Download Monsoon over Australia (Region – IV)

Chapter 7
Monsoon over Australia (Region – IV)
7.1 Introduction – Location and Physical Features
Situated in the southern hemisphere between latitudes about 10 and 43◦ S and
longitudes 113 and 153◦ E and surrounded by oceans, the continent of Australia
experiences its summer monsoon from about December to March and winter monsoon from May to October. A map showing the geographical location and physical
features of the continent and surrounding areas is presented in Fig. 7.1.
The oceans around Australia are: the Indian Ocean in the west, the Pacific Ocean
in the east, the Great Australian Bight in the south, and a series of seas, such as
the Timor, Arafura, and Coral Seas in the north. The Gulf of Carpentaria which lies
between the Northern Territory and the York Peninsula also lies in the north. The
land-sea configurations of the northern and the southern coasts of Australia maintain
a quasi-stationary wave in the fields of temperature, pressure and circulation along
these coasts, especially during Australian summer.
Orography plays an important role in the climate of Australia. The Great
Dividing Range which runs more or less parallel to its eastern coast divides the
moderately cool oceanic climate on one side from the dry desert climate on the
other. However, the southern part of the mountains including the Australian Blue
Alps experiences moderate rainfall almost throughout the year. The mighty MurrayDarling River rises in these mountains and flows westward to make the southeastern
part of the continent fertile and abundantly habitable.
A significant impact on the continent’s weather and climate is made by synopticscale disturbances in the form of depressions and cyclones. They develop in the
quasi-stationary waves when traveling E’ly or W’ly waves of the southern hemisphere interact with them. Most of them form over the warmer waters of the oceans
around Australia. In the north, the oceanic areas which are most likely to breed these
disturbances are the Timor and Arafura seas, the Gulf of Carpentaria and the Coral
Sea. The southern parts of the continent are affected by the eastward-propagating
subtropical/midlatitude baroclinic waves. The continent is also affected by ENSO
events, though irregularly, once every 2–5 years.
K. Saha, Tropical Circulation Systems and Monsoons,
C Springer-Verlag Berlin Heidelberg 2010
DOI 10.1007/978-3-642-03373-5_7, 171
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Fig. 7.1 Geographical location and physical features of Australia Territorial boundaries are indicated by long-dashed lines, deserts by dots, mountains by hats, and depressions and cyclones by
open circles
7.2 Early Studies
The early studies of Australian summer monsoon date back to the sixties and early
seventies of the last century (e.g., Troup, 1961; Berson and Troup, 1961; Gentilli,
1971). But the same were conducted with limited surface and upper-air data.
The data situation improved during the FGGE Northern Hemisphere Winter
Monsoon Experiment (WMONEX), 1978–1979, following which there was a spurt
in research activity with studies undertaken on such diverse topics as onset and
structure of the summer monsoon, divergent circulations, active and break monsoons, intraseasonal oscillations, tropical midlatitude interactions, depressions and
cyclones, effect of ENSO on Australian rainfall and other related weather phenomena. The results of some of these studies (e.g., Sumi and Murakami, 1981;
Murakami and Sumi, 1982; Nicholls et al., 1982, 1984a,b; Davidson et al., 1983,
1984; McBride, 1983; McBride and Nicholls, 1983; Love and Garden, 1984;
Pittock, 1984; Love, 1985a,b; Holland and Nicholls, 1985) are available in an
excellent review by McBride (1987). Holland (1984a,b,c) made a special study
of the climatology and structure of the tropical cyclones which form in the
Australian/SW Pacific region. Westward-propagating tropical disturbances often
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recurve into higher latitudes where they come under the influence of eastwardpropagating midlatitude disturbances of the southern hemisphere.
All the above-mentioned studies contributed significantly to our knowledge
and understanding of the Australian summer monsoon. A more recent review of
Australian summer and winter monsoons has been provided by Saha and Saha
(2001a).
7.3 Climate of Australia and Surrounding Oceans
A major portion of the continent lies over the subtropical belt, so it is mostly the
northern part (north of about 20◦ S) that experiences the impact of a monsoonal-type
of climate. The rest of the continent, barring the coastal regions, located over the
subtropical belt, experiences generally dry and desert-like climate almost throughout the year. In fact, some of the world’s extensive deserts, viz., the West Australian
desert, the Gibson Desert, and the Simpson Desert, are all located over this continent. However, a narrow belt along the southern coast of Australia, especially the
southwestern portion of Western Australia and the southeastern States of Victoria,
New South Wales and Southern Queensland which jut out into the southern oceans
enjoy mild climate. These latter areas are affected by eastward-propagating midlatitude baroclinic wave disturbances of the southern hemisphere, more frequently
during winter than summer, and experience occasional spells of cool, rainy weather.
In this respect, the southeastern states which extend to higher latitudes feel the
impact of these disturbances to greater extent and enjoy much milder climates with
heavier rainfall. The island of Tasmania which lies further poleward has cool, rainy
climate almost throughout the year.
7.3.1 Ocean Surface Temperature (SST, C)
Figure 7.2 shows the mean ocean surface temperature around Australia during (a)
February and (b) August (Courtesy: NCEP Reanalysis).
In February (Fig. 7.2a), two prominent areas of warm SST (≥28◦ C, shaded) stand
out; one rather a narrow zone over the equatorial eastern Indian ocean extending
from near equator southeastward to a wider area near the northwestern coast of
Australia, and the other a very extensive area of equatorial western Pacific Ocean
near New Guinea. These warm areas extend poleward to about 20◦ S. The warm
zone near New Guinea area covers a wide area across the Coral Sea and extends
eastsoutheastward over the SW Pacific Ocean. The SST off the coast of Western
Australia is much lower than that off the coasts of Queensland and New South Wales
at the same latitude.
In August, the whole thermal pattern appears to have shifted northward by a few
degrees of latitude, so that most of the warm areas now lie north of the continent.
The large warm SST area near New Guinea has also shifted northward.
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Fig. 7.2 Climatological (1971–2000) SST (C) around Australia: (a) February and (b) August
7.3.2 Air Temperatures
Mean air temperatures at two pressure surfaces, viz., 925 and 300 hPa, over
Australia and surrounding oceans during February and August, obtained from
NCEP Reanalysis, are shown in Fig. 7.3(a,b) respectively.
In February, the land surface and the lower troposphere over the continent is very
warm with a pronounced temperature maximum centered over Western Australia.
Two warm ridges may be seen, one over the southern part of Western Australia
and the other over Queensland, New South Wales and Victoria areas. Temperatures
drop rather slowly towards the equator, but steeply poleward. However, in the nearequatorial Indonesian region, there appears to be a gradual increase of temperature
from west to east, resulting in a well-marked warm area over New Guinea and
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Fig. 7.3 Mean air temperature (◦ C) over Australia and surrounding regions at 925 and 300 hPa
(a) February and (b) August (Courtesy: NCEP/NCAR Reanalysis Project)
adjoining Coral Sea area which appears to extend further eastward to the dateline or
even beyond.
In the upper troposphere (300 hPa) during February, a belt of warm area is
located over the northern part of the continent with a temperature maximum
along about 15◦ S. The temperature gradient towards the equator is almost negligible, but that to higher latitudes appears to be quite steep. The east–west
gradient of temperature in the lower troposphere almost disappears in the upper
troposphere.
In August, which represents the Austral winter season, the whole thermal field
in the lower and the upper troposphere appears to have moved northward and
temperatures over the continent as well as the adjoining oceans to west and east
are much lower. Further, there appears to be no real temperature maximum now
over the continent except a weak warm area over the ocean close to its northern
coast.
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7.3.3 Atmospheric Pressure (Isobaric Height)
Consistent with the distribution of temperature shown in Fig. 7.3(a,b), the distribution of geopotential height (gpm) at 925 hPa during February and August is shown
in Fig. 7.4.
According to Fig. 7.4(a), a deep low pressure trough oriented in a WSW-ENE
direction is located at 925 hPa over northwestern Australia. It is the summer
Fig. 7.4 Isobaric height (m) fields at 925 hPa over Australia and surrounding areas during (a)
February and (b) August (Courtesy: NCEP Reanalysis)
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177
‘heat low’ (H.L.) trough over Australia. Other well-marked troughs of low pressure
at 925 hPa include:
(i) The equatorial trough over the eastern Indian Ocean to the west of the
Australian heat low;
(ii) An approximately N–S oriented trough over Western Australia; a poleward
extension of the ‘heat low’ trough;
(iii) A north-south oriented trough of low pressure over eastern Australia. This
trough is often called the ‘Cloncurry trough’, since it is associated with a low
pressure over the Cloncurry area of Queensland; Two additional troughs of low
pressure appear over the oceans. These are:
(iv) A prominent trough of low pressure over the Southwest Pacific Ocean, extending from New Guinea area eastsoutheastward across the Coral Sea region;
(v) A low pressure trough off the east coast of Australia.
In February, the ridge of the subtropical high pressure appears to lie along about
35◦ S, with a high pressure cell each over the Great Australian Bight and the Tasman
Sea. A ridge of the Tasman Sea high pressure cell appears to extend equatorward
along the Great Dividing Range.
In February, a strong pressure gradient exists at 925 hPa between the heat low
over Australia and the cold high pressure areas over the oceans to west, east and
south. Pressure generally increases northward with only a small gradient over the
equatorial region. In the equatorial region, pressure appears to decrease generally
from west to east.
In august, with change of season, the ridge of the subtropical high pressure
over Australia appears to have moved equatorward by about 7–10◦ of latitude and
runs along about 28◦ S, with much steeper pressure gradient towards the pole than
towards the equator. A pressure minimum appears over the equatorial region.
7.3.4 Wind and Circulation
Figure 7.5 shows the mean wind field and circulation over Australia and neighborhood at 925 and 300 hPa: (a) February, (b) August.
The February wind field at 925 hPa shows the following features:
(i) A well-defined cyclonic circulation around the ‘heat low’ over northwestern
Australia;
(ii) A broad band of strong cross-equatorial flow from northern to southern hemisphere over the longitudes of Indonesia; NE-ly tradewinds, after crossing the
equator, turn anti-clockwise and blow as W/NW-ly tradewinds. These are the
deflected monsoon winds;
(iii) The deflected northwesterlies converge into the cyclonic circulations around
the heat lows over Australia and New Guinea area, producing well-defined
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Fig. 7.5 Mean wind field and circulation over Australia and surrounding regions at 925 and
300 hPa (a) February and (b) August (Courtesy: NCEP/NCAR Reanalysis Project) but for wind
and circulation
ITCZ and TCZ. It is the TCZ which extends eastsoutheastward from New
Guinea area across the Coral Sea region which has come to be known as the
SW Pacific Convergence Zone (SPCZ).
At 300 hPa in February, the windfield shows an anticyclonic circulation over
northern Australia with its ridge running along about 18◦ S. This means that the low
level W/NW-ly trade winds are overlain by upper-air easterlies over the region to
the north of Australia. Poleward of the ridge, the flow is generally westerly.
The August wind field at 925 hPa shows the following circulation features
(i) A cross-equatorial flow from the Australian region to the northern hemisphere
(Note the reversal of the flow direction from SE to SW);
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(ii) A general northward shift of the axis of the subtropical anticyclone over the
Great Australian Bight from about 35◦ S to about 28◦ S over Australia;
(iii) Strong E/SE-ly tradewinds sweeping across most of northern Australia, with
westerlies to the south of the subtropical ridge.
At 300 hPa in August, the axis of the anticyclonic circulation appears to have
shifted northward to the extreme northern part of Australia with strong easterlies
to its north and westerlies to the south. The westerlies attain high jet speeds during
southern winter.
Two aspects of the atmospheric circulation over the Australian region are
noteworthy. These are:
(1) Cross-equatorial airflow of the lower troposphere; and
(2) The upper-tropospheric subtropical jetstream.
7.3.4.1 Low-Level Cross-Equatorial Airflow
Figure 7.6 presents the equatorial distribution of monthly mean v, the meridional
component of the wind, between 105 and 150◦ E: (a) February and (b) August.
The February distribution (Fig. 7.6a) reveals the existence of three longitudinal
segments of northerlies, one extending from the eastern part of the Bay of Bengal
to about 105◦ E, the other from about 110◦ E to about 135◦ E and a third from about
140◦ E further eastward. Of these, the first two are strong and deep, extending from
surface to almost 300 hPa, while the third has a layer of southerlies between about
700 and 500 hPa. All the segments have southerlies above about 300 hPa.
The August distribution (Fig. 7.6b) shows that barring a few exceptions, crossequatorial flows occur over almost the same longitudinal segments as during
February, but in the reverse direction, that is, from southern to the northern hemisphere. Here also, more than one layer of cross-equatorial flow are involved.
Strong southerlies blow below about 700 hPa over all the segments, but the
layer is overlain by a shallow layer of northerlies around 600 hPa. But a layer
of southerlies reappears in midtroposphere, with a deep layer of northerlies
above.
7.3.4.2 Jetstreams
Loewe and Radok (1950) computed the meridional profile of the zonal component
of the wind from the distribution of temperature by assuming geostrophic approximation. They found that in both the seasons, the W’ly jetstream occurred at a height
of about 12–14 km, but the speed of the jetstreams during winter (July) was much
higher than that during the summer (January). Their computed results appear to be in
substantial agreement with actual observations obtained later from various sources,
such as rawins, etc. The accuracy of their computations is also confirmed by IIOE
datasets presented by Ramage and Raman (1972) and later observations.
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Fig. 7.6 Vertical profile of v, the meridional component of the wind (m s–1 ), along the equator
between 105 and 150◦ E: (a) February and (b) August (Saha and Saha, 2000)
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Fig. 7.7 Jetstreams over
Australia during (a) January,
(b) July (after Loewe and
Radok, 1950)
The subtropical jetstreams over Australia, first computed by Loewe and Radok
(1950), are presented in Fig. 7.7: (a) for January and (b) for July, in which the
isolines show the latitudinal distributions of temperature (◦ C) (dashed) and zonal
geostrophic windspeed (m s–1 ) (continuous lines) at various pressure surfaces from
1000 to 50 mb over the southern latitudes from the equator to 50◦ S. The distributions
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show that the winter jetstream is not only stronger but also occurs much closer to
the equator than the location of the summer jetstream.
Such seasonal movements of the subtropical jetstreams are also observed in the
northern hemisphere and the winter jetstreams which occur closer to the equator are
much stronger than the summer jetstream.
7.4 Monsoon over Australia
7.4.1 Onset of Monsoon
According to Troup (1961) who studied the onset of summer monsoon at Darwin,
a change in the gradient-level (900 hPa) wind direction to northwesterly marks the
beginning of the monsoon season at the station. He found that the change is followed
by a spurt in convective activity and precipitation. He also found that the intensity
of monsoon rainfall was highly correlated with the intensity of the cross-equatorial
flow from the northern hemisphere. The thermal wind over the region in Australian
summer being easterly, the northwesterly flow at low level changed over to easterlies
at some level in midtroposphere.
While the above view of onset of summer monsoon over Australia appears to
be widely accepted, Davidson et al. (1983) put forward an alternative view that the
onset of summer monsoon over Australia is strongly influenced by synoptic events
in the subtropics of the southern hemisphere. They observed that a dramatic increase
in the intensity of convective activity and precipitation at Darwin occurred during
the movement of eastward-propagating large-amplitude midlatitude baroclinic wave
disturbances across the Australian region. They defined monsoon onset by the firsttime appearance of the gradient-level westerly wind at Darwin, following Troup
(1961). The actual date of onset was taken to be the date at which there was largescale increase in convective activity, as long as it occurred within 5 days of the
appearance of the gradient-level northwesterly wind. Their conclusion was based
on the results of a study of monsoon onset during the 6 years 1971, 1973, and
1976–1979 in which they found that in each of the years examined the enhancement
of tropical convection could be attributed to an interaction between a largeamplitude subtropical/midlatitude baroclinic wave and the monsoon trough and its
eastward movement along latitude about 10◦ S towards Darwin.
In the present text, we are led by Fig. 7.6(a) to put forward a hypothesis that
monsoon advances and sets in over Australia in the same manner and retaining the
same wave structure as it does over other parts of the tropics, for example, the Indian
Subcontinent, Eastern Asia, Africa, and South America. In the case of Australia, the
advance of the monsoon wave is spearheaded by three cross-equatorial currents of
strong northwesterlies which after flowing over the Maritime Continent converge
into the heat lows of the Australian region.
While the main heat low lies over the Australian continent, the low pressures over
the equatorial eastern Indian Ocean, and the New Guinea area, are the other two heat
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Fig. 7.8 Schematic showing the principal cross-equatorial aircurrents (thick continuous lines with
arrow) involved in monsoon onset over Australia and other neighboring regions Streamlines (thin
continuous lines with arrows) show directions of air motion around troughs (thick dashed lines) of
low pressure. L denotes Low Pressure; H, High pressure
lows. The circulation features around these heat lows are shown by a schematic in
Fig. 7.8.
Figure 7.8 shows how the principal aircurrents, after crossing the equator near
longitudes 105, 130 and 150◦ E first diverge and then converge into the circulations
around the heat lows over the different regions. Note that two principal aircurrents
converge into the circulation around the heat low over the Australian mainland. It is
the arrival of the principal aircurrents that appears to signal the onset of the summer
monsoon with its convective activity and rainfall over a region.
7.4.2 Co-existence of Monsoon and Hadley
Circulations – Interhemispheric Movement
The monsoon circulation as a perturbation in the tradewind circulation over the
Australian region and its co-existence with the Hadley circulations of the two
hemispheres stands out in a meridional-vertical section through the heat low over
Australia, shown schematically in Fig. 7.9.
The resultant streamlines, shown in Fig. 7.9, were arrived at by using computed values of v (the meridional component of the wind) and ω (the vertical
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Fig. 7.9 Resultant streamlines (constructed from computed v and ω values) along 135◦ E, showing
the linkage between the circulations of the two hemispheres and the Monsoon circulation over
Australia during Australian summer. The arrow shows the direction of airflow. The location of the
monsoon trough zone is indicated by a thick dashed line sloping equatorward with height (after
Saha and Saha, 2000, 2001a)
p-velocity) from an 18 year (1979–1996) mean NCEP/NCAR reanalysis, after suitably scaling the ω-values, along 135◦ E, a meridian passing through the heat low
over Australia. The results shown in Fig. 7.9 clearly reveal the interhemispheric
structure of the monsoon circulation in co-existence with the Hadley circulations of
the two hemispheres.
During Australian summer, the cool, humid monsoon winds diverging from the
subtropical high pressure of the northern hemisphere cross the equator and converge
into the circulation around the heat low over Australia. The field is reversed during
Australian winter when similar winds diverging from the subtropical high pressure
belt of the southern hemisphere cross the equator and converge into the circulations
around the heat lows over Eastern Asia and the Indian Subcontinent.
The direction and magnitude of cross-equatorial fluxes of air in the two seasons
are presented in Table 7.1. For comparison, similar information available in respect
of the Western Indian Ocean region and the equator as a whole is also included in
the table.
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Table 7.1 Magnitudes of estimated and computed cross-equatorial fluxes of air (Unit: 1012 metric
tons (day)–1 ) (Plus sign indicates S’ly, minus N’ly)
Equatorial sector
Tropospheric layer
January/
February
July/
August
(i) Australian section (105–150◦ E)
(Saha and Saha, 2000)
(ii) Western Indian Ocean (42–75◦ E)
(Findlater, 1969a, b)
(Saha, 1970)
(iii) Estimated total across the whole
equator (5◦ N–5◦ S)
(Rao, 1964)
Lower (sfc-300 hPa)
Upper (300–50 hPa)
–3.14
+1.93
+2.68
–4.00
Lower (sfc-400 hPa)
Lower (sfc-400 hPa)
Lower (sfc-500 hPa)
–2.28
+7.68
+5.03
+16.20
–18.49
Table 7.1 highlights the magnitudes and direction of the seasonal movements of airmasses between the hemispheres during both Australian summer
(January/February) and winter (July/August). According to Table 7.1, during
February, the total cross-equatorial flux amounts to –3.14 in the lower troposphere
and 2.68 in the upper troposphere. In August, the figures change to 1.93 and –4.00
respectively.
It is evident from Fig. 7.9 and Table 7.1 that during Austral summer (February)
the lower-tropospheric monsoon trough where the circulations from the two hemispheres converge slopes equatorward with height up to about 500 hPa and that the
Hadley circulation of the winter hemisphere makes considerable inroads into the
summer hemisphere both in the lower and the upper troposphere. It is mostly around
this sloping line between the equator and latitude 20◦ S that low-level convergence
supported by upper-air divergence leads to penetrative convection and precipitation
over Australia. The involvement of the Hadley circulations of the two hemispheres
in the formation of monsoon circulation over Australia stands out in Fig. 7.9. It
connects the monsoon circulation over Australia with that over Asia.
7.4.3 Summer Monsoon Rainfall over Australia
The distributions of mean February rainfall (mm day–1 ) and observed outgoing
longwave radiation (OLR) over Australia and surrounding oceans, as obtained
from Reanalysis, are presented in Fig. 7.10(a,b) respectively. They show concentration of heavy rainfall exceeding 6 mm day–1 along the northern and northeastern
coasts of the continent as well as over the adjoining oceanic areas. The area of
heavy rainfall extends northward to about 5◦ N. There are several pockets of heavier
rainfall exceeding 10 mm day–1 along the equatorial zone of Indonesia as well as
the oceanic area to the north and northeast of Australia, especially the New Guinea
area.
The rainfall rates are well supported by OLR values. In general, low OLR values
(≤220 Wm–2 ) indicate penetrative convection and high rainfall rates. Three areas
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Fig. 7.10 Distribution of February (a) Mean rainfall (mm day–1 ) and (b) Outgoing Longwave
radiation (OLR) (Wm–2 ) over Australia and surrounding areas (from NCEP/NCAR Reanalysis)
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187
of low OLR with values of 190 Wm–2 or less appear over the equatorial region of
eastern Indian Ocean, Southern Borneo, and the New Guinea area.
By contrast, very high values of OLR exceeding 260 Wm–2 prevail over the subtropical belt especially Southern and Western Australia. An extensive area of the
eastern Indian Ocean off the coast of Western Australia is an extremely dry area
within the subtropical belt with high OLR values.
7.5 Annual Rainfall of Australia and Its Seasonal Variability
7.5.1 Annual Rainfall
The inadequacy of water resources in Australia which is a major problem of
the continent is well reflected by the distribution of its annual rainfall, shown in
Fig. 7.11.
Fig. 7.11 Mean annual rainfall (mm) of Australia during period, 1911–1940 (after Bureau of
Meteorology, 1962)
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According to an estimate by Gentilli (1971), the annual rainfall of Australia
leaves 37% of the land with less than 250 mm, 57% with less than 375 mm, and 68%
with less than 500 mm of rain (cumulative percentages). Statewise, on the average,
83% of Southern Australia, 58% of Western Australia, 25% of Northern Territory,
20% of New South Wales, and 13% of Queensland receive less than 250 mm of rain
in the year. Only 0.5% of South Australia, 5.5% of Western Australia, 16% of New
South Wales, 17% of Northern Territory, 23% of Queensland, and 27% of Victoria
receive more than 750 mm of rain in the year. Only in Tasmania, rainfall appears to
be plentiful with more than half the island receiving more than 1000 mm in the year.
The driest area in Australia is located in Southern Australia, around Lake Eyre,
where the average rainfall is less than even 125 mm in the year. On account of
extremely low rainfall, vast tracts of the continent suffer from frequent droughts
7.5.2 Seasonal Variability
A study by Andrews (1932, 1933) reveals the seasonal variation, i.e., the percentage
of the total annual rain that falls in a particular season. His findings for the four
seasons, viz, Summer (A), Autumn (B), Winter (C), and Spring (D), are shown in
Fig. 7.12.
Fig. 7.12 Seasonal concentration (%) of mean annual rainfall over Australia: Summer (a),
Autumn (b), Winter (c), and Spring (d) (after Andrews, 1932, 1933)
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7.6 Variability of Australian Rainfall with ENSO
A possible relationship between Australian rainfall and the Southern Oscillation
has been the subject matter of much research in recent years (e.g., McBride and
Nicholls, 1983; Allan, 1983). The studies conducted so far reveal that years of
extreme values of the Southern Oscillation Index (SOI), coincide with those of
widespread very high or very low values of rainfall over the Australian tropics.
McBride (1987) quotes an example from two contrasting years of rainfall, namely,
1983 and 1974, when the January SOI was highly negative and positive respectively.
In January 1983, an SOI of –29.8 corresponded to a distribution of widespread much
below average rainfall over northern Australia, whereas in January 1974 with a value
of +21.7 for SOI, rainfall was very much above average over a much wider area of
tropical Australia. Here, the SOI which is usually given by the difference in sea
level pressure between Tahiti and Darwin divided by the standard deviation of that
quantity is a measure of the seesaw type oscillation in surface pressure between the
equatorial eastern Pacific Ocean and the Western Pacific-Eastern Indian Oceans, as
originally defined by Sir Gilbert Walker and called the Southern Oscillation.
However, the relationship appears to exist in years of highly extreme values of
SOI only. Over a period of many years in a row, the relationship appears to be weak.
7.7 Tropical Disturbances in the Australian
Region – Depressions and Cyclones
In Australia, interest in studies of atmospheric disturbances began quite early last
century and has continuously grown since then, as demonstrated by several studies (e.g., Bureau of Meteorology, 1956, 1978; Keenan, 1981, 1982; McBride and
Keenan, 1982; Holland, 1984a,b,c; Lajoie and Butterworth, 1984; Nicholls, 1984b),
especially after the MONEX, 1978–1979. The contributions made by these studies
have thrown light on several aspects of the formation and behaviour of monsoon
depressions and tropical cyclones in the Australian region and adjoining the SW
Pacific Ocean. However, notwithstanding great advances made, uncertainties remain
in several areas relating to these disturbances, especially their development and
movement that need further study. With the availability of satellite data and global
data analysis, we have now much greater opportunity to observe and study these
disturbances than ever before.
Paterson and Bate (2001) who carried out a detailed study of tropical cyclones
over the South Pacific and Southeast Indian Ocean during the cyclone season
(November–April), 1999–2000, found that the number of tropical cyclones formed
during the season to the west of 105◦ E, between 105 and 165◦ E, and to the east of
165◦ E was 10, 11 and 5 respectively against a climatological average of 12.7, 9.6
and 5.6 over the respective basins. Figure 7.13 shows the tracks of the cyclones that
formed between longitudes 100 and 180◦ E during the season.
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Fig. 7.13 Tracks of named tropical depressions and cyclones which formed during the summer
season (December–April) of 1999–2000 between 100 and 180◦ E. Name of the disturbance is given
at the location of its first detection. The arrow shows the direction of its movement (Paterson and
Bate, 2001)
Paterson and Bate furnished particulars of these cyclones stating the date each
was first identified as a low, the date it turned into a cyclone, the date it reached
maximum intensity and the date it ended its tropical cyclone phase. Those interested
in these details may look up their original paper.
As shown in Fig. 7.13, after formation, most of the cyclonic disturbances tend
to move with the dominant airstream in which they are embedded, and gradually
recurve towards the south as they move towards the belt of the midlatitude westerlies. But a small percentage of them do not recurve but keep moving in their original
direction till they move over a cold ocean or come under the influence of some other
disturbances.
McBride and Keenan (1982) who carried out a case-by-case study of tropical cyclone development over a period of 5 years found that in 84% of the cases
examined, the precyclone cloud cluster when it first appeared was located on the
gradient-level monsoon trough or shear line. The cloud cluster associated with
the developed cyclone also was fully developed. As a fully developed cloud also,
97% of them were on the monsoon shear line. This close association of monsoon
depressions and cyclones with the monsoon trough suggests that the development,
7.8
Tropical-Midlatitude Interaction in the Australian Region
191
intensification and movement of the cyclone are probably governed by fluctuations
of the aircurrents that converge at the troughline.
McBride and Zehr (1981) and McBride and Keenan (1982) found that a strengthening of monsoon westerlies equatorward of a disturbance was favorable for its
development into a tropical cyclone in many cases. According to Love (1985a,b),
the strengthening of the westerlies usually followed cold surges in the South China
Sea.
Studies reveal that a monsoon disturbance may undergo several transformations
during its life period. In most cases, it starts off as a low or depression in or around
a monsoon trough and takes a day or two to develop into a tropical cyclone over a
warm ocean surface. As long as it remains over a warm ocean and other environmental conditions continue to be favorable, the cyclone rages in full fury but on entering
land or a cold SST anomaly area it rapidly transforms itself back again into a depression or low pressure system. A reverse transformation of a low or depression into
a tropical cyclone appears to occur when a low or depression after extensive traveling over land enters a warm ocean. Studies by Paterson and Bate (2001), referred
to earlier in this section report several cases of such transformations in the life of a
tropical disturbance in the Australian region.
7.8 Tropical-Midlatitude Interaction in the Australian Region
During Australian summer, wave disturbances in midlatitude baroclinic westerlies
of the southern hemisphere which usually move along latitudes south of about
35◦ S at surface during Austral summer often develop large amplitudes and interact
with the monsoon circulation over the continent and adjoining oceans. An example of such an interaction during onset of monsoon was discussed by Davidson
et al. (1983). These wave disturbances also interact with tropical depressions and
cyclones which may come under their influence dung their eastward movement
along the southern parts of the continent. There are several cases on record of such
interactions in the past (see, e.g., McBride, 1987; Saha and Saha, 2001a). The interactions affect tropical convection and lead to enhanced rainfall in certain sectors,
especially over the southwestern and southeastern parts of the continent.
Saha and Saha (2001a) discuss the life history of two tropical disturbances,
‘BOBBY’ and ‘JASON’, particulars of which including their dates of initial
formation and movement and later recurvature are given in Fig. 7.14.
Of the two tropical disturbances, Bobby had a chequered career. Starting life
as a simple low pressure wave in the monsoon trough zone (TCZ) near Darwin in
Arnhem Land on 18 February 1995, Bobby continued to move southwestward as
a closed ‘Low’ along the northwestern coast of Australia but on emerging over the
adjoining ocean after 2 days of land travel it rapidly developed into a depression on
21 February and a tropical cyclone the following day. From 22 February onward,
it gradually came under the influence of a midlatitude W-ly trough which was
approaching Australia from the west. On 24 February, the westerly trough reached
192
7 Monsoon over Australia
Fig. 7.14 Dates, approximate locations, central pressures, intensities, and tracks of two tropical
disturbances in the Australian region (Courtesy: NCEP/NCAR Reanalysis)
the extreme southwestern part of the continent and interacted directly with Bobby
which was then centered near Onslow at about 21◦ S, 115◦ E. The interaction led to
a coupling of the two wave disturbances and for the following 2 days they moved
together over the sandy deserts of Western Australia. However, the umbilical cord
was soon broken and Bobby moved away southeastward across the southern coast
of Australia. A satellite view of Bobby when it was located over the desert area is
in Fig. 7.15.
The coupling between Bobby and the quasi-stationary wave is clearly suggested
by the analyses in Fig. 7.16(a,b) respectively.
Through massive warm air advection in the southeast and cold air advection
in the northwest of the coupled trough, an isallobaric gradient forced the tropical cyclone to recurve and move in a southeasterly direction as the trough in the
midlatitude westerlies in the south moved away eastward in the following 4 days.
7.8.1 Northerly and Southerly Bursters
During the movement of midlatitude W’ly waves across Southern Australia,
cyclonic and anticyclonic circulations associated with these waves follow each other
in quick succession as they move eastward, with gale-force winds blowing from the
7.8
Tropical-Midlatitude Interaction in the Australian Region
193
Fig. 7.15 NOAA-12 satellite view of the tropical cyclone ‘BOBBY’ over the desert of southwestern Australia on 26 February 1995
hot desert lands to the southern oceans ahead of a cyclonic circulation and cool
humid winds blowing from ocean to land in its rear when the cyclonic circulation
is replaced by anticyclonic circulation. Locally, these winds are known as Northerly
and Southerly Bursters respectively, because of the suddenness with which they
set in, or one replaces the other. They can occur in any part of the year, but are
more common during summer. The southeastern parts of Australia and Africa are
particularly vulnerable to occurrence of bursters.
According to the Meteorological Glossary (Second Edition) of the American
Meteorological Society (2000), a southerly burster is ‘a sudden shift of wind to
the southeast in the south and southeast parts of Australia, especially frequent on
the coast of New South Wales near Sydney in summer.
It occurs in the rear of a trough of low pressure that is followed by the rapid
advance of an anticyclone from west Australia. After some days of hot, dry northerly
wind, cumulus clouds approach from the south, the wind drops to calm and then sets
in suddenly from the south, sometimes reaching gale force. Temperature at Sydney
has fallen from 38 to 18◦ C in 30 min. The average summer frequency of bursters at
Sydney is 32. Similar winds are experienced in the east of South Africa, especially
near Durban.
194
7 Monsoon over Australia
Fig. 7.16 Analyses of (a) temperature and (b) wind at 925 hPa during interaction of tropical cyclone ‘Bobby’ with an eastward-propagating midlatitude W’ly trough disturbance over the
Australian region, 12 GMT, 24 February 1995