T h e 'physical cause' proposed by Halley was essentially the difference b e t w e e n
the surface temperatures
of land
and
ocean, coupled with the spin o f the earth.
W i n d s , h e suggested, were c a u s e d by air
rising over hot land in s u m m e r , and their
direction was i n f l u e n c e d by the p l a n e ť s
rotation — t h e C o r i o l i s effect.
H a l l e ' s ideas still hold good, and they
a c c o u n t for p e r h a p s 9 0 % o f a m o n s o o n ' s
structure. Unfortunately, they are m u c h
t o o g e n e r a l i z e d to e x p l a i n w i n d s a n d r a i n s
i n detail. E v e n now, hundreds of years and
millions
of
research
dollars
later,
m e t e o r o l o g i s t s are j u s t b e g i n n i n g to work
out the fine tuning that determines mon­
s o o n w e a t h e r day b y day.
Seasonal weather
T h e w o r d ' m o n s o o n ' is u s e d i n s e v e r a l
ways. It possibly c o m e s from a n
Arab
w o r d m e a n i n g ' s e a s o n ' , a n d it c a n b e d e ­
fined as a period o f heavy s u m m e r rainfall
Last July and August, as much of Australia prepared for
a dry, dusty spring during one of the most severe
droughts on record, the River Ganges, swollen by the
monsoon rains, burst its banks.
i n a c o u n t r y w i t h dry w i n t e r s . A n o t h e r
definition hinges o n a reversal o f wind
direction t h r o u g h about 180° t w i c e a year.
B o t h definitions fail to d e s c r i b e t h e par­
ticular idiosyncracies of classic
mon­
soons; seasonal wind reversals even occur
i n M e l b o u r n e ! I t s e e m s best to restrict t h e
In India's most populous State, Uttar
fed l i o n w i l l i g n o r e p o t e n t i a l p r e y , a n d a
word to w e a t h e r patterns that satisfy b o t h
Pradesh, already ravaged by floods, t h e
m o n s o o n will bring rain only on certain
definitions. T h e s e form two major mon­
death toll passed 1 0 0 0 .
days.
At s u c h times the two countries s e e m
soon
T h e q u a n t i t y o f i n f o r m a t i o n r e q u i r e d to
systems: the
African
and
the
Asian—Australasian.
worlds apart, yet the I n d i a n m o n s o o n is
understand weather systems is e n o r m o u s ,
I n s u m m e r , the sun warms both land
part o f a w e a t h e r s y s t e m t h a t a l s o b r i n g s
but s o m e o f the key ideas date b a c k c e n ­
and sea, but the land surface b e c o m e s
Australia its c y c l o n e s — like T r a c y ,
turies. I n 1 6 8 6 the British scientist Ed­
hotter than the surface o f the ocean. T h i s
w h i c h d e v a s t a t e d D a r w i n at C h r i s t m a s
m u n d H a l l e y , w h o gave h i s n a m e to the
is b e c a u s e h e a t diffuses d o w n t h r o u g h t h e
1 9 7 4 , and Kerry, whose destructive and
w e l l - k n o w n c o m e t , contributed a paper to
l a n d o n l y s l o w l y , b u t it i s c a r r i e d b e l o w
wayward b e h a v i o u r is described on p a g e
t h e Philosophical
t h e surface o f the sea by vertical c u r r e n t s .
17.
Royal
T h e r e is little w e c a n do to c h a n g e the
Society
Transactions
of
the
under the title ' A n Histori­
cal A c c o u n t of the T r a d e Winds,
v e n t s t h e w a t e r c o o l i n g a s r a p i d l y as t h e
land, a n d so it is the sea that is the w a r m e r .
both
M o n s o o n s , observable in the S e a between
hemispheres would benefit enormously
and near t h e T r o p i c k s , with an attempt to
from accurate forecasting o f both the vio­
assign the Physical cause o f the
lent storms that flatten h o m e s and
Winds'.
weather,
but
people
living
vital rains on w h i c h m u c h
in
the
of south­
eastern Asia's rice crops, and therefore its
h u m a n populations, depend.
T h e m e t e o r o l o g i s t trying to
predict
weather is like a zoologist studying
a
species o f wild animal. H e begins in the
field, w a t c h i n g , recording, and
making
c o u n t l e s s o b s e r v a t i o n s . B a c k at h i s d e s k
h e sifts h i s a c c u m u l a t e d
information,
l o o k i n g for r e c u r r i n g patterns that will
transform shapeless heaps of unrelated
facts i n t o orderly r a n k s governed by rela­
tively few major ideas.
O n c e h e is familiar with the patterns,
t h e s c i e n t i s t c a n predict h o w a n a n i m a l or
a m o n s o o n will behave, provided
he
k n o w s its detailed c i r c u m s t a n c e s : a well-
18
I n winter, t h e o c e a n ' s stored h e a t pre­
and
same
The
sea's slow temperature
changes
have a n o t h e r effect: they delay the sea­
s o n a l w e a t h e r c y c l e , p u t t i n g i t 6— 8 w e e k s
T h e s u m m e r w e a t h e r in e a c h h e m i s p h e r e is greatly influenced by winds f r o m t h e o t h e r
h e m i s p h e r e . A s t h e y c r o s s t h e e q u a t o r t h e s e winds t u r n t h r o u g h a b o u t 9 0 ° . T h e s h a d e d
a r e a is t h e region o f m o n s o o n a l r a i n s .
S c i e n t i s t s d e b a t e t h e r e a s o n for t h i s dif­
ference. O n e likely explanation is that the
by D r Peter Webster o f the D i v i s i o n o f
Southern Hemisphere lacks a land mass
A t m o s p h e r i c P h y s i c s . H e spends m u c h o f
the size o f A s i a , w h i c h draws air farther
his time building and testing mathemati­
from the equator. Another, m o r e conten­
cal models of m o n s o o n s — models that
tious, hypothesis is that the
incorporate his ideas on the
northern
m o n s o o n is partly i n f l u e n c e d by
H i m a l a y a s , w h i c h a c t as an
the
elevated
source of heat. T h i s huge mountain range
In J a n u a r y , m o i s t u r e - l a d e n winds bring
r a i n t o n o r t h e r n A u s t r a l i a . In J u l y t h e
c i r c u l a t i o n i s r e v e r s e d a n d air d e s c e n d s ,
o v e r n o r t h e r n A u s t r a l i a , w a r m i n g u p a s it
d o e s s o a n d t h e r e f o r e inhibiting r a i n .
Australia's tropical north, is co-ordinated'
has no southern counterpart.
underlying
recurrent patterns and that, i f successful,
could eventually help achieve accurate
forecasting.
J u s t as zoologists m a k e e x p e d i t i o n s to
A u s t r a l i a ' s ' t o p e n d ' i s f a m o u s for i t s
observe their subjects in the wild, so D r
summer rains, but these amount, in a
Webster enjoys monsoon-watching in the
s e n s e , to other c o u n t r i e s ' left-overs. I n
field. H e participated i n a n d h e l p e d to
particular, the high mountains and warm
organize two international m o n s o o n e x ­
seas o f Indonesia interrupt the m o n s o o n
p e r i m e n t s ( M O N E X for s h o r t ) , o n e i n
climax o f the summer 'weť in northern
o n its j o u r n e y south, trapping m o s t o f the
D e c e m b e r 1 9 7 8 to F e b r u a r y 1 9 7 9 c e n t r e d
A u s t r a l i a c o m e s n o t at t h e s o l s t i c e o n
rain.
b e h i n d the solar cycle. T h i s is why the
about D e c e m b e r 2 1 , but in January and
on the S o u t h China S e a , and the other the
I n any case, the Australian
monsoon
following July studying the northwards
m o n s o o n in the Bay o f Bengal.
February. T h e r e is a similar t i m e lag in
delivers less rain than does the A s i a n
the Northern Hemisphere: south-eastern
m o n s o o n blowing in the other half of the
B o t h t h e s e r e s e a r c h efforts f o r m e d p a r t
A s i a e x p e r i e n c e s its h e a v i e s t m o n s o o n
year. T h i s is b e c a u s e the northern m o n ­
o f the G l o b a l Weather E x p e r i m e n t (see
soon crosses larger expanses of ocean on
Ecos
its j o u r n e y to south-eastern A s i a .
s u r e m e n t s u s i n g aircraft, b u o y s , ships,
rains in late July and August.
W h y does the m o n s o o n bring rain? A s
1 8 ) and involved co-ordinated
mea­
t h e l a n d h e a t s u p i n s u m m e r , it h e a t s t h e
Indian monsoons and s u m m e r rains in
and radar contributed by various nations,
air above it, w h i c h b e c o m e s less dense and
northern Australia both belong, therefore,
including the United States of America
therefore rises.
t o o n e v a s t s y s t e m o f air c u r r e n t s stradd­
and the Soviet U n i o n .
T h e vast body o f information
from
T h e n , as H a l j e y p u t it, ' i t i s n e c e s s a r y
ling the equator and linking whole con­
that the c o o l e r and m o r e d e n s e air should
tinents and o c e a n s . F o r accurate forecast­
t h e s e two intensive field e x e r c i s e s and
r u n thither to restore the e q u i l i b r i u m ' . S o
ing, scientists need information
from other sources, including the geo­
an onshore wind, which may have a fetch
w e a t h e r d e t a i l s o v e r as m u c h a s p o s s i b l e
stationary meteorological satellites (see
of many thousands
of the monsoon's range — horizontal and
p a g e 1 6 ) , is grist to t h e m o d e l l i n g m i l l .
o f k i l o m e t r e s , de­
velops up to an altitude of about 1 0 0 0 m.
about
W h e n h e b e g a n his study o f m o n s o o n s ,
vertical.
T h i s air, b r i n g i n g water vapour that has
D r Webster set about understanding the
International research
large-scale phenomena, and particularly
its
S e v e r a l c o u n t r i e s a r e c o l l a b o r a t i n g to c o l ­
the t i m i n g o f the rains. I n their studies o f
water-holding capacity drops. T h e e x c e s s
lect s u c h i n f o r m a t i o n , a n d CSlRO's c o n ­
the interactions between land and sea,
water c o n d e n s e s and falls as rain. T h e
tribution is a tropical m e t e o r o l o g y pro­
Halley and subsequent m o n s o o n model­
a t m o s p h e r i c c i r c u i t is c o m p l e t e d by air
gram involving the Division of Atmo­
lers had considered only the influence o f
returning to the winter h e m i s p h e r e
spheric Physics, w h i c h concentrates on
the oceans south of Asia, but D r Webster
theoretical work, and
Australian
found t h e o c e a n to t h e east (the northern
D u r i n g the Australian winter, air is ris­
Numerical Meteorology Research Centre
P a c i f i c ) to b e a n e q u a l l y i m p o r t a n t c a u s e
i n g , and u n l o a d i n g its water, over south­
( A N M R C ) , which carries out analyses o f
o f the observed lag between solstice and
eastern A s i a . T h e air sinks over Australia;
c o l l e c t e d observations. T h e A N M R C is
monsoon.
as it does s o , it u s u a l l y w a r m s up and c a n
r u n j o i n t l y b y CSIRO a n d t h e D e p a r t m e n t
comfortably h o l d all the water vapour it
of S c i e n c e and T e c h n o l o g y .
evaporated from the sea, i n turn rises over
the land. A s it r i s e s , it c o o l s and
at
about 12 0 0 0 m.
meets.
T h i s is w h y winter rains are rare i n
tropical Australia. C o l d fronts m a y bring
rain to the south and c e n t r e o f t h e c o n t i ­
n e n t , b u t t h e n o r t h s t a y s dry.
Reverse cycle
T h e w h o l e h u g e c y c l e o f air revolves in
the opposite direction in our
summer.
N o w the winds blow towards
Australia at l o w level. T h e
northern
Australian
m o n s o o n is n o t an e x a c t m i r r o r - i m a g e o f
the Indian one, however, and a
suggests why. I n the Northern
map
Hemi­
sphere m o s t o f t h e m o n s o o n rain falls
between latitudes 3 0 ° and 4 0 ° , but
the
southern m o n s o o n chiefly drenches the
region within 20° o f the equator.
the
H e then tackled the mystery o f the 'ac­
tive' a n d ' ' b r e a k
5
periods within a mon­
T h e program, w h o s e a i m is the de­
s o o n . A n active period lasts for perhaps a
v e l o p m e n t o f forecasting t e c h n i q u e s for
fortnight and includes a cluster o f about
I n s u m m e r h o t land ' p u m p s ' a i r u p w a r d s , w h e r e it c o o l s and d r o p s rain. T h e s o a k e d
g r o u n d is c o o l e d b y e v a p o r a t i o n , a n d s t o p s 'pumping' until it h a s dried o u t .
S c i e n t i s t s do n o t yet fully
understand
cold surges. It s e e m s that the air pressure
gradually builds up over S i b e r i a before
suddenly falling and expelling a
huge
c o o l southerly gust. S u r g e s are hard to
predict, a n d at p r e s e n t s c i e n t i s t s u s e t w o
main clues.
A surge is always preceded by a rise in
p r e s s u r e o v e r S i b e r i a , a n d by t h e p a s s a g e
o f a trough from E u r o p e or a n o t h e r m i d latitude region. Forecasters in
Hong
K o n g k n o w that a surge will reach t h e m
3—4 days after a t r o u g h a r r i v e s o v e r L a k e
Baikal.
D u r i n g our winter, similar (but weaker)
it easier to predict the o n s e t a n d end o f
cold
a c t i v e p e r i o d s t h a n t o say w h e t h e r
Chinese meteorologists use
surges
head
northwards,
and
ralia's m o n s o o n brings a s u m m e r
η t o t h e S o u t h Alligator R i v e r r e g i o n ,
hern Territory.
w i l l fall o n a n y p a r t i c u l a r day w i t h i n a n
s u r g e s to h e l p p r e d i c t r a i n f a l l i n
ϊ or four 'disturbances', e a c h of w h i c h
active period. I n p r a c t i c e this is n o t too
monsoon. D r Webster thinks Australian
gs r a i n . A b r e a k i s a dry s p e l l b e t w e e n
important; it is the 2 - w e e k forecasts that
forecasters m a y eventually b e able to u s e
e m o n s o o n periods.
matter to farmers, n o t the 2 4 - h o u r o n e s .
S i b e r i a n s u r g e s t o p r e d i c t at l e a s t s o m e o f
G e r m i n a t i n g rice can tolerate short inter­
our s u m m e r rainfall.
r
rain
Australian
their
ι the rains are intermittent
vals b e t w e e n downpours, but i f the crop is
:curred t o D r W e b s t e r t h a t w h e n r a i n
s o w n j u s t before a break period it m a y w e l l
M O N E X f i e l d w o r k g a v e h i m t h e i d e a for
the wet ground below gradually cools
fail.
a model o f surge formation, i n w h i c h an
rater
evaporates from its surface. A
T h e Australian m o n s o o n is more c o m ­
Information
collected during
incoming trough
the
interacts with a jet
mountains,
stream between the Himalayas and Japan
tped b y h o t l a n d , a n d a s t h e l a n d c o o l s
forests, and warm seas o f Indonesia c o m ­
at a b o u t 1 0 0 0 0 m . T h e t r o u g h shifts and
he p u m p slows down.
plicate
a
accelerates the j e t stream eastwards until
cross-equatorial flow o f
it is so p o s i t i o n e d that it e x p e l s from over
soon is like a current o f air
For a
being
while
plex than the Asian. T h e
what
would
otherwise
be
e is a break in the rains, until the land
straightforward
dried o u t s u f f i c i e n t l y t o r a i s e i t s t e m p -
air, and tropical Australia is d o w n w i n d o f
S i b e r i a c o l d air that h a s slowly b u i l t u p in
ure a n d r e s t a r t t h e p u m p ,
Indonesia. D r Webster needs a substan­
the preceding days.
r Webster developed a mathematical
tial body o f meteorological information,
lei f o r t h e s o u t h - e a s t e r n A s i a n m o n -
including analyses of satellite records, be­
scale' pattern o f m o n s o o n weather
l incorporating this idea, and a c o m -
fore h e c a n c o n s t r u c t a model o f northern
has so great an i n f l u e n c e on agriculture,
:r t o l d h i m w h a t r a i n p a t t e r n t h e m o d e l
Australia's summer weather.
Id c r e a t e . T h e a n s w e r w a s a s e r i e s o f
ire p e r i o d s a b o u t
1 0 — 2 0 days
apart,
that
a n d t h i s i s w h y s c i e n t i s t s a r e k e e n t o de­
S u c h a model would benefit forecasting
in other parts o f Australia, too.
C o l d surges form part o f the ' m a c r o -
The
v e l o p a t e c h n i q u e for f o r e c a s t i n g t h e m .
T h e r e i s a n o t h e r r e a s o n for w a n t i n g to
i r a t e d b y dry b r e a k s ,
northern 'weť extends well south in some
understand surges: they s e e m to b e c o n ­
ach active period was caused by a
years, s u c h as 1 9 7 4 , w h e n the c e n t r e o f t h e
nected with the formation of tropical cyc­
hwards-moving, rain-bearing cell o f
c o n t i n e n t r e c e i v e d t o r r e n t i a l r a i n s t h a t fil­
l o n e s . D r J o h n M c B r i d e o f A N M R C is
w h i c h gradually w e a k e n e d as it travel-
led L a k e Eyre.
using
farther
inland
and crossed increas-
W h a t ' s m o r e , the m o n s o o n influences
y c o o l e r l a n d at h i g h e r latitudes. T h e
weather patterns in the southern coastal
I soaked by a cell's rain gradually dried
districts where most Australians live, and
statistical analysis to test
the
strength o f this connection.
W h a t triggers a cyclone?
warmed up, heating the lower atmo-
s o m e o f the unpredicted rains in this reg­
Australia lies in o n e o f the world's four
;re a n d c r e a t i n g a fresh s u r g e o f m o i s t
i o n s e e m to b e associated w i t h the rapid
major tropical cyclone regions, the others
r o m t h e o c e a n . T h i s w i n d d e v e l o p e dm o v e m e n t o f m o i s t a i r o u t o f t h e t r o p i c s .
a new northwards-moving
cell.
7hile D r W e b s t e r and D r L a n g C h o u
T h e Siberian connection
Our
weather
is
further
affected
by
:he N a v a l P o s t - g r a d u a t e
School
at
nterey,
writing
a
p h e n o m e n a called cold surges. T h e s e are
California, were
they
pulses o f c o o l air that enter the m o n s o o n
rd o f a d e t a i l e d s t u d y o f a i r c i r c u l a t i o n
flow; during the Australian s u m m e r they
ing t h e A s i a n m o n s o o n by D r K . S i k k a
take the form o f bursts o f c o o l air pouring
inical paper
about the model,
D r S. Gadgil, o f the Indian Institute
Science. T h e s e on-the-spot
IS m a t c h e d t h e c o m p u t e r
observa-
out o f n o r t h e r n A s i a , w e a k e n i n g as they
travel south.
predictions
H o n g K o n g e x p e r i e n c e s a b o u t four or
1 e n o u g h to satisfy D r W e b s t e r a n d D r
five c o l d s u r g e s a m o u t h , e a c h c a u s i n g t h e
3U t h a t t h e y w e r e o n t h e r i g h t t h e o r e t i -
temperature to drop by up to 1 0 ° C for 1 - 2
lines .
days, but only s o m e o f the surges r e a c h
Ising t h e p r i n c i p l e s o f t h e Webster—
Australia, where they usually have a less
au m o d e l , m e t e o r o l o g i s t s s h o u l d f i n d
d r a m a t i c effect.
0 d
being the north-western Pacific, areas o f
the Indian O c e a n , and the North Atlantic.
C l o u d s o v e r t h e S o u t h C h i n a S e a during
the M O N E X study.
M o s t tropical cyclones form within about
10° o f the equator, and the activity o f
A u s t r a l i a n c y c l o n e s is c o n c e n t r a t e d
on
two areas: in the Coral S e a and off the
north-west shelf.
A s yet meteorologists
cannot'explain
what triggers a c y c l o n e but, as D r W e b s t e r
p u t s it, ' t h e m o n s o o n i s t h e e n v e l o p e i n
w h i c h c y c l o n e s reside', and so w e m u s t
u n d e r s t a n d the m o n s o o n i f we are to pre­
dict cyclones.
D r M c B r i d e ' s studies o f a large n u m b e r
of
Northern
Hemisphere
cyclones
suggest that certain configurations o f the
large-scale tropical flow o f air s e e m to
increase their likelihood. I f the analysis of
Southern Hemisphere cyclones confirms
this idea, then accurate forecasting o f the
Australian
monsoon will become ex­
tremely important — the m o n s o o n deter­
mines these configurations.
Weather
forecasting
is
based, on
m a t h e m a t i c a l m o d e l s that e m b o d y
past
experience. T h e computer analyses the
current meteorological situation in
the
light o f t h e m o d e l . W h y , t h e n , a r e f o r e ­
c a s t s sometimesKinaccurate? W a s H a l l e y
right w h e n h e wrote nearly 3 0 0 years ago
'there is n o g e n e r a l rule but admits o f
some exceptions'?
In a sense he was, but scientists believe
the exceptions can be explained too. T h e
model
must
be
refined.
Australian
weather-forecasting models ignore events
north o f the equator.
T h i s approach reflects the former belief
that the equator was a barrier between two
independent
weather systems.
Studying
m o n s o o n s h a s c h a n g e d all that. I n
the
world's m o n s o o n regions, one o f w h i c h
embraces Australia, the weather is con­
siderably d e t e r m i n e d by events t h e other
side o f the equator. ' W e n e e d ' , D r W e b s t e r
urges, 'a global model.'
I f s u c h a m o d e l is to improve weather
forecasting i n Australia's temperate reg­
i o n s a n d s o r t o u t t h e day-to-day v a g a r i e s o f
the A u s t r a l i a n m o n s o o n , it m u s t be b o t h
c a r e f u l l y b u i l t a n d t h o r o u g h l y 'fed' w i t h
meteorological observations. Scientists
h o p e that satellites will help to provide the
large body of information they need. T h e
other requirement, careful
construction
o f the m o d e l , is the m a i n a i m o f the tropi­
cal meteorology program.
John
Seymour
M o r e about the topic
M e c h a n i s m s affecting the state, evolu­
tion, and transition of the
planetary
o
t
scale monsoon. P.J. Webster,
Lang
t
C h o u , a n d K a M i n g L a u . Pure
and
t
Applied
Geophysics,
1977, 1 4 6 3 - 9 1 .
21