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
The dry season, from May through to October exhibits dry, cool
weather with little rain, low humidity and wider-ranging
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.
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.
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
The tropical savannas region of northern Australia experiences
many of nature’s more extreme phenomena; including droughts,
floods, tropical cyclones, severe storms and bushfires.
Damage from Cyclone Larry at the lower Johnson near Innisfail,
north Queensland, 2006 Photo: Kate O'Donnell
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
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.
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
Bureau of Meteorology 1989, Climate of
Australia, October 1989, Australian Government Publishing Service,
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,
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
Tapper, N. & Hurry, L. 1993, Australia's
Weather Patterns - An Introductory Guide, Dellasta Pty Ltd, Mt.