Like the SAO, the theoretical understanding of theQBO is that it is forced by momentum transfer byvertically propagating waves forced in the loweratmosphere, interacting with the mean flow. Thedynamical theory of the QBO is explained in detailelsewhere.

See also

Middle Atmosphere: Planetary Waves; PolarVortex;Quasi-Biennial Oscillation; Semiannual Oscillation.

Quasi-geostrophic Theory. Wave Mean-FlowInteraction.

Further Reading

Andrews DG, Holton JR and Leovy CB (1987) MiddleAtmosphere Dynamics. New York: Academic Press.

Brasseur G and Solomon S (1986) Aeronomy of the MiddleAtmosphere. Boston, MA: Reidel.

Labitzke KG and van Loon H (1999) The Stratosphere:Phenomena, History and Relevance. Berlin: Springer-Verlag.

MIRAGES

See OPTICS, ATMOSPHERIC: Optical Phenomena

MONSOON

Contents

Overview

Dynamical Theory

ENSO–Monsoon Interactions

Prediction

Overview

J Slingo, University of Reading, Reading, UK

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Monsoon derives from the Arabic word ‘mausam’,meaning season, and in its broadest definition describesthose climates that are seasonally arid. As shown inFigure 1, many regions of the tropics and subtropicsexperience a rainy summer season and a dry winterseason, although regions close to the Equator can oftenexperience two rainy seasons; e.g., equatorial eastAfrica with its ‘long’ (March–May) and ‘short’ (Octo-ber–December) rains. The main driver of this markedseasonality in rainfall is the change in the distributionofsurface heating between winter and summer, primarilyassociated with seasonal variations in the position ofthe sun. Because of this close relationshipwith the solarseasonal cycle, the start of the rainy season often beginswith remarkable regularity each year.

Although Figure 1 shows that many regions areseasonally arid, the more precise definition of amonsoon climate, as proposed by Ramage, identifiesSouth Asia, Australia, and Africa as having distinctmonsoons. Ramage’s criteria for a monsoon to existare as follows:

1. Prevailing wind direction shifts by at least 1201between January and July.

2. Prevailing wind direction persists for at least 40%of the time in January and July.

3. Mean wind exceeds 3m s�1 in either month.4. Fewer than one cyclone–anticyclone alternation

occurs every 2 years in eithermonth in a 51 latitude–longitude rectangle.

These criteria essentially demand that a monsoon becharacterized by a wind regime that is steady, sus-tained and therefore inherently driven by the season-ally evolving boundary conditions, such as land orocean surface temperatures. It excludes most extra-tropical regions that are characterized by synopticweather systems with alternating cyclonic–anti-cyclonic circulations.

MONSOONS / Overview 1365

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Figure 1 Mean precipitation distributions for northern winter (January–February; upper panel) and northern summer (July–August;

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Project.)

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Figure2 Meanwinds at 925hPa for northernwinter (January–February; upper panel) and northern summer (July–August; lower panel)

at the height of the monsoon season (units: ms�1). (Data source: Reanalyses from European Centre for Medium Range Weather

Forecasts for 1979–93.)

1366 MONSOONS / Overview

Based on these criteria, Figure 2 shows that thismajor reversal in the seasonalwind regimes only occursover (1) India and South-East Asia, (2) northernAustralia, and (3) West and central Africa. These threeregions constitute the major monsoons of the globalcirculation. Although the Americas and South Africaalso experience a strong seasonal cycle in rainfall, theprevailingwind direction is largely unchanged betweenwinter and summer (Figure 2), and so strictly speakingcannot be classified as monsoon regions. The name‘monsoon’ is often used to denote the rainy season, butin fact can relate to both extremes of the seasonal cycle.The termwinter monsoon is used in South-East Asia todescribe the dry, north-easterly winds that prevail overthe northern IndianOcean and South China Sea duringboreal winter (Figure 2, upper panel).

For a monsoon to be established, a thermal contrastbetween the land and ocean must exist. This occurswhen large land masses, such as Asia, Africa, andAustralia, heat up rapidly during the spring andsummer (Figure 3). Since the thermal inertia of theland is much less than the surrounding oceans, thecontinents respondmuch more rapidly to the seasonalcycle in solar heating, setting up large temperaturegradients. These hot land masses draw humid air infrom the surrounding oceans, like amassive sea breeze

(Figure 2). As themoisture-laden air reaches the warmland, it rises, the moisture condenses, and the rainyseason begins. By contrast, in winter the land becomesmuch cooler than the surrounding oceans and cold,dry air then flows from the land out over the ocean.Often the two monsoons, winter and summer, areclosely linked with the winter monsoon of onehemisphere feeding the summermonsoon of the other.For example, in the Asian–Australian monsoon sys-tem, the dry air from the winter continent flows acrossthe Equator toward the summer hemisphere (Figure2), picking up moisture from the warm oceans andfeeding themonsoon rains over the summer continent.

A critical factor that determines the generation of amonsoon is the geographical orientation of the oceansand continents. The strongest monsoons occur wherethere is a pronounced north–south distribution in landand ocean that can take advantage of the north–southprogression of the solar seasonal cycle. As Figure 3shows, the largest land–sea temperature contrastsoccur over the seasonally arid regions ofNorth Africa,India, and Australia, during the months preceding thesummer monsoon. These very warm temperatureslead to the development of thermal lowswhich serve topull in air from surrounding regions. Once themonsoon is established and the rains begin, the land

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Figure 3 Surface temperatures for early austral summer (November–December; upper panel) and boreal summer (May–June; lower

panel) at the onset of themonsoon (units: 1C). (Data source:Reanalyses fromEuropeanCentre forMediumRangeWeather Forecasts for

1979–93.)

MONSOONS / Overview 1367

surface temperatures tend to cool due to the increasedsoil wetness, but the atmospheric warming from latentheat release associatedwith themonsoon rains (Figure1)maintains the low-pressure regions (Figure 4) whichcontinue to drive the monsoon winds.

Although this classic description of monsoonsprovides the fundamental basis for their existence,there are important regional differences associatedwith the shape of continents, orography (particularlymountain barriers), and ocean temperatures. For the

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Figure 4 Mean sea-level pressure for northern winter (January–February; upper panel) and northern summer (July–August; lower

panel) at the height of the monsoon season (units: hPa). (Data source: Reanalyses from European Centre for Medium Range Weather

Forecasts for 1979–93.)

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Figure 5 Mean temperature between 200 and 500hPa for northern winter (January–February; upper panel) and northern summer

(July–August; lower panel) at the height of the monsoon season (units: K). (Data source: Reanalyses from European Centre for Medium

Range Weather Forecasts for 1979–93.)

1368 MONSOONS / Overview

Asian summermonsoon, the Tibetan plateau acts as anelevated heat source which clearly influences theestablishment and maintenance of the monsoon circu-lation. The seasonal heating of the plateau leads to areversal of the meridional temperature gradient whichextends throughout the troposphere (Figure 5). Thisreversal is instrumental in triggering the large-scaleseasonal change in the circulation over East Asia, withthe poleward transition of the subtropical jet and theonset of the monsoon over the Indian subcontinent.

The importance of this deep warm core over SouthAsia is demonstrated in Figure 6, which shows thewinds in the free troposphere at 700 hPa. These windscan be compared with the boundary layer winds at925 hPa in Figure 2. It is only for the domain of theAsian monsoon that the seasonal reversal of the windsis seen extending above the boundary layer into thefree troposphere. This is important because it is onlywhen there is advection of moist air through asubstantial depth of the troposphere that sustainedmonsoon rainfall is achieved.

The significance of the Tibetan plateau is furtherdemonstrated when a comparison is made between thesummer monsoons of South Asia, northern Australia,and West Africa. In the absence of an orographic heatsource, the seasonal reversal of the meridional temper-ature gradient through the depth of the troposphere isbarely evident over Australia and West Africa. This is

despite the very substantial surface warming of Aus-tralia andNorthAfrica shown in Figure 3. The effect ofthe confinement of the seasonal reversal in the merid-ional temperature gradient to the near-surface layersover Africa and Australia can be seen in the winds at700hPa (Figure 6). Unlike South-East Asia, there is noseasonal reversal of thewinds in the free troposphere sothat a deep moist layer is not established in the sameway as over South Asia. Consequently, the monsoonrainsofWestAfricaandAustraliaarenotas intense,nordo they extend as far polewards.

Monsoons are crucial elements of the global circu-lation and monsoon rainfall provides the water neededby over 60%of the world’s population. Understandingandpredictinghowmonsoonsmay change fromyear toyear, and the result of globalwarming are key scientific,economic and societal issues. The management ofwater resources is a top priority for monsoon-affectedcountries to enable the population to survive from onerainy season to the next. Food production in seasonallyarid areas is also inherently risky. By the end of the dryseason, the soil is parched and planting cannot beginuntil the rains arrive. A late or weak monsoon can leadto a short or poor growing season and hence low yields.Agricultural failure has a profound effect on theeconomy ofmonsoon-affected countries, such as India,where farming accounts for 30% of the gross domesticproduct and 67% of the workforce.

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Figure6 Meanwinds at 700hPa for northernwinter (January–February; upper panel) and northern summer (July–August; lower panel)

at the height of the monsoon season (units: ms�1). (Data source: Reanalyses from European Centre for Medium Range Weather

Forecasts for 1979–93.)

MONSOONS / Overview 1369

See also

Monsoon: Dynamical Theory; ENSO–Monsoon Interac-tions; Prediction.TropicalMeteorology:Tropical Climates.

Further Reading

Fein JS and Stephens PL (eds.) (1987)Monsoons. NewYork,USA: Wiley-Interscience.

Hastenrath S (1994) Climate Dynamics of the Tropics: AnUpdated Edition of Climate and Circulation of theTropics. Norwell, MA: Kluwer.

Pant GB and Rupa Kumar K (1997) Climates of SouthAsia. Belhaven Studies in Climatology. Chichester:Wiley.

Ramage C (1971) Monsoon Meteorology. InternationalGeophysics Series, vol. 15. San Diego, CA: AcademicPress.

Webster PJ, Magana VO, Palmer TN, et al. (1998) Mon-soons: processes, predictability, and the prospects forprediction. Journal of Geophysical Research 103(C7):14 451–14 510.

Dynamical Theory

P J Webster and J Fasullo, University of Colorado –Boulder, Boulder, CO, USA

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Elements of a Monsoon Circulation

A monsoon is a circulation system with certain well-defined characteristics. During summer, lower tropo-spheric winds flow toward heated continents awayfrom the colder oceanic regions of the winter hemi-sphere. In the upper troposphere the flow is reversed,with flow from the summer to the winter hemisphere.Precipitation generally occurs during summer, cen-tered in time on either side of the summer solstice andlocated over the heated continents and the adjacentoceans and seas in the vicinity of a trough of lowpressure referred to as the ‘monsoon trough’. Mostsummer rainfall is associated with synoptic distur-bances that propagate through the region. However,these disturbances are grouped in periods lasting from10 to 30 days. Such envelopes of disturbed weatherand heavy rainfall are referred to as ‘active periods ofthe monsoon’. The intervening periods of mini-drought are referred to as ‘monsoon breaks’. Thelocation of the monsoon trough and axis of heavymonsoon precipitation is generally well poleward ofthe position of the oceanic intertropical convergencezone (ITCZ), within which the majority of tropicaloceanic precipitation occurs. For example, the rainfallassociated with the South Asian monsoon falls at thesame latitudes as the great deserts of the planet.

Monsoon systems are associated with colocatedpairs of continents such as Asia and Australia, orcontinents straddling the Equator such as north-westand south-west Africa, and North and South Americadefining, respectively, the Asian–Australian monsoonsystem, the West African monsoon, and the Americanmonsoon. Each system is different in terms of intensityand circulation characteristics. For example, the

northern arm of the American monsoon is a relativelyweak counterpart of the othermajormonsoon systemsand there does not appear to be a discernible cross-equatorial component during the summer. In thatsense, the North and South American monsoons maybe thought of as almost separate entities. Rainfall thatoccurs over the continents that span the Equator (e.g.,equatorial Africa and South America, and Indonesia)is not strictly monsoonal and possesses double rainfallmaxima occurring with the equinoxes. Monsoonclimates, on the other hand, possess a single solstitialrainfall maximum, while solstices demark the dryseasons for equatorial climates.

Basic Driving Mechanisms of theMonsoon

It is helpful to consider first a simple prototypegeography that will allow us to identify the basicelements of amonsoon system.The geographicalmodelwe adopt is an oceanic planet with a continental capextending from the subtropics to the pole in onehemisphere. After establishing the important processesthat drive the monsoon for this simple geography, wewill return to the consideration of local influences.

Monsoons arise from the development of cross-equatorial pressure gradients produced ormodified bythe following physical properties of, or processesassociated with, the land–ocean–atmosphere system:differential heating of land and ocean produced by thedifferent heat capacity of land and water; the differentmanner in which heat is transferred vertically andstored in the ocean and the land; modification ofdifferential heating by moist processes; the generationof meridional pressure-gradient forces resulting fromthe differential heating; and the meridional transportof heat in the ocean by dynamical processes. Each ofthese processes and properties has to be consideredrelative to the rotation of the planet, and the influence

1370 MONSOON / Dynamical Theory