The weather and climate of the tropics: Part 6 - Monsoons
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The weather and climate
of the tropics:
Part 6 – Monsoons
J. F. P. Galvin fall, such that (ii) southwesterly winds bring troposphere, forming the Equatorial Easterly
Weather – May 2008, Vol. 63, No. 5
a moist flow and copious rainfall across Jet stream (EEJ), centred over the southern
Met Office, Exeter the Indian subcontinent. This by no means Bay of Bengal. The EEJ may extend from
Seasonal monsoon circulations of the describes the progress of the Asian summer the Philippines to the Gulf of Guinea at its
atmosphere are seen over all the tropical monsoon, however. The main complexity maximum extent and usually lies between
continents (Figure 1). The most significant arises first from the distribution of moun- about 10 °N and the equator. It forms the
is that over Asia, although west Africa, sou- tains across the subcontinent, secondly from southern limb of an upper atmospheric cir-
thern Africa, northern Australia and parts the various water masses of the Indian Ocean culation around a warm dome, usually cen-
of South America are also dependent on and thirdly from the broad-scale dynamics of tred over Tibet. During the month, humid,
summer monsoons for the majority of their the atmosphere. but predominantly dry, oppressive weather
annual rainfall. Initially the northern branch of the inter- sets in across southern Asia (Figure 2(a)).
Monsoons are generally characterized by tropical convergence zone (ITCZ) streng- Afternoons can be very hot under almost
a reversal of winds near the surface twice thens over the equatorial Indian Ocean. cloudless skies, in particular over central
per year and the development of large- During April, moist southwesterly winds and northern India with maximum tempera-
scale deep circulation systems (Figure 2). set in at low levels south of it and the sub- tures often exceeding 45 °C. By this time,
As a result, for most places within the area tropical jet stream (STJ) moves northwards, the southwesterlies have an origin in the
of the monsoon circulation, one season is eventually well to the north of the Himalaya southern hemisphere as south-east Trade
wet (summer) and the other dry (winter). (Galvin, 2007b). Winds (as described in Parts 1 and 2: Galvin,
There are many variations, however, and In May, the surface of the Arabian Sea and 2007a, b).1 As this flow becomes established,
this generalization hides important local Bay of Bengal becomes very warm (>29 °C), outbreaks of deep convection develop in
weather variations. allowing convection to spread northwards Nepal, Bhutan, the Indian states of Assam
and the ITCZ to become diffuse. During and Arunchal Pradesh, at times spreading
the month, Thailand and Burma see wet across the Gangetic plains and Bangladesh.
The summer monsoon over weather as the moist southwesterlies deep-
southern Asia en. Over much of India, however, it is only 1
There is a temperature inversion associated with
Over southern Asia in summer, the distor- after sufficiently deep southwesterly winds the Mascarene high during the winter season.
tion of the line of surface convergence of in the lower troposphere can become estab- This provides a ‘lid’ to convection over the south-
western tropical Indian Ocean. The Mascarene
the Trade Winds (the meteorological equa- lished that the summer monsoon sets in.
Islands include Mauritius, La Réunion and the
tor: ME) extends thousands of kilometres Before the south-west monsoon becomes
Seychelles; they lie close to the equator in the
north of the equator. In theory, the conver- established, easterlies must strengthen aloft western Indian Ocean. Relatively high pressure
gence line forms the leading edge of the in response to the warming and deepen- is seen in this area, in particular at medium and
south-west monsoon, but in truth the story ing of the troposphere over southern Asia high levels, during the northern winter, associa-
is much more complex. (Galvin and Lakshminarayanan, 2006). These ted with mass descent. This high must move
Elementary accounts of the monsoon state reach jet-stream strength (>40 ms−1) near south before the Asian summer monsoon can
that: (i) the heating of Asia causes pressure to the top of the warmed and thus deepened become established.
Figure 1. Land areas that have the majority of their rainfall in summer, associated with the poleward motion of deep convection. Where appropriate,
low-level wind directions that carry moist warm air are indicated. In areas where there are no arrows, winds are relatively dry, or are weak
(as over South America). Shaded areas show the normal maximum extent of deep convection (where 850-hPa θw ≥ 22 °C). 129
The weather and climate of the tropics – Part 6
(a) (b)
Weather – May 2008, Vol. 63, No. 5
(c) (d)
Figure 2. The progress of the Asian monsoon. Streamlines indicate the direction and, by their spacing, the strength of mean monthly surface wind; colours
indicate mean monthly air temperature at 1000-hpa (approximately 100 m a.s.l.), ranging from white:
The weather and climate of the tropics – Part 6
Weather – May 2008, Vol. 63, No. 5
Figure 3. (a) Cross-section of the summer monsoon flow over the Indian Ocean and south Asia between 2 °N and 32 °N from radiosonde profiles
at 1200 UTC on 15 July 2006. The tropopause is shown ; the top of the low-level moist zone ; and the level of the top of westerly winds .
(b) Corresponding 500-hPa height with rainfall data for June–September adapted from Rao (1976).
Although the moist monsoon flow is ulti- spreads across India from the south-east.3 explain the progress of the monsoon than
mately blocked by the Himalaya, heating of Hence, we need a different mechanism to the simple motion of moist southwesterly
the Tibetan plateau causes a diurnal cycle winds. Indeed the moist flow across India
3
of winds along valleys crossing the moun- It may seem strange that the south-west mon- is partially blocked by the Western Ghats,
tain chain. Daytime anabatic winds may soon is often associated with southeasterly the mountains that mark the west coast
become particularly strong, locally reaching winds. Prior to the MONEX experiments of the of India (Patwardhan and Asnani, 2000).
1970s, it was generally assumed that monsoon
gale force along valleys such as that of the There is copious summer rainfall on the
rains advanced on predominantly southwesterly
Sun Kosi in Nepal (Nigel Bolton, personal western slopes of these mountains, but
winds, as indicated by the climatological average
communication). These valleys are deep, pressure gradient across south Asia in summer.
the area to their lee remains largely dry, in
incised several thousand metres into the As described in the text, however, it is necessary a rain shadow, the southwesterly air flow
mountain barrier. for winds from other directions, predominantly having lost some of its humidity. In order
Despite the evident northward progress the south-east, to become established for suffi- to bring monsoon rains across much of
of the summer monsoon, its advance is ciently moist low-level air to advance across both the rest of India, two mechanisms appear
not straightforward and rainfall usually India and East Asia. important. 131
northern India and Nepal, into southern
The weather and climate of the tropics – Part 6
Pakistan. Associated weather systems are
usually carried by easterly waves. Some of
these disturbances seem to have an ori-
gin in tropical revolving storms over the
South China Sea, the vorticity from which
is maintained at high levels and can cross
the south-eastern part of the Himalaya. As
the low-pressure areas cross northern India,
southeasterly winds develop in their wake,
bringing warm moist air from the northern
Bay of Bengal. Thus rainfall is greater in the
east than in the west and is periodic – in
particular across northern parts – spells typi-
cally lasting a few days, followed by several
Weather – May 2008, Vol. 63, No. 5
days of dry weather.
The summer monsoon over
east Asia
East of the Himalaya, the monsoon ‘plum’
rains spread across mainland South-East
Asia and China in three stages. During April,
as fruit trees come into bloom, a shallow
humid southwesterly wind moves across
southern parts. This is followed by south
to southeasterly winds that bring rainfall
as a form of frontal system, known as the
Mei-Yu front, as far north as the Yellow River
during June (Figure 5). Finally, the south-
easterlies reach around 45 °N, bringing most
of the area’s annual rainfall total during July
(Asnani, 1993). Ascent to the uplands of
inland China aids the release of latent heat
and rainfall is copious in central areas, locally
amounting to more than the totals in the
south during the summer.4 Winds must back
at low levels for deep humid air to spread
across the region, since southwesterlies are
partially blocked by high ground in Sumatra,
peninsular Malaysia, Thailand, Burma and
south-western China to depths between 1
and 5 km. These mountain chains have fre-
quent rains, but an area thousands of kilo-
metres in their lee are in rain shadow.
Variations of rainfall in the
Asian summer monsoon
The summer monsoon is highly significant
in terms of rainfall across all the continents
Figure 4. Infra-red, visible and water-vapour satellite images from Meteosat 5 of the south-west on which it develops. Most of the Indian
monsoon over Asia at 0600 UTC on 21 July 2007. In the infra-red picture, the cold cirrus clouds over subcontinent receives about 80% of its rain-
the Bay of Bengal and India form elongated streaks, where the cloud tops are sheared off in the EEJ fall under its influence, mainly between June
(a). The deepest cloud, most likely to produce precipitation is shown as bright white in the visible and September. The depth of southwest-
image (b). The deep humid zone is indicated by light shades in the water-vapour image and can be erly winds at the core of the Asian monsoon
seen to extend as far north as the Himalayan peaks (c). The most humid air is distorted north across reaches about 5 km. However, the depth of
India and Africa by the monsoon flows. Dry air remains over parts of south-west Asia, notably much the most humid air is no more than about
of Pakistan, although a bulge of moist air can be see across much of the Arabian peninsula. The 3 km (Asnani, 1993). Along the southern
remnant ITCZ can be seen over the southern Indian Ocean, near 7 °S. (Courtesy University of Dundee ranges of the Himalaya, very large amounts
Satellite Receiving Station.) of rainfall are deposited with some of the
The first is the development of the pre- the Bay of Bengal and much of southern highest global rainfall totals recorded. For
monsoon trough at the surface over the India, supplying a reservoir of moisture for instance, Cherrapunji in the Assam region of
Arabian Sea. The flow ahead of this trough, convective development. 4
The mean June–August rainfall at Xi’an in cen-
which may contain a tropical depression The second important mechanism is the tral China is 750 mm, whereas that at Haikou on
or cyclone, brings southeasterly winds and motion of small, low-pressure systems from Hainan Island (the extreme south of the country)
132 moisture from the equatorial zone across east to west across Bangladesh, Bhutan, is 650 mm.
close to the surface and, in some places,
The weather and climate of the tropics – Part 6
in particular the bay between Al Ghaydah,
Yemen and Şalālah, Oman, there is persist-
ent low cloud and drizzle, associated with
the Khareef wind. In the relative shelter of
the Şalālah Plain, which is surrounded by
mountains, this weather is very popular
with many an Arab holidaymaker (Membery,
personal communication)! However, the
increased moisture in the boundary layer
also allows deep convection to develop
from scattered high-based cumulonimbus
clouds over the Yemeni and Saudi Arabian
mountains.
Weather – May 2008, Vol. 63, No. 5
The Asian winter monsoon
As Asia cools, the subtropical high becomes
re-established over the south of the conti-
nent and northeasterly winds bring dry wea-
ther across all areas north of about 10 °N.
The establishment of the winter monsoon
takes several months (Figures 2(c) and (d)),
Figure 5. The Mei-Yu front over southern China at 1200 UTC on 12 June 2007 observed by MTSAT.
however; areas of heavy rainfall gradually
This front, the leading edge of the east-Asian summer monsoon, moves north gradually during the
retreat south in a rather erratic fashion
summer and can be seen between Burma and Kyushu, Japan. Along its northern edge, where warm
between September and November.
moist air has advanced over cooler air near the surface, is a band of altostratus and altocumulus. At From late September, the southwesterly
its southern edge is a line of cumulonimbus with a cloud-top temperature around – 80 °C, indicating winds at low levels decrease, allowing the
an altitude of about 16 km. The deep convection formed over land where the 850-hPa θw was above Arabian Sea to warm. Indeed, much of the
23 °C. (Courtesy University of Dundee Satellite Receiving Station.) northern Indian Ocean warms, generating
periodic widespread deep convection and
north-east India, at an altitude of 1313 m, on India, through south-west Pakistan and Iran, heavy, thundery rain. Occasional tropical
the southern slopes of the Khāsi Hills, has an across the Arabian Peninsula. Nevertheless, depressions or cyclones can be expected after
annual total of 10 824 mm of precipitation,5 some parts of south-west Asia have relatively the southwesterly winds become light over
more than 5000 mm of it falling in June and large populations, in particular in coastal the Arabian Sea and Bay of Bengal between
July.6 As the moist air reaches these ranges, areas, where trade and fisheries have long September and December (Galvin, 2008c).
surplus moisture is deposited as rainfall been important industries. During October, the south-east of India
(Dhar and Nandargi, 2005). Away from the These industries are significantly affected has another period of rainfall as northeast-
Himalayan slopes, the zone north of 15 °N by the monsoons, in particular in summer, erly winds become established. Warm moist
also receives the highest rainfall accumula- when strong winds make sailing both diffi- air is carried inland from the Bay of Bengal
tions during the summer monsoon. North of cult and hazardous. Even in these days of and a broad zone of cumulonimbus devel-
25 °N, in particular over land, heavy rainfall is motorised vessels, high seas may be peril- ops inland of the coast – in particular on
somewhat more limited than further south ous (J. C. Galvin, personal communication7) the Eastern Ghats – over an area as much as
(Asnani, 1993). and many traders across the Indian Ocean several hundred kilometres across (Asnani,
Figure 4 shows monsoon activity across the are likely to be confined to port for at least 1993). This is an important period of rainfall
Indian Ocean and Asia in July 2006. The main part of the summer season. The increased for this region.
area of convection can be seen between numbers of fish that the monsoon-related During the same month, however, the
Thailand and northern India with more limit- upwelling brings, however, means that fish- re-established cool north to northeasterly
ed convective cells evident in southern India, ing tends to peak in summer off Arabia flow across Japan, Korea and China begins
on the Himalayan slopes and across Tibet. Air and Pakistan, despite the rough seas. Most to interact with the re-established westerly
in the upper troposphere can be seen to be fishing vessels of the area are small and rela- flow above it, associated with the return of
very moist in all these areas, whilst the Thar tively fragile. Membery (2001) gave an excel- the STJ, close to 30 °N. Initially, there is
Desert of India and Pakistan remains under lent review of the effects of tropical cyclones convection to middle-tropospheric levels,
the influence of subsided dry air. and the monsoon on historical trade around but maritime stratocumulus soon forms as
Although moist southwesterlies are the Indian Ocean. cooling of the boundary layer continues,
observed across the whole of the Arabian Sea One rather strange aspect of the sum- at times accompanied by stratus in areas
during the Asian summer monsoon, stability mer monsoon along the south coast of the near to the coast (Galvin and Walker, 2007).
and the influence of surrounding mountain Arabian Peninsula is the effect of upwelling This brings slight rain and drizzle at times to
chains are sufficient to keep rainfall to very cool water along this coast. As well as pro- many areas, including Hong Kong.
low levels in coastal Yemen, Oman, Iran and moting a good fishery, the upwelling water The cooling of China, away from the coast,
Pakistan. This results in a desert or semi- cools the overlying air. This causes conden- leads to a strong interaction of airstreams
desert climate stretching from north-west sation from the moist flow to be trapped and a pall of deep altostratus is common in
7
Much of my father’s service in the Royal Navy the lee of the north-south ranges that form
5
1961–1990 total; this is more than five times the aboard ship was spent on the Arabian Sea and the eastern Himalaya from late November
annual total for most British uplands! the roughest weather he encountered was dur- or early December. Widespread rain or snow
6
The extreme annual rainfall at Cherrapunji ing the south-west monsoon of 1942 on the falls from this cloud, in particular over higher
is more than twice this total, as given by destroyer HMAS Launceston – sufficient to make ground. Much of central southern China
Guhathakurta (2007). most of the crew sick! endures cloudy conditions in winter, rather 133
than summer, even though the heaviest This monsoon circulation has many simi- the EEJ extends west across much of the
The weather and climate of the tropics – Part 6
precipitation (some of it very heavy) falls larities to that in Asia, but also significant continent providing the divergence needed
during the south-west monsoon. differences, largely due to the differences in to generate deep convection.
The thicker cloud that forms over China orography and the land-mass distribution of This monsoon flow is like a giant sea breeze,
can be a significant hazard to aviation. Much these two regions. It is also less extensive, the although on a much larger scale. Its great
of this cloud deck is at a temperature near monsoon flow reaching no more than about scale means that stability cannot be main-
0 °C, even when several thousand feet deep. 20oN of the equator and deep convection only tained as it passes inland, in particular late in
Moderate, occasionally severe icing may be occasionally seen north of 15oN. The monsoon the season. As it warms, under the divergent
encountered by aircraft flying at modest flow and deep instability spread inland much upper-tropospheric flow, deep convection
height through this cloud layer. Such icing more quickly here than over India, however, brings thundery rain, most of the energy
of the airframe or engine can cause loss of since there are no ranges of mountains to for which is from the moist low-level flow
airspeed and stability; ultimately, loss of the impede the inland flow. Initially, convection no more than about 4 km deep. The tops of
aircraft (Galvin, 2008b). is scattered, but by late June, as a warm these cumulonimbus clouds frequently reach
By late December, the ITCZ is recogniz- dome develops over north Africa strengthen- 14 km or more with a peak in deep convec-
able as two bands of cloud across the Indian ing the easterly winds above about 3 km, the tion from late afternoon to the early hours
Weather – May 2008, Vol. 63, No. 5
Ocean, close to 10 °N and 10 °S. southwesterly winds at low levels bring more of the morning, although associated layer
Much of the winter season is dry across the frequent rains north. clouds may remain for days, bringing gener-
whole of south Asia, including parts of sou- Within the broad-scale structure, there are ally cloudy conditions with occasional slight
thern China, Burma, northern Thailand, Laos, notable variations in the northward extent of rain. Most of the monsoon rainfall occurs well
Cambodia and Vietnam. However, although the deep humid zone on a daily and monthly to the south of the ME, typically from near the
areas of large-scale convection are absent, scale. In the east, the zone has a limited coast of the Gulf of Guinea to about 10oN.
slight showers may still affect windward northward extent, usually south of Lake Chad Further north, where the depth of moist air
coasts. A north-west monsoon brings cool (c. 12oN), which is about 600 km south of its is rarely more than about 2 km, deep convec-
continental air from central China across northern edge across most of west Africa, tion occurs less often and much is associated
much of the region. Drier weather also although occasionally it reaches 20oN. To the with mesoscale convective systems. These
spreads briefly into the northern and central east of Lake Chad, the humid air south of the will be discussed in Part 8.
Philippines during March and early April. How- ITF has a somewhat different character: it has A conceptual model of the structure of
ever, monsoon variation is less well marked a greater instability, but is somewhat drier, cloud masses and air masses across the west
over the extreme south of the region and the following a long land track. This air is not a African monsoon zones is shown as Figure 6.
island nations of south-east Asia, where rain true monsoon, due to its origin mainly over This shows the change southwards from (A)
can be expected for much of the year. the central African land mass. hot-dry (hazy) desert to (B) humid-stable
The relatively cool weather of the win- To the south of the ME is a moist flow that transitional to (C) convective, then (D) vigor-
ter monsoon can be uncomfortable for the has crossed the equator. Initially this was a ously convective, (E) deep-tropical stable
population – in particular those who live in dry southeasterly from the southern African and shallow tropical unstable then (F) shal-
coastal locations, such as Hong Kong, where mainland. However, it picks up moisture as it low tropical stable conditions. It can be seen
cool spells require warm clothing and build- flows towards the south-facing coast of the that embedded convection is most likely in
ings to be heated. Indeed, warnings of cold Gulf of Guinea. The change of direction of the centre of zone D. Layer clouds are well
are issued to the Hong Kong fire brigade if the Coriolis force as the southeasterly flow broken in the north of this zone (or may be
night-time temperatures are expected to crosses the equator causes it to become absent) and, although deep and precipitat-
fall below 10 °C, when the fire risk becomes southwesterly. ing, the layer clouds in the south of the zone
significant, as the rooms of otherwise-unhea- Between April and June, while the Gulf of do not contain cumulonimbus.
ted flats are warmed using paraffin stoves Guinea remains warm, this flow is unstable
(B.-Y. Lee, personal communication). and rains may be copious over the sea, as The west African winter
well as on land, although the northward
advance of deep convection is periodic, monsoon
The west African summer since the pressure gradient between the As the year wanes and high-level easter-
monsoon warm Gulf and northern Africa is relatively lies decrease in strength, the ME moves
weak, or absent. The wind strengthens as the slowly southward. The Sahel becomes dry
Over west Africa, the ITCZ is distorted and pressure gradient increases from south to in October as the moist convective zone
moves north from the coast of the Gulf north as the summer progresses. This causes moves south. Steadily more and more of
of Guinea as the Sahara desert becomes cooling of the surface water by overturning, the savannah lands become dry. At its fur-
increasingly hot during the lengthening days. the resulting positive feedback causing fur- thest excursion south, the ME lies along the
The ME moves north, a few degrees behind ther strengthening of the pressure gradient, south-facing coast (~5oN), so that there is a
the position of the sun overhead at midday in turn causing further cooling by overturn- dry northeasterly flow on most days from
during April and May. The distortion is so ing. Thus by late June, the flow across the the central Sahara. The flow is generally hot
strong that the ME becomes separated from Gulf is relatively cool, sensible heat having by day and into the evenings, but may be
the area of deep convection south of the being used for evaporation to form exten- rather cool following nights of clear skies
inter-tropical front (ITF) and associated thun- sive cumulus and stratocumulus. Given this over the desert, the effect seen most often
dery rain. In the early part of the monsoon cooling, it is necessary for the moist mon- across the drier areas of savannah and
season, convection occurs periodically over soon flow to be heated and divergence to semi-desert.
land and continues over the Gulf of Guinea. be present aloft for there to be significant However, the coastal zone does not become
In June it becomes confined to land, north of deep convection with associated heavy completely dry during the winter. There are
5 °N, as the waters of the Gulf cool. By early rains, which are largely confined to the land, occasional incursions of sea air, which, by the
August, the ME lies at its northward extreme, since the moist flow is capped over the sea. latter part of winter, is unstable. The reduced
between 1500 and 2000 km from the coast of In this respect, the west African monsoon is strength or absence of southwesterly winds
the Gulf of Guinea, over northern Africa.8 rather different from that in southern Asia. weakens the overturning of the ocean and a
8
A full discussion of this process is given in By June, the troposphere has expanded over warm Guinea current becomes established
Leroux (2001), Ch. 1. the Sahara Desert as north Africa warms and (Galvin, 2008a, Figure 1(a)). Cloudy skies are
134
The weather and climate of the tropics – Part 6
Weather – May 2008, Vol. 63, No. 5
Figure 6. Meridional cross-section of the troposphere over west Africa in northern summer (adapted from Leroux,1970; 1972). SL indicates a squall line
associated with an African Easterly Wave. Zone A lies north of the ME; the (transitional) zone between the ME and the ITF indicated by B; C is a shallow
convective (capped) zone; D is the main convective region of the monsoon under the influence of upper-level diffluence; E is an area of thick layer clouds,
associated with long periods of monsoon rain; F sees the return to capped shallow convection with a transition to layer clouds as surface divergence
becomes re-established. This zone dominates over the Gulf of Guinea and along the Guinea coast in high summer when the total breadth of the monsoon
transition and monsoon rains reaches 1000 km or more.
relatively common throughout the year in of the predominantly dry Sahara desert. The there is a flow from the Indian Ocean and
this coastal zone, bringing occasional slight first is the extensive Ethiopian plateau. Cloudy precipitation may occur over high ground.
rain or showers. Deep convection occurs skies and rainfall are present for much of the The Ethiopian plateau also generates some
south of 5oN across central Africa through year over this large area of high ground. A rainfall, although this is mainly somewhat
the winter months. peak in rainfall occurs as the ME moves north further south than in the northern summer
across the plateau and deep convection from months.
Rainfall and the monsoons in increasingly moist air becomes widespread.
North of the plateau, however, the weather
eastern north Africa remains predominantly dry. The northern sum- Monsoons in southern Africa
It might reasonably be thought that all of mer brings moist air from the Indian Ocean The broad area of convection spreads south
east Africa across, and to north and south of north across Somalia, although the strong during the summer across much of the
the equator would see significant periodic southwesterly winds of the Asian monsoon south of the continent, across the savan-
rainfall from the monsoons, but Somalia, are offshore and the weather is mainly dry nahs. Most of the south of this continent
Kenya and Tanzania frequently suffer over the lowlands of the Horn of Africa. has a long dry winter monsoon season. The
droughts and have become notably dry in Secondly, there are complex interactions ME is most notable over east Africa, reach-
recent years, away from their highland inte- between air masses from the Atlantic and ing Madagascar and eastern coastal areas
riors. This is largely due to the very shallow Indian Oceans. The one from the Indian of Mozambique by November. It is a very
moist boundary layer frequently seen over Ocean contains more moisture and ascent diffuse feature further west. As in north-
the western equatorial Indian Ocean (Kiangi, up the ranges of the East African Highlands eastern Africa, however, the southern
1989) and the relatively low temperature of releases its latent heat. This leaves relatively African monsoon is not a classical form.
the western equatorial Indian Ocean. In turn, warm dry air at high levels above the moist, Over land, the situation is complicated
warming of the western Indian Ocean has a but stable Atlantic air. This stability restricts first by the presence of high ground, form-
profound effect on rainfall, usually bringing rainfall north and west of the East African ing a ‘ring’ around most of the land mass
floods, as in Somalia in 1997 (Camberlin and Highlands, with rainfall only developing in and secondly by the convergence of air from
Philippon, 2001) and 2006. the presence of additional forcing. the Indian and Atlantic Oceans. As described
However, there are significant complications In the northern winter, northeasterly winds above, Indian Ocean air contains more mois-
in the rainfall pattern in eastern Africa, south are generally dry, although near the equator ture than that from the Atlantic (Leroux,
135
2001), so air from the north-east can release vice versa. There is no upper-tropospheric times, leading to local flash flooding, as
The weather and climate of the tropics – Part 6
more rainfall from instability than air from jet stream (upper-tropospheric winds are occurred in Channel Country, Warrego and
the west. The inland motion of moist air is westerlies) and the depth of the tropopause central west Queensland on 20 and 21
restricted by orography, since the leading remains modest, rarely reaching more than January 2007.
edge of the monsoon flow is no more than about 16 km. An unusual phenomenon precedes the
about 2 km deep. Over central Africa, it is In the provinces of north-east Brazil, as in onset of the Australian monsoon rains in
only when the moist air can flow over the east Africa, the effect of varying off-shore the Gulf of Carpentaria between late August
highlands that deep convection can occur. water temperature affects the climate. The and early November as humidity rises. Here
The East African Highlands, either side of cool equatorial counter-current brings peri- a complex interaction of orography, a moist
the Rift Valley absorb much of this moisture, odic drought and has been a cause of a shallow cool boundary layer and sea breez-
even when the line of convergence of sur- mass exodus of the population (Hastenrath, es sets up the ‘Morning Glory’ cloud roll
face air is well to the west. 1991). as the ME makes a halting advance south
In general, the southward movement of the across the Gulf (Figure 7). The phenom-
monsoon flow during November is accompa- enon is still poorly understood in detail,
nied by a short period of rain. By the end of The Australian summer but development appears dependent on
monsoon the shallow monsoon boundary layer being
Weather – May 2008, Vol. 63, No. 5
the month, it has reached southern Tanzania
and northern Mozambique. Once again, as drawn over Cape York and up its moun-
The distortion of the ITCZ into a mon-
summer wanes, the rainfall area retreats tains by a sea breeze. As this sea breeze
soon flow is perhaps less well marked
north from mid-January to reach northern subsides, it is overturned to form a wave
over northern Australia than in most other
Kenya and Uganda by the end of February, trapped beneath the Trade-Wind inversion.
monsoonal climates and the expansion of
bringing a period of ‘long rains’. This, in turn, This is carried across the Gulf, accompa-
the troposphere is modest, compared with
clears north during April and May, precipita- nied by a roll cloud, close to the line of
that over Asia or north Africa. However,
tion eventually becoming largely restricted surface-wind convergence. It brings a rapid
easterlies strengthen near the equator,
to equatorial Africa. wind increase to around 13 ms−1 and rising
over New Guinea and Java, occasionally
Despite the effect of orography, over humidity. In some cases, the convergence
reaching jet-stream strength, in response
south-western and southern Africa convec- may be accompanied by a line of deep
to the warming of the Australian land mass.
tion at the leading edge of the monsoon convection, but in most cases the roll cloud,
Moist oppressive air precedes convective
often forms a band extending from about which may be up to 1000 km long, brings a
downpours southwards into the marginal
10oS on the coast of Angola, arcing south- stable flow. The overturning is due to west-
semi-desert areas of the country in sum-
eastwards to reach the Mozambique coast erly winds above the inversion, so the cloud
mer. Almost all of northern Australia is
near 30oS when the monsoon reaches its rolls backwards and generates both lift and
dependent on summer rainfall and, as a
peak. Off shore, the warm Mozambique turbulence (Jackson et al., 2002). This can
result, has woody savannah vegetation,
Current, between Madagascar and the main- be a major hazard to light aircraft at low
despite the proximity of parts of the north
land, allows the ME to retain characteristics levels, but the lift ahead of the roll allows
coast of Australia to the equator (the lati-
similar to those of the ITCZ. gliders to ‘surf’ at speed for several hundred
tude of Cape York is less than 11 °S). The ME
kilometres (Thomas, 2003).
reaches about 20–25 °S across Australia at
The South American- the height of the monsoon in late January,
Acknowledgements
its greatest poleward extension usually
Caribbean monsoons in the east of the country, occasionally to With thanks to Nigel Bolton, Richard Young,
Over South America, there is a gradual 25 °S or more. The extensive areas of high Chris Tyson, Clive Jones, Clive Wilson, Martin
northward and southward motion of moist ground in central and northern Australia, Young and Gavin Pretor-Pinney. Dedicated
unstable air. In the southern-hemisphere and the Great Dividing Range along the to my father (15 August 1915 – 5 January
summer, it moves steadily south, crossing east coast, enhance monsoon rains at 2007).
Bolivia, Paraguay, northern Argentina and
Uruguay by late December. The area close
to the equator does not experience a dry
season, although the Caribbean and much
of Central America is mainly dry during the
Northern Hemisphere winter.
In the southern summer, convergence of
a northwesterly wind with southeasterlies
cannot generate the same level of deep
convection as elsewhere, since air from the
north must follow a long land track across
South America, the Andes preventing air
from the Pacific spreading across the con-
tinent. Northwesterlies over south-eastern
Brazil form a relatively dry continental flow.
Here, much of the rainfall comes from mes-
oscale convective complexes (Laing and
Fritsch, 1997).
In the northern summer, the Caribbean
experiences its wet season, although here
it is mainly the result of destabilisation of
the atmosphere as the ITCZ moves north,
rather than a true monsoon, since there
is not a marked reversal of wind direction Figure 7. The ‘Morning Glory’ roll cloud near Burketown, Queensland, Australia, seen from the air.
136 and the flow is from land to sea, rather than (© Russell White.)The weather and climate of the tropics – Part 6
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DOI: 10.1002/wea.230
Cumulonimbus with mamma seen on 13 March 2008 at 1600 UTC from Lossiemouth, Scotland. (© Roger Kingham.) 137You can also read
