern Australia as a whole (defined as the combined areas of Queensland, New South Wales, Victoria and Tasmania), the four-year period from June 2001 to May 2005 was the driest June to May four-year period on record (ABS 2006).

Further reading

Bureau of Meteorology 1989, Climate of Australia, October 1989, Australian Government Publishing Service, Canberra.

Bureau of Meteorology 1996, Kimberley, Western Australia - Climatic Survey January 1996, Australian Government Publishing Service, Canberra.

Colls, K. & Whitaker, R. 1990, The Australian Weather Book, Child & Associates Publishing Pty Ltd, NSW, Australia.

Gentilli, J. 1972, Australian Climate Patterns, Thomas Nelson (Australia) Limited.

Partridge, I. (ed.) 1991, Will it Rain? El Nino & the Southern Oscillation, Information series Q191028, Queensland Department of Primary Industries, Queensland Government.

Tapper, N. & Hurry, L. 1993, Australia's Weather Patterns - An Introductory Guide, Dellasta Pty Ltd, Mt. Waverly, Victoria.