Chapter 7 Monsoon over Australia (Region – IV) Weng/course... · 2015-04-06 · Monsoon over Australia (Region – IV) 7.1 Introduction – Location and Physical Features Situated

Chapter 7Monsoon over Australia (Region – IV)

7.1 Introduction – Location and Physical Features

Situated in the southern hemisphere between latitudes about 10 and 43◦S andlongitudes 113 and 153◦E and surrounded by oceans, the continent of Australiaexperiences its summer monsoon from about December to March and winter mon-soon from May to October. A map showing the geographical location and physicalfeatures 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 Oceanin the east, the Great Australian Bight in the south, and a series of seas, such asthe Timor, Arafura, and Coral Seas in the north. The Gulf of Carpentaria which liesbetween the Northern Territory and the York Peninsula also lies in the north. Theland-sea configurations of the northern and the southern coasts of Australia maintaina quasi-stationary wave in the fields of temperature, pressure and circulation alongthese coasts, especially during Australian summer.

Orography plays an important role in the climate of Australia. The GreatDividing Range which runs more or less parallel to its eastern coast divides themoderately cool oceanic climate on one side from the dry desert climate on theother. However, the southern part of the mountains including the Australian BlueAlps experiences moderate rainfall almost throughout the year. The mighty Murray-Darling River rises in these mountains and flows westward to make the southeasternpart of the continent fertile and abundantly habitable.

A significant impact on the continent’s weather and climate is made by synoptic-scale disturbances in the form of depressions and cyclones. They develop in thequasi-stationary waves when traveling E’ly or W’ly waves of the southern hemi-sphere interact with them. Most of them form over the warmer waters of the oceansaround Australia. In the north, the oceanic areas which are most likely to breed thesedisturbances are the Timor and Arafura seas, the Gulf of Carpentaria and the CoralSea. The southern parts of the continent are affected by the eastward-propagatingsubtropical/midlatitude baroclinic waves. The continent is also affected by ENSOevents, though irregularly, once every 2–5 years.

171K. Saha, Tropical Circulation Systems and Monsoons,DOI 10.1007/978-3-642-03373-5_7, C© Springer-Verlag Berlin Heidelberg 2010

172 7 Monsoon over Australia

Fig. 7.1 Geographical location and physical features of Australia Territorial boundaries are indi-cated by long-dashed lines, deserts by dots, mountains by hats, and depressions and cyclones byopen circles

7.2 Early Studies

The early studies of Australian summer monsoon date back to the sixties and earlyseventies 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 WinterMonsoon Experiment (WMONEX), 1978–1979, following which there was a spurtin research activity with studies undertaken on such diverse topics as onset andstructure of the summer monsoon, divergent circulations, active and break mon-soons, intraseasonal oscillations, tropical midlatitude interactions, depressions andcyclones, effect of ENSO on Australian rainfall and other related weather phe-nomena. 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 anexcellent review by McBride (1987). Holland (1984a,b,c) made a special studyof the climatology and structure of the tropical cyclones which form in theAustralian/SW Pacific region. Westward-propagating tropical disturbances often

7.3 Climate of Australia and Surrounding Oceans 173

recurve into higher latitudes where they come under the influence of eastward-propagating midlatitude disturbances of the southern hemisphere.

All the above-mentioned studies contributed significantly to our knowledgeand understanding of the Australian summer monsoon. A more recent review ofAustralian 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 thenorthern part (north of about 20◦S) that experiences the impact of a monsoonal-typeof climate. The rest of the continent, barring the coastal regions, located over thesubtropical belt, experiences generally dry and desert-like climate almost through-out the year. In fact, some of the world’s extensive deserts, viz., the West Australiandesert, the Gibson Desert, and the Simpson Desert, are all located over this conti-nent. However, a narrow belt along the southern coast of Australia, especially thesouthwestern portion of Western Australia and the southeastern States of Victoria,New South Wales and Southern Queensland which jut out into the southern oceansenjoy mild climate. These latter areas are affected by eastward-propagating mid-latitude baroclinic wave disturbances of the southern hemisphere, more frequentlyduring winter than summer, and experience occasional spells of cool, rainy weather.In this respect, the southeastern states which extend to higher latitudes feel theimpact of these disturbances to greater extent and enjoy much milder climates withheavier rainfall. The island of Tasmania which lies further poleward has cool, rainyclimate 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) standout; one rather a narrow zone over the equatorial eastern Indian ocean extendingfrom near equator southeastward to a wider area near the northwestern coast ofAustralia, and the other a very extensive area of equatorial western Pacific Oceannear New Guinea. These warm areas extend poleward to about 20◦S. The warmzone near New Guinea area covers a wide area across the Coral Sea and extendseastsoutheastward over the SW Pacific Ocean. The SST off the coast of WesternAustralia is much lower than that off the coasts of Queensland and New South Walesat the same latitude.

In August, the whole thermal pattern appears to have shifted northward by a fewdegrees 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.


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, overAustralia and surrounding oceans during February and August, obtained fromNCEP Reanalysis, are shown in Fig. 7.3(a,b) respectively.

In February, the land surface and the lower troposphere over the continent is verywarm with a pronounced temperature maximum centered over Western Australia.Two warm ridges may be seen, one over the southern part of Western Australiaand the other over Queensland, New South Wales and Victoria areas. Temperaturesdrop rather slowly towards the equator, but steeply poleward. However, in the near-equatorial Indonesian region, there appears to be a gradual increase of temperaturefrom west to east, resulting in a well-marked warm area over New Guinea and


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 oreven beyond.

In the upper troposphere (300 hPa) during February, a belt of warm area islocated over the northern part of the continent with a temperature maximumalong about 15◦S. The temperature gradient towards the equator is almost neg-ligible, but that to higher latitudes appears to be quite steep. The east–westgradient of temperature in the lower troposphere almost disappears in the uppertroposphere.

In August, which represents the Austral winter season, the whole thermal fieldin the lower and the upper troposphere appears to have moved northward andtemperatures over the continent as well as the adjoining oceans to west and eastare much lower. Further, there appears to be no real temperature maximum nowover the continent except a weak warm area over the ocean close to its northerncoast.


7.3.3 Atmospheric Pressure (Isobaric Height)

Consistent with the distribution of temperature shown in Fig. 7.3(a,b), the distribu-tion of geopotential height (gpm) at 925 hPa during February and August is shownin Fig. 7.4.

According to Fig. 7.4(a), a deep low pressure trough oriented in a WSW-ENEdirection 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)


‘heat low’ (H.L.) trough over Australia. Other well-marked troughs of low pressureat 925 hPa include:

(i) The equatorial trough over the eastern Indian Ocean to the west of theAustralian heat low;

(ii) An approximately N–S oriented trough over Western Australia; a polewardextension of the ‘heat low’ trough;

(iii) A north-south oriented trough of low pressure over eastern Australia. Thistrough is often called the ‘Cloncurry trough’, since it is associated with a lowpressure over the Cloncurry area of Queensland; Two additional troughs of lowpressure appear over the oceans. These are:

(iv) A prominent trough of low pressure over the Southwest Pacific Ocean, extend-ing 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 about35◦S, with a high pressure cell each over the Great Australian Bight and the TasmanSea. A ridge of the Tasman Sea high pressure cell appears to extend equatorwardalong the Great Dividing Range.

In February, a strong pressure gradient exists at 925 hPa between the heat lowover Australia and the cold high pressure areas over the oceans to west, east andsouth. Pressure generally increases northward with only a small gradient over theequatorial region. In the equatorial region, pressure appears to decrease generallyfrom west to east.

In august, with change of season, the ridge of the subtropical high pressureover Australia appears to have moved equatorward by about 7–10◦ of latitude andruns along about 28◦S, with much steeper pressure gradient towards the pole thantowards 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 neighbor-hood 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 northwesternAustralia;

(ii) A broad band of strong cross-equatorial flow from northern to southern hemi-sphere over the longitudes of Indonesia; NE-ly tradewinds, after crossing theequator, turn anti-clockwise and blow as W/NW-ly tradewinds. These are thedeflected monsoon winds;

(iii) The deflected northwesterlies converge into the cyclonic circulations aroundthe heat lows over Australia and New Guinea area, producing well-defined


Fig. 7.5 Mean wind field and circulation over Australia and surrounding regions at 925 and300 hPa (a) February and (b) August (Courtesy: NCEP/NCAR Reanalysis Project) but for windand circulation

ITCZ and TCZ. It is the TCZ which extends eastsoutheastward from NewGuinea area across the Coral Sea region which has come to be known as theSW Pacific Convergence Zone (SPCZ).

At 300 hPa in February, the windfield shows an anticyclonic circulation overnorthern Australia with its ridge running along about 18◦S. This means that the lowlevel W/NW-ly trade winds are overlain by upper-air easterlies over the region tothe 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);


(ii) A general northward shift of the axis of the subtropical anticyclone over theGreat Australian Bight from about 35◦S to about 28◦S over Australia;

(iii) Strong E/SE-ly tradewinds sweeping across most of northern Australia, withwesterlies to the south of the subtropical ridge.

At 300 hPa in August, the axis of the anticyclonic circulation appears to haveshifted northward to the extreme northern part of Australia with strong easterliesto its north and westerlies to the south. The westerlies attain high jet speeds duringsouthern winter.

Two aspects of the atmospheric circulation over the Australian region arenoteworthy. 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 meridionalcomponent of the wind, between 105 and 150◦E: (a) February and (b) August.

The February distribution (Fig. 7.6a) reveals the existence of three longitudinalsegments of northerlies, one extending from the eastern part of the Bay of Bengalto about 105◦E, the other from about 110◦E to about 135◦E and a third from about140◦E further eastward. Of these, the first two are strong and deep, extending fromsurface to almost 300 hPa, while the third has a layer of southerlies between about700 and 500 hPa. All the segments have southerlies above about 300 hPa.

The August distribution (Fig. 7.6b) shows that barring a few exceptions, cross-equatorial flows occur over almost the same longitudinal segments as duringFebruary, but in the reverse direction, that is, from southern to the northern hemi-sphere. Here also, more than one layer of cross-equatorial flow are involved.Strong southerlies blow below about 700 hPa over all the segments, but thelayer is overlain by a shallow layer of northerlies around 600 hPa. But a layerof southerlies reappears in midtroposphere, with a deep layer of northerliesabove.

7.3.4.2 Jetstreams

Loewe and Radok (1950) computed the meridional profile of the zonal componentof the wind from the distribution of temperature by assuming geostrophic approxi-mation. They found that in both the seasons, the W’ly jetstream occurred at a heightof about 12–14 km, but the speed of the jetstreams during winter (July) was muchhigher than that during the summer (January). Their computed results appear to be insubstantial agreement with actual observations obtained later from various sources,such as rawins, etc. The accuracy of their computations is also confirmed by IIOEdatasets presented by Ramage and Raman (1972) and later observations.


Fig. 7.6 Vertical profile of v, the meridional component of the wind (m s–1), along the equatorbetween 105 and 150◦E: (a) February and (b) August (Saha and Saha, 2000)


Fig. 7.7 Jetstreams overAustralia during (a) January,(b) July (after Loewe andRadok, 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 theisolines show the latitudinal distributions of temperature (◦C) (dashed) and zonalgeostrophic windspeed (m s–1) (continuous lines) at various pressure surfaces from1000 to 50 mb over the southern latitudes from the equator to 50◦S. The distributions


show that the winter jetstream is not only stronger but also occurs much closer tothe equator than the location of the summer jetstream.

Such seasonal movements of the subtropical jetstreams are also observed in thenorthern hemisphere and the winter jetstreams which occur closer to the equator aremuch 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 thebeginning of the monsoon season at the station. He found that the change is followedby a spurt in convective activity and precipitation. He also found that the intensityof monsoon rainfall was highly correlated with the intensity of the cross-equatorialflow from the northern hemisphere. The thermal wind over the region in Australiansummer being easterly, the northwesterly flow at low level changed over to easterliesat some level in midtroposphere.

While the above view of onset of summer monsoon over Australia appears tobe widely accepted, Davidson et al. (1983) put forward an alternative view that theonset of summer monsoon over Australia is strongly influenced by synoptic eventsin the subtropics of the southern hemisphere. They observed that a dramatic increasein the intensity of convective activity and precipitation at Darwin occurred duringthe movement of eastward-propagating large-amplitude midlatitude baroclinic wavedisturbances across the Australian region. They defined monsoon onset by the first-time 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 large-scale increase in convective activity, as long as it occurred within 5 days of theappearance of the gradient-level northwesterly wind. Their conclusion was basedon the results of a study of monsoon onset during the 6 years 1971, 1973, and1976–1979 in which they found that in each of the years examined the enhancementof tropical convection could be attributed to an interaction between a large-amplitude subtropical/midlatitude baroclinic wave and the monsoon trough and itseastward 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 thatmonsoon advances and sets in over Australia in the same manner and retaining thesame wave structure as it does over other parts of the tropics, for example, the IndianSubcontinent, Eastern Asia, Africa, and South America. In the case of Australia, theadvance of the monsoon wave is spearheaded by three cross-equatorial currents ofstrong northwesterlies which after flowing over the Maritime Continent convergeinto the heat lows of the Australian region.

While the main heat low lies over the Australian continent, the low pressures overthe equatorial eastern Indian Ocean, and the New Guinea area, are the other two heat

7.4 Monsoon over Australia 183

Fig. 7.8 Schematic showing the principal cross-equatorial aircurrents (thick continuous lines witharrow) involved in monsoon onset over Australia and other neighboring regions Streamlines (thincontinuous lines with arrows) show directions of air motion around troughs (thick dashed lines) oflow pressure. L denotes Low Pressure; H, High pressure

lows. The circulation features around these heat lows are shown by a schematic inFig. 7.8.

Figure 7.8 shows how the principal aircurrents, after crossing the equator nearlongitudes 105, 130 and 150◦E first diverge and then converge into the circulationsaround the heat lows over the different regions. Note that two principal aircurrentsconverge into the circulation around the heat low over the Australian mainland. It isthe arrival of the principal aircurrents that appears to signal the onset of the summermonsoon with its convective activity and rainfall over a region.

7.4.2 Co-existence of Monsoon and HadleyCirculations – Interhemispheric Movement

The monsoon circulation as a perturbation in the tradewind circulation over theAustralian region and its co-existence with the Hadley circulations of the twohemispheres stands out in a meridional-vertical section through the heat low overAustralia, shown schematically in Fig. 7.9.

The resultant streamlines, shown in Fig. 7.9, were arrived at by using com-puted values of v (the meridional component of the wind) and ω (the vertical


Fig. 7.9 Resultant streamlines (constructed from computed v and ω values) along 135◦E, showingthe linkage between the circulations of the two hemispheres and the Monsoon circulation overAustralia during Australian summer. The arrow shows the direction of airflow. The location of themonsoon trough zone is indicated by a thick dashed line sloping equatorward with height (afterSaha and Saha, 2000, 2001a)

p-velocity) from an 18 year (1979–1996) mean NCEP/NCAR reanalysis, after suit-ably scaling the ω-values, along 135◦E, a meridian passing through the heat lowover Australia. The results shown in Fig. 7.9 clearly reveal the interhemisphericstructure of the monsoon circulation in co-existence with the Hadley circulations ofthe two hemispheres.

During Australian summer, the cool, humid monsoon winds diverging from thesubtropical high pressure of the northern hemisphere cross the equator and convergeinto the circulation around the heat low over Australia. The field is reversed duringAustralian winter when similar winds diverging from the subtropical high pressurebelt of the southern hemisphere cross the equator and converge into the circulationsaround the heat lows over Eastern Asia and the Indian Subcontinent.

The direction and magnitude of cross-equatorial fluxes of air in the two seasonsare presented in Table 7.1. For comparison, similar information available in respectof the Western Indian Ocean region and the equator as a whole is also included inthe table.

7.4 Monsoon over Australia 185

Table 7.1 Magnitudes of estimated and computed cross-equatorial fluxes of air (Unit: 1012 metrictons (day)–1) (Plus sign indicates S’ly, minus N’ly)

Equatorial sector Tropospheric layerJanuary/February

July/August

(i) Australian section (105–150◦E) Lower (sfc-300 hPa) –3.14 +2.68(Saha and Saha, 2000) Upper (300–50 hPa) +1.93 –4.00

(ii) Western Indian Ocean (42–75◦E)(Findlater, 1969a, b) Lower (sfc-400 hPa) –2.28 +7.68(Saha, 1970) Lower (sfc-400 hPa) +5.03

(iii) Estimated total across the whole Lower (sfc-500 hPa) –18.49 +16.20equator (5◦N–5◦S)(Rao, 1964)

Table 7.1 highlights the magnitudes and direction of the seasonal move-ments of airmasses between the hemispheres during both Australian summer(January/February) and winter (July/August). According to Table 7.1, duringFebruary, the total cross-equatorial flux amounts to –3.14 in the lower troposphereand 2.68 in the upper troposphere. In August, the figures change to 1.93 and –4.00respectively.

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 hemi-spheres converge slopes equatorward with height up to about 500 hPa and that theHadley circulation of the winter hemisphere makes considerable inroads into thesummer hemisphere both in the lower and the upper troposphere. It is mostly aroundthis sloping line between the equator and latitude 20◦S that low-level convergencesupported by upper-air divergence leads to penetrative convection and precipitationover Australia. The involvement of the Hadley circulations of the two hemispheresin the formation of monsoon circulation over Australia stands out in Fig. 7.9. Itconnects 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 outgoinglongwave radiation (OLR) over Australia and surrounding oceans, as obtainedfrom Reanalysis, are presented in Fig. 7.10(a,b) respectively. They show concen-tration of heavy rainfall exceeding 6 mm day–1 along the northern and northeasterncoasts of the continent as well as over the adjoining oceanic areas. The area ofheavy rainfall extends northward to about 5◦N. There are several pockets of heavierrainfall exceeding 10 mm day–1 along the equatorial zone of Indonesia as well asthe oceanic area to the north and northeast of Australia, especially the New Guineaarea.

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


Fig. 7.10 Distribution of February (a) Mean rainfall (mm day–1) and (b) Outgoing Longwaveradiation (OLR) (Wm–2) over Australia and surrounding areas (from NCEP/NCAR Reanalysis)

7.5 Annual Rainfall of Australia and Its Seasonal Variability 187

of low OLR with values of 190 Wm–2 or less appear over the equatorial region ofeastern Indian Ocean, Southern Borneo, and the New Guinea area.

By contrast, very high values of OLR exceeding 260 Wm–2 prevail over the sub-tropical belt especially Southern and Western Australia. An extensive area of theeastern Indian Ocean off the coast of Western Australia is an extremely dry areawithin 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 ofthe continent is well reflected by the distribution of its annual rainfall, shown inFig. 7.11.

Fig. 7.11 Mean annual rainfall (mm) of Australia during period, 1911–1940 (after Bureau ofMeteorology, 1962)


According to an estimate by Gentilli (1971), the annual rainfall of Australialeaves 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 rainin the year. Only 0.5% of South Australia, 5.5% of Western Australia, 16% of NewSouth Wales, 17% of Northern Territory, 23% of Queensland, and 27% of Victoriareceive more than 750 mm of rain in the year. Only in Tasmania, rainfall appears tobe 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 ofextremely 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 percentageof the total annual rain that falls in a particular season. His findings for the fourseasons, viz, Summer (A), Autumn (B), Winter (C), and Spring (D), are shown inFig. 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)

7.7 Tropical Disturbances in the Australian Region – Depressions and Cyclones 189

7.6 Variability of Australian Rainfall with ENSO

A possible relationship between Australian rainfall and the Southern Oscillationhas been the subject matter of much research in recent years (e.g., McBride andNicholls, 1983; Allan, 1983). The studies conducted so far reveal that years ofextreme values of the Southern Oscillation Index (SOI), coincide with those ofwidespread 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 muchbelow average rainfall over northern Australia, whereas in January 1974 with a valueof +21.7 for SOI, rainfall was very much above average over a much wider area oftropical Australia. Here, the SOI which is usually given by the difference in sealevel pressure between Tahiti and Darwin divided by the standard deviation of thatquantity is a measure of the seesaw type oscillation in surface pressure between theequatorial eastern Pacific Ocean and the Western Pacific-Eastern Indian Oceans, asoriginally defined by Sir Gilbert Walker and called the Southern Oscillation.

However, the relationship appears to exist in years of highly extreme values ofSOI only. Over a period of many years in a row, the relationship appears to be weak.

7.7 Tropical Disturbances in the AustralianRegion – Depressions and Cyclones

In Australia, interest in studies of atmospheric disturbances began quite early lastcentury and has continuously grown since then, as demonstrated by several stud-ies (e.g., Bureau of Meteorology, 1956, 1978; Keenan, 1981, 1982; McBride andKeenan, 1982; Holland, 1984a,b,c; Lajoie and Butterworth, 1984; Nicholls, 1984b),especially after the MONEX, 1978–1979. The contributions made by these studieshave thrown light on several aspects of the formation and behaviour of monsoondepressions and tropical cyclones in the Australian region and adjoining the SWPacific Ocean. However, notwithstanding great advances made, uncertainties remainin several areas relating to these disturbances, especially their development andmovement that need further study. With the availability of satellite data and globaldata analysis, we have now much greater opportunity to observe and study thesedisturbances than ever before.

Paterson and Bate (2001) who carried out a detailed study of tropical cyclonesover the South Pacific and Southeast Indian Ocean during the cyclone season(November–April), 1999–2000, found that the number of tropical cyclones formedduring the season to the west of 105◦E, between 105 and 165◦E, and to the east of165◦E was 10, 11 and 5 respectively against a climatological average of 12.7, 9.6and 5.6 over the respective basins. Figure 7.13 shows the tracks of the cyclones thatformed between longitudes 100 and 180◦E during the season.


Fig. 7.13 Tracks of named tropical depressions and cyclones which formed during the summerseason (December–April) of 1999–2000 between 100 and 180◦E. Name of the disturbance is givenat the location of its first detection. The arrow shows the direction of its movement (Paterson andBate, 2001)

Paterson and Bate furnished particulars of these cyclones stating the date eachwas first identified as a low, the date it turned into a cyclone, the date it reachedmaximum intensity and the date it ended its tropical cyclone phase. Those interestedin these details may look up their original paper.

As shown in Fig. 7.13, after formation, most of the cyclonic disturbances tendto move with the dominant airstream in which they are embedded, and graduallyrecurve towards the south as they move towards the belt of the midlatitude wester-lies. But a small percentage of them do not recurve but keep moving in their originaldirection till they move over a cold ocean or come under the influence of some otherdisturbances.

McBride and Keenan (1982) who carried out a case-by-case study of tropi-cal cyclone development over a period of 5 years found that in 84% of the casesexamined, the precyclone cloud cluster when it first appeared was located on thegradient-level monsoon trough or shear line. The cloud cluster associated withthe 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 monsoondepressions 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 fluctuationsof the aircurrents that converge at the troughline.

McBride and Zehr (1981) and McBride and Keenan (1982) found that a strength-ening of monsoon westerlies equatorward of a disturbance was favorable for itsdevelopment into a tropical cyclone in many cases. According to Love (1985a,b),the strengthening of the westerlies usually followed cold surges in the South ChinaSea.

Studies reveal that a monsoon disturbance may undergo several transformationsduring its life period. In most cases, it starts off as a low or depression in or arounda monsoon trough and takes a day or two to develop into a tropical cyclone over awarm ocean surface. As long as it remains over a warm ocean and other environmen-tal conditions continue to be favorable, the cyclone rages in full fury but on enteringland or a cold SST anomaly area it rapidly transforms itself back again into a depres-sion or low pressure system. A reverse transformation of a low or depression intoa tropical cyclone appears to occur when a low or depression after extensive trav-eling over land enters a warm ocean. Studies by Paterson and Bate (2001), referredto earlier in this section report several cases of such transformations in the life of atropical disturbance in the Australian region.

7.8 Tropical-Midlatitude Interaction in the Australian Region

During Australian summer, wave disturbances in midlatitude baroclinic westerliesof the southern hemisphere which usually move along latitudes south of about35◦S at surface during Austral summer often develop large amplitudes and interactwith the monsoon circulation over the continent and adjoining oceans. An exam-ple of such an interaction during onset of monsoon was discussed by Davidsonet al. (1983). These wave disturbances also interact with tropical depressions andcyclones which may come under their influence dung their eastward movementalong the southern parts of the continent. There are several cases on record of suchinteractions in the past (see, e.g., McBride, 1987; Saha and Saha, 2001a). The inter-actions 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 initialformation and movement and later recurvature are given in Fig. 7.14.

Of the two tropical disturbances, Bobby had a chequered career. Starting lifeas a simple low pressure wave in the monsoon trough zone (TCZ) near Darwin inArnhem Land on 18 February 1995, Bobby continued to move southwestward asa closed ‘Low’ along the northwestern coast of Australia but on emerging over theadjoining ocean after 2 days of land travel it rapidly developed into a depression on21 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 wasapproaching Australia from the west. On 24 February, the westerly trough reached


Fig. 7.14 Dates, approximate locations, central pressures, intensities, and tracks of two tropicaldisturbances in the Australian region (Courtesy: NCEP/NCAR Reanalysis)

the extreme southwestern part of the continent and interacted directly with Bobbywhich was then centered near Onslow at about 21◦S, 115◦E. The interaction led toa coupling of the two wave disturbances and for the following 2 days they movedtogether over the sandy deserts of Western Australia. However, the umbilical cordwas soon broken and Bobby moved away southeastward across the southern coastof Australia. A satellite view of Bobby when it was located over the desert area isin Fig. 7.15.

The coupling between Bobby and the quasi-stationary wave is clearly suggestedby the analyses in Fig. 7.16(a,b) respectively.

Through massive warm air advection in the southeast and cold air advectionin the northwest of the coupled trough, an isallobaric gradient forced the tropi-cal cyclone to recurve and move in a southeasterly direction as the trough in themidlatitude 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 otherin 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 southwest-ern Australia on 26 February 1995

hot desert lands to the southern oceans ahead of a cyclonic circulation and coolhumid winds blowing from ocean to land in its rear when the cyclonic circulationis replaced by anticyclonic circulation. Locally, these winds are known as Northerlyand Southerly Bursters respectively, because of the suddenness with which theyset in, or one replaces the other. They can occur in any part of the year, but aremore common during summer. The southeastern parts of Australia and Africa areparticularly vulnerable to occurrence of bursters.

According to the Meteorological Glossary (Second Edition) of the AmericanMeteorological Society (2000), a southerly burster is ‘a sudden shift of wind tothe southeast in the south and southeast parts of Australia, especially frequent onthe 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 rapidadvance of an anticyclone from west Australia. After some days of hot, dry northerlywind, cumulus clouds approach from the south, the wind drops to calm and then setsin suddenly from the south, sometimes reaching gale force. Temperature at Sydneyhas fallen from 38 to 18◦C in 30 min. The average summer frequency of bursters atSydney is 32. Similar winds are experienced in the east of South Africa, especiallynear Durban.


Fig. 7.16 Analyses of (a) temperature and (b) wind at 925 hPa during interaction of tropi-cal cyclone ‘Bobby’ with an eastward-propagating midlatitude W’ly trough disturbance over theAustralian region, 12 GMT, 24 February 1995

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Chapter 7 Monsoon over Australia (Region – IV) Weng/course... · 2015-04-06 · Monsoon over Australia (Region – IV) 7.1 Introduction – Location and Physical Features Situated