70

THE CROSS-EQUATORIAL PRESSURE GRADIENT ANDSUMMER MONSOON RAINFALL IN NORTHERN AND

CENTRAL AUSTRALIA IN JANUARY 1974

The Flinders Institute for Atmospheric and Marine SciencesThe Flinders University of South Australia

Bedford Park, S.A. 5042

ABSTRACTThe occurrence of the Australian monsoon is examined in relation to the

cross-equatorial gradient which stretched across the equator between thewinter-hemisphere subtropical anticyclone and t he summer-hemispheremonsoon low pressure system situated over the Australian continent i nJanuary 1974. The pressure gradient is calculated in two parts, both parts ofthe same sign directed from north to south. The first part is expressed as thedifference in pressure between 25 'N and the equator, and the second part asthe difference in pressure between the equator and 25 'S. The day by daysequence of these gradients is compared with daily rainfall values recorded atselected stations i n north and central Australia i n January 1974. Thediagnostic results support the conclusions obtained from numerical trajectorycalculations, that a pronounced cross-equatorial gradient is favourable torainfall in the summer hemisphere. In particular, a strong pressure gradient inthe northern winter hemisphere directed southwards appears to be associatedwith substantial rainfall in the northern part of the area studied, between 11 'Sand 18 'S, whereas when a strong gradient extends further southwards into theinterior of the continent, heavy rainfall occurs between 18 'S and 25 'S, and itbecomes less rainy north of 18 'S.

INTRODUCTIONThe monsoon is commonly defined in terms

of a seasonal reversal i n direction o f theclimatically established mean vector w ind(Ramage, 1979). In this work the monsoon isinterpreted as the occurrence o f spells o fcontinuous rain, which although enhanced byconvection, i s primari ly triggered b y t h edynamics of the meridional flow. Examinationof climatic charts of mean sea level barometricpressure (Lockwood, 1974; Riehl, 1979) suggestthat there are only two major monsoon regionsin the world, the Asian and the Australian. In

A. H. Gordon

Weather and Climate (1986) 6: 70-76

these regions recognizable cross-equatorialpressure gradients exist. For example, in Julythe mean difference in pressure along the 60 "Emeridian between 20 'N and 20 'S latitude is 20mb, while in January along the 130 "E meridianbetween 20 'S and 20 'N i t is 16 mb. I t isprobably the vast zonal extent of the meridionalpressure gradient rather than its slightly greatermagnitude which results in the Indian monsoondeveloping a greater intensity than its southernhemisphere counterpart.

In Bombay the mean monthly rainfall in-creases from 18 mm in May to 483 mm in June

The Australian Monsoon

and 610 mm in July, while the cross-equatorialpressure gradient as defined above has reacheda mean value of between 2 and 3 mb per 5 ' oflatitude. In Darwin the mean monthly rainfallincreases from 12 mm in September to 385 mmin January.

Gordon and Taylor (1966, 1975), and Gordon(1967), computed trajectories f o r selectedperiods during which a cross equatorial pres-sure gradient occurred, stretching f rom thewinter hemisphere sub-tropical anticyclone tothe summer hemisphere monsoon trough or lowpressure region. The computations showed thattrajectories computed by the method describedtended to converge in the summer hemispherebetween 10 ' and 20 ' latitude, depending on thevalues of the initial wind, pressure gradient andthe frictional dissipation constant used. Afurther example, but on a shorter time scale,was quoted by Gordon (1973) covering the areaof Luzon in the Philippines. Some 800 mm ofrain fell in Manila during two days when apressure gradient o f 2 mb per 5 ' of latitudeextended f rom the winter t o the southernhemisphere.

In recent years attention has been directed tothe summer monsoon in Australia. Davidson,McBride and McAveney (1984) suggest that dayto day changes in tropical convective activityare physically linked to day to day changes inthe subtropical high cells and that the secret tothe understanding and prediction of changes inmonsoon activity on a day to day time scalemay lie in the understanding o f the detailedstructure o f t h e subtropical anticyclone.Davidson (1984) presented composite pressuremaps for clear and cloudy monsoon periods.From the maps he found that the cross-equatorial f l ow between 100"E and 140 "Elongitude is slightly stronger for the cloudyphase. The total pressure differences between20 'N and 20 'S along the 130 'E meridian were13 mb and 9 mb, respectively. I t should benoted tha t t h e period studied, December1978-January 1979, was not a well markedmonsoon such as January 1974.

ANALYSIS OF JANUARY 1974 PRESSUREGRADIENT AND RAINFALL DATA

The author was interested in examining i fthere was any day to day connection betweenthe meridional cross-equatorial gradient andrainfall which would support the trajectorycalculations already referred to in the previous

71

section. He paid a visit to Darwin and extractedpressure values f rom the original workingcharts fo r January 1974 for 5 ' intervals o flatitude and longitude between 25 'N and 25 'Sand 120 "E and 140 'E. A mean value was thencalculated for the belts 25 'N to the equator,and from the equator to 25 'S for an averagelongitude of 130 'E. I t is emphasised that boththe northern and the southern hemispheregradients were of the same sign, that is, directedfrom north to south. January 1974 was a monthduring which the Australian monsoon wasstrongly pronounced. It was an extraordinarilywet month in the centre of the continent where,for example, 500 mm fell at Vaughan Springs,some 350 km northwest of Alice Springs. LakeEyre was transformed into an inland sea in oneof its most spectacular fillings.

Fig. I shows the day to day variation in thepressure gradient as measured by the differencein pressure i n m b between 25 'N and theequator (solid line), and between the equatorand 25 'S (broken line), together with dailyrainfall totals, f o r three selected stations innorthern Australia. The vertical lines representdaily rainfall amounts in mm. The stationschosen are Snake Bay (11 '25'S, 130 "O'E) onthe northern fringe o f Australian territory,Ayers Rock (25 '20'S, 131 '4'E) in the centre ofthe continent, and Banka Banka (18 '48'S,134 '1'E), about half way between the formertwo stations. The percentages of rainfall in thetwo periods 1-18 January and 19-31 Januaryhave been weighted according to the number ofdays in each period.

It is clearly seen that the pressure gradientduring the first part of the month (1-18 Januaryinclusive) was greater in the winter hemispherewhile that during the latter part of the month(19-31 January) was greater in the summerhemisphere. It can also be seen that rainfall wasmore frequent and heavier at Snake Bay duringthe first part of the month than during the latterpart o f the month (with the exception o f 31January) and again more frequent at AyersRock during the latter part of the month thanduring the first part. The mid-way station,Banka Banka, shows a fairly even distributionof rain throughout the whole month.

The overall relation between the winterhemisphere and summer hemisphere pressuregradients and the rainfall i s shown morespecifically i n Fig. 2 . The ordinate shows

72 T h e Australian Monsoon

20 -

15 -

5

Apmb

20

15

10

JAN 1-18 7 8 %JAN 19-31 2 2 %

WEIGHTED

JAN 1 - 1 8 5 3 %JAN 19-31 4 7 %

WEIGHTED

15 2 0 2 5 3 0JANUARY 1 9 7 4

10 1 5 2 0JANUARY 1974

1 I 1

(a)

2 5

M

- 100

- 7 5

- 5 0

25

3 0

(c)

M

100

25

ApAlb

20 •

15

10

JAN 1-18 8 %JAN 19-31 9 2 %

WEIGHTED

5 1 0 1 5 2 0 2 5 3 0JANUARY 1 9 7 4

(b)

rnm

- 100

- 75

- 5 0

- 2 5

Fig. 1: Daily variation o f the meridional pressure gradientexpressed as the difference in mb between 25'N and the75equator (solid line) and between the equator and 25'S(pecked line). Daily rainfall in mm shown by the verticallines. Data for January 1974.(a) Snake Bay, 11'25'S(b) Ayers Rock, 25'20'S and(c) Banka Banka, 18'48'S.The percentages o f rainfall in the two periods have been5 0weighted according to the number of days in each period.

The Australian Monsoon 7 3

as

Station Latitude Longitudemm of rainin January

1974

1 Darwin 12'23'S 130'44'E 189.72 Snake Bay 11'25'S 130 '40'E 618.03 Minjilang 11'9'S 132'35'E 432.34 Elcho Island 12'2'S 135'34'E 379.75 Yirrkala Mission 12'15'S 136 '53'E 247.06 Port Keats Mission 14'14'S 129'31'E 671.07 El Sharana 13'31'S 132 '31'E 265.48 Mountain Valley 14'5'S 133'49'E 154.29 Larrimah 15'35'S 133'13'E 229.1

10 Borroloola 16 '41'S 136 '18'E 265.611 Timber Creek 15'39'S 130 '29'E 278.012 Rabbit Flat 20'13'S 1301'E 324.013 Banka Banka 18°48S 134'1'E 334.714 Avon Downs 20 '21'S 137'29'E 635.015 Jervois 22 '57'S 136 '9'E 240.716 Barrow Creek 21'32S 133'53'E 383.417 Vaughan Springs 22 '18'S 130 '51'E 502.018 Ayers Rock 25'20'S 131'4'E 242.019 Wave Hill 17 '27'S 130'50'E 308.2

10

2

: 1 5

20

25

25 50PERCENT

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Fig. 2: Variation with latitude in northern Australia of thepercentage of total rainfall in January 1974 which occurredwhen the meridional pressure gradient north of the equatorexceeded the meridional pressure gradient south o f theequator. The pressure gradient was directed from north tosouth in both hemispheres (high pressure to the north andlow pressure to the south).

degrees of latitude representing the meridionallocations of 19 stations (Table 1) at which dailyrainfall has been reported during January 1974.The abscissa represents the percent of the totalmonthly rainfall amount which occurred duringthe first part o f the month (1-18 January),weighted so that the distribution is referred toequal periods. There appear to be two narrowbelts of latitude at which rainfall was greatestduring the first part o f the month, at about11 'S and 16 'S. Rainfall was a maximum duringthe latter part of the month south of 20 'S.

TRAJECTORY COMPUTATIONSIt was thought that i t would be useful to

check if there were any dynamic reasons whichwould explain the profile exhibited in Fig. 2 andalso the broad results shown in Fig. 1. A meansof doing this was to compute trajectories of airparcels starting from different latitudes and seewhere they went. I f there was significant con-vergence of the trajectories it would be expectedthat vertical mot ion would occur i n theconvergent area, causing precipitation. Theconvergence o f trajectories moving from thewinter hemisphere and crossing the equatorinto t h e summer hemisphere under t h einfluence o f a continuous pressure gradientdirected f r o m t h e winter t o the summer

TABLE 1: LIST OF STATIONS FROM WHICH DAILYRAINFALL AMOUNTS HAVE BEEN OBTAINED.

hemisphere has already been discussed anddocumented. T h e mathematical techniqueinvolving the solution o f a pair o f ordinarydifferential equations, and the stepwise timeiteration o f the resulting algebraic equationshave been described i n detail (Gordon andTaylor, 1966, 1975). The method shows theimportance o f the variation o f the Coriolisparameter with latitude, the beta effect, ontypical monsoon rainfall situations. Analyticsolution o f t h e problem has a lso beeninvestigated (Byron-Scott and Gordon, 1985).

In Fig. 3 three idealized cases have beenstudied, simulating typical meridional pressuregradients corresponding to the cases shown inFig. 1 . T h e vertical l ines represent t h emeridional extent o f t h e trajectories a smeasured along the ordinate. The abscissa, ineffect, represents different starting latitudesfrom 20 'N to 20 'S. Thus 9 trajectories havebeen considered starting from 9 different lati-tudes. The pressure gradient is assumed to bezonal as is, o f course, the Coriolis parameter.Thus t h e trajectories a r e longitudinallyindependent, a reasonable assumption for the

74

INITIAL L AT I T U D E INITIAL LATITUDE20 20(a) (b)8

15 z 15 110 10

5 5

00

a 0

11:-4 5 5

10 10

15 •5 15

20 1 20

25 255

width o f longitude studied. The presence o fmonsoon troughs or low pressure centres wouldnaturally ampl i fy t he convergence i n t hepreferred locations. The horizontal lines aredrawn normal to the end points o f the tra-jectories so that the closeness of the horizontallines relative to their initial distance apart of 5of latitude, wi l l represent a measure o f con-vergence. In Fig. 3(a) a pressure gradient of 4mb per 5 ' of latitude has been assumed northof the equator and a pressure gradient of I mbper 5 ' of latitude south of the equator. Con-vergence is displayed by the density of the hori-zontal lines occurs at about 12 'S and at about17 'S. This result agrees quite well with therainfall maxima shown in Fig. 2 for the firstpart of the month when the pressure gradient isgreater north of the equator than south of theequator. Fig. 3(b) shows the case where thepressure gradients f o r t h e northern a n dsouthern hemispheres have been reversed. I nthis case strong convergence occurs furthersouth. Fig. 3(c) shows the case where thepressure gradient is 4 mb/5 ' of latitude in both

hemispheres. In this case convergence is spreadout between about 18 'S and 25 'S latitude andis not so tightly concentrated as in Fig. 3(b).These particular cases were computed f o rfrictionless motion and f o r parcels startingfrom rest. However, t h e introduction o ffriction i n t h e boundary layer i n t o t h eequations and the use of some sub-geostrophicinitial velocities greater than zero do not affectthe overall result of convergence, but do shiftthe belts marginally. See Gordon and Taylor(1966) for discussion o f relationship betweenconvergence of the trajectories and consequentconvergence of airflow in the boundary layerand vertical motion.

Fig. 4 shows a typical family of trajectoriesfor the case of frictionless motion where parcelsstart with an easterly velocity of 5 m sec. Thelatter velocity is almost geostrophic at 25 'N fora pressure gradient of 2 mb per 5 ' of latitude,but sub-geostrophic f o r latitudes nearer theequator. Convergence occurs between 100 and15 'S. Trajectories f o r the exact conditions

Fig. 3: Meridional extent of trajectories start ng every 5' latitude from 20W to 20'S.(a) pressure gradient of 4 mb/5' latitude north o f the equator and I mb/5' latitude south o f the equator.(b) pressure gradient of I mb/5' latitude north of the equator and 4 mb/5' latitude south o f the equator.(c) pressure gradient o f 4 mb/5' latitude in both summer and winter hemispheres.

The Australian Monsoon

INITIAL LATITUDE

The Australian Monsoon

LONGITUDE

Fig. 4: Computed trajectories in the Central Pacific for theconditions shown. Initial winds were 5 m sec-1 from theeast; 12-hour ly pos i t ions a re ind icated b y sma l l opencircles. T h e zonal pressure gradient was very smallthroughout the area of interest. (After Gordon and Taylor.1966).

given in Fig. 3 show very similar patterns tothose in Fig. 4. For an in-depth discussion ofthe dynamical beta effect on trajectories in thetropics, see Gordon (1985).

Fig. 5 shows the mean topography o f the1000 mb surface f o r January 1974 fo r theAustralasian region. A difference of 60 m isequivalent to about 8 mb. It is noted that thereis a cross-equatorial gradient o f the contoursbetween the winter sub-tropical high and thecontinental low. Although the presence o f amonsoon depression over the continent mayhave helped to induce rainfall, the theoreticalcalculations referred t o indicate tha t t hemeridional pressure gradient can explain heavyrainfall i n the summer hemisphere tropicswithout the need t o invoke zonal pressuregradients.

CONCLUSIONSThe results of the diagnostic and numerical

analysis o f the relation between monsoon

75

Fig. 5 : Mean topography fo r the 1000 mb surface forJanuary 1974 for the Attstralasian region.

rainfall a n d a cross-equatorial winter t osummer hemisphere pressure gradient duringJanuary 1974 in the Australian region supportprevious work on the subject for other regions.Although i t is not useful to relate individualdays, periods o f two to three weeks duringwhich pressure gradients continuously straddlethe equator relate positively to the occurrenceof substantial monsoon-type rainfall i n thesummer hemisphere. The belts o f latitude atwhich rain occurs depend upon the strength ofthe pressure gradient itself, and also whetherthe gradient i s stronger i n t h e w in te rhemisphere than in the southern hemisphereand vice-versa. Broadly speaking, a strongergradient in the winter hemisphere causes a beltof rainfall at about 12 'S, whereas i f a stronggradient penetrates deeply into the summerhemisphere the belt o f heavy continuous rainshifts to about 20 '-25

ACKNOWLEDGEMENTSThe author is indebted to Dr. Geoffrey Love,

Bureau of Meteorology, Darwin, N.T. for hisassistance in providing access to the workingcharts for the period studied.

REFERENCESBryon-Scott, R. A . D . and A . H . Gordon, 1985: The

critical trajectory for cross-equatorial flow and onset ofthe Australian Summer monsoon. Extended abstracts ofthe Second ABRMS Australian Conference on TropicalMeteorology, Perth, W.A. pp. 9-11.

Davidson, N . E. , 1984: Short-term fluctuations in the

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Australian monsoon during winter MONEX, MonthlyWeather Review, 112, 1697-1708.

Davidson, N. E., J. L. McBride and B. J. McAveney, 1984:Divergent circulations during the onset of the 1978-79Australian monsoon, Monthly Weather Review, 112,1684-1696.

Gordon, A . H. , 1985: Lagrangian interpretations o f at-mospheric motion. Ph.D. thesis. Flinders University ofSouth Australia.

Gordon, A . H. , 1967: A Lagrangian approach to prob-lems in tropical meteorology, Weather, 22, 11, 455-468.

Gordon, A. H., 1973: The Great Philippine floods of 1972,Weather, 28, 404-415.

Gordon, A . H . and R . C . Taylor, 1966: Lagrangian

The Australian Monsoon

dynamics and low latitude weather, H.I .G. Report,66-12, Honolulu, Hawaii, 32 pp.

Gordon, A. H. and R. C. Taylor, 1975: Computations ofsurface layer air parcel trajectories and weather in theoceanic tropics. International Indian Ocean ExpeditionMonographs, 7 , East-west press, Honolulu, Hawaii,112 pp.

Lockwood, J . , 1974: W o r l d Climatology, A rno ld ,London, 330 pp.

Ramage, C. S., 1971: Monsoon Meteorology, AcademyPress, London and New York, 296 pp.

Riehl, H . , 1979: Climate and Weather in the tropics,Academic Press, London and New York, 611 pp.