About Monsoons

Over the years many mysteries of the monsoons have been unravelled but still much remains to be done. The theories regarding the monsoons are generally divided into following two broad categories:

1. Classical Theory, and

2. Modem Theories.

1. Classical Theory:
Although monsoons are mentioned in our old scriptures like the Rig Veda and in the writings of several Greek and Buddhist scholars, the credit for first scientific studies of the monsoon winds goes to the Arabs. Near about the tenth century, Al Masudi, an Arab explorer from Baghdad, gave an account of the reversal of ocean currents and the monsoon winds over the north Indian Ocean. Date of commencement of monsoons at several places was reported by Sidi Ali in 1554 A.D.

In 1686 the famous Englishman Sir Edmund Hailey explained the monsoon as resulting from thermal contrasts between continents and oceans due to their differential heating. Accordingly, Hailey conceived summer and winter monsoons depending upon the
season.

(a) Summer Monsoon:
In summer the sun shines vertically over the Tropic of Cancer resulting in high temperature and low pressure in Central Asia while the pressure is still sufficiently high over Arabian Sea and Bay of Bengal. This induces air flow from sea to land and brings heavy rainfall to India and her neighbouring countries.

(b) Winter Monsoon:
In winter the sun shines vertically over the Tropic of Capricorn. The north western part of India grows colder than Arabian Sea and Bay of Bengal and the flow of the monsoon is reversed (Fig. 5.1).

Hailey’s ideas are basically the same as those involved in land and sea breezes except that in the case of the monsoon day and night are replaced by summer and winter, and the narrow coastal strip and adjacent sea are replaced by large portions of continents and oceans.

2. Modern Theories:

Hailey’s classical theory based on differential heating of land and water as the main driving force of the monsoon winds dominated the scene for about three centuries. However, the monsoons do not develop equally everywhere and the thermal concept of Hailey fails to explain the intricacies of the monsoons. Besides differential heating, the development of monsoon
is influenced by the shape of the continents, orography, and the conditions of air circulation in the upper troposphere.

Therefore, Hailey’s theory has lost much of its significance and modern theories based on air masses and jet streams are becoming more relevant. Although Hailey’s ideas have not yet been out-rightly rejected, studies during the last five decades have thrown much light on the genesis of the monsoons.

During these years, Flohn, Thompson, Stephenson, Frost, M.T. Yin, Hwang, Takahashi, E. Palmen, C. Newton and Indian meteorologists including P. Koteswaram, Krishnan, Raman, Ramanathan, Krishna Murti, Rama Rattan, Ramaswami, Anant Krishnan,etc. have contributed a lot to the study of the monsoon winds.

Air Mass Theory:
The southeast trade winds in the southern hemisphere and the northeast trade winds in the northern hemisphere meet each other near the equator. The meeting place of these winds is known as the Inter-Tropical Convergence Zone (ITCZ).

Satellite imagery reveals that this is the region of ascending air, maximum clouds and heavy rainfall. The location of ITCZ  shifts north and south of equator with the change of season. In the summer season, the sun shines vertically over the Tropic of Cancer and the ITCZ shifts northwards.

The southeast trade winds of the southern hemisphere cross the equator and start flowing from southwest to northeast direction under the influence of coriolis force (Fig. 5.2). These displaced trade winds are called south-west monsoons when they blow over the Indian sub-continent. The front where the south-west monsoons meet the north-east trade winds is known as the Monsoon Front.

In the month of July the ITCZ shifts to 20°- 25° N latitude and is located in the Indo-Gangetic Plain and the south-west monsoons blow from the Arabian Sea and the Bay of Bengal (Fig. 5.3). The ITCZ in this position is often called the Monsoon Trough.

H. Flohn of the German Weather Bureau, while rejecting the classical theory of origin of monsoons suggested that the tropical monsoon of tropical Asia is simply a modification of the planetary winds of the tropics. He thinks of the thermal low of northern India and the accompanying monsoon as simply an unusually great northward displacement of the Northern Inter-TropicalConvergence Zone (NITCZ). The seasonal shift of the ITCZ has given the concept of Northern Inter- Tropical Convergence Zone (NITCZ) in summer (July) and Southern Inter-Tropical Convergence Zone (SITCZ) in winter (Jan.). The fact that the NITCZ is drawn to about 30° latitude may be associated with the unusually high temperature over north India.

According to this interpretation the main westerly current of the monsoon is simply the expanded equatorial westerlies which lie embedded in the great mass of tropical easterlies or the trade winds. NITCZ is the zone of clouds and heavy
rainfall.

Jet Stream Theory: 
Jet stream is a band of fast moving air from west to east usually found in the middle latitudes in the upper troposphere at a height of about 12 km. The wind speeds in a westerly jet stream are commonly 150 to 300 km p.h. with extreme values reaching 400 km p.h. Jet stream is the latest theory regarding the origin of the monsoons and has earned world wide acclaim from the meteorologists.

M.T. Yin (1949), while discussing the origin of the monsoons expressed the opinion that the burst of monsoon depends upon the upper air circulation. The low latitude upper air trough shifts from 90° E to 80° E longitude in response to the northward shift of the western jet stream in summer. The southern jet becomes active and heavy rainfall is caused by south-west monsoons.

Yin’s ideas are well recognised by Pierre Pedelaborde (1963), in his book entitled ‘The Monsoon’. The map, showing the seasonal shift of the westerly jet stream, has been reproduced in figure 5.4. It shows that in winter the western jet stream flows along the southern slopes of the Himalayas but in summer it shifts northwards, rather dramatically, and flows along the northern edge of the Tibet Plateau. The periodic movements of the Jet stream are often indicators of the onset and subsequent withdrawal of the monsoon.

P. Koteswaram (1952), put forward his ideas about the monsoon winds based on his studies of upper air circulation. He has tried to establish a relationship between the monsoons and the atmospheric conditions prevailing over Tibet Plateau.Tibet is an ellipsoidal plateau at an altitude of about 4,000 m above sea level with an area of about 4.5 million sq km.
This plateau is surrounded by mountain ranges which rise 6,000 – 8,000 m above sea level. It gets heated in summer and is 2°C to3°C warmer than the air over the adjoining regions.

Koteswaram, supported by Flohn, feels that because the Tibet Plateau is a source of heat for the atmosphere, it generates an area of rising air. During its ascent the air spreads outwards and gradually sinks over the equatorial part of the Indian Ocean.

At this stage, the ascending air is deflected to the right by the earth’s rotation and moves in an anti-clockwise direction leading to anticyclonic conditions in the upper troposphere over Tibet around 300-200 mb (9 to 12 km). It finally approaches the west coast of India as a return current from a south-westerly direction and is termed as equatorial westerlies (Fig. 5.5).It picks up moisture from the Indian Ocean and causes copious rainfall in India and adjoining countries.

The south-west monsoon in southern Asia is overlain by strong upper easterlies with a pronounced jet at 100 to 200 mb.These easterly winds, which often record speeds exceeding 100 knot are known as the Easterly Jet Stream of the tropics.

The Easterly Jet Stream was first inferred by P. Koteswaram and P.R. Krishna in 1952 and aroused considerable interest among tropical meteorologists. A careful study of the jets would suggest that the core of the easterly jet is at 13 km (150 mb) while that of the westerly jet is at 9 km. Over India, the axis of the strongest winds in the easterly jet may extend from the southern tip of the peninsula to about 20° N latitude. In this jet stream wind speeds exceeding 100 knot may be recorded.

Figure 5.6 shows the axis of the easterly jet at 12 km (200 mb). The figure shows that there is the subtropical westerly jet to the north of the Himalayas besides the easterly jet over the peninsular India. It has already been made clear in Fig. 5.4 that the westerly jet stream is located along the southern slopes of the Himalayas in winter but it suddenly shifts to the north with the onset of the monsoon.

The periodic movements of the sub-tropical jet stream provide a useful indication of the onset and subsequent withdrawal of the monsoon. In fact, northward movement of the subtropical jet is the first indication of the onset of the monsoon over India.

Recent observations have revealed that the intensity and duration of heating of Tibet Plateau has a direct bearing on the amount of rainfall in India by the monsoons. When the summer temperature of air over Tibet remains high for a sufficiently long time, it helps in strengthening the easterly jet and results in heavy rainfall in India.

The easterly jet does not come into existence if the snow over the Tibet Plateau does not melt. This hampers the occurrence of rainfall in India. Therefore, any year of thick and widespread snow over Tibet will be followed by a year of weak monsoon and less rainfall.

Thomson (1951), Flohn, (1960) and Stephenson (1965) have expressed more or less similar views But Flohn’s concept is widely accepted. These ideas can be explained by considering the winter and the summer conditions over large parts of Asia.

Winter:
This is the season of outblowing surface winds but aloft the westerly airflow dominates. The upper westerlies are split into two distinct currents by the topographical obstacle of the Tibet Plateau, one flowing to the north and the other to the south of the plateau. The two branches reunite off the east coast of China (Fig. 5.7).

The southern branch over northern India corresponds with a strong latitudinal thermal gradient which, along with other factors, is responsible for the development of southerly jet. The southern branch is stronger, with an average speed of about 240 km p.h. at 200 mb compared with 70 to 90 km p.h. of the northern branch.

Air subsiding beneath this upper westerly current gives dry out blowing northerly winds from the subtropical anticyclone over northwestern India and Pakistan. The surface winds blow from northwest over most parts of northern India.

The upper jet is responsible for steering of the western depressions from the Mediterranean Sea. Some of the depressions continue eastwards, redeveloping in the zone of jet stream confluence about 30° N, 105° E beyond the area of subsidence in the immediate lee of Tibet.

Summer:
With the beginning of summer in the month of March, the upper westerlies start their northward march, but whereas the northerly jet strengthens and begins to extend across central China and into Japan, the southerly branch remains positioned south of Tibet, although weakening in intensity.

The weather over northern India becomes hot, dry and squally due to larger incoming solar radiation. By the end of May the southern jet begins to break and later it is diverted to the north of Tibet Plateau. Over India, the Equatorial Trough pushes northwards with the weakening of the upper westerlies south of Tibet, but the burst of the monsoon does not take place until the upper-air circulation has switched to its summer pattern (Fig. 5.8). The low level changes are related to the high level easterly jet stream over southern Asia about 15° N latitude.

T.N. Krishnamurti used data of the upper atmosphere to calculate the patterns of divergence and convergence at 200 mb for the period of June-August, 1967. He observed an area of strong divergence at 200 mb over northern India and Tibet, which coincides with the upper-level divergence associated with the easterly jet.

Similarly he found a northerly component to the flow from this region which represents the upper branch of the Hadley cell.
These happenings are closely related to the Indian monsoon. S. Rama Rattan opined that the development of monsoon winds is deeply connected with the jet stream in addition to the differential heating of land and sea.

The upper air circulation in summer has anticyclonic pattern between 40° N and 20° S whereas cyclonic conditions prevail at the surface. Western and eastern jets flow to the north and south of the Himalayas respectively. The eastern jet becomes powerful and is stationed at 15° N latitude. This results in more active south-west monsoon and heavy rainfall is caused.Raman and Ramanathan while discussing the tropical easterly jet stream suggested that the easterly winds become very active in the upper troposphere after the beginning of the rainy season. The latent heat produced due to cloud cover results into inversion of temperature and causes rainfall.

Ananth Krishnan is of the opinion that the south-west monsoons are deeply influenced by the subtropical cyclones in the upper troposphere between 20° and 25° N latitudes. These winds start developing in the beginning of the summer season and shift to 30° N about 5-6 weeks later.

Besides intensive heat between 20° and 40° N latitudes gives further strength to the south-west monsoons. S. Parthasarthy in his essay on ‘Trying to solve the Monsoon Riddle’ expressed the view that the monsoons are influenced by the north-east trade winds. A weak north-east trade wind results in weak monsoon and leads to drought conditions.

The Indian monsoons, particularly the south-west monsoons, have generated a lot of interest among the meteorologists all over the world. Concerted efforts on data collection and of intensive studies of monsoon regimes by various meteorological services and organisations from different nations have been made during the last four decades.

Much has been done but much more is yet to be done. The first attempt was done during International India Ocean Expedition (IIOE) from 1962 to 1965. It was organised jointly by the International Council of Scientific Unions. (ICSU), Scientific Committee on Ocean Research (SCOR) and UNESCO with World Meteorological Organisation (WMO) joining the meteorology programme.

Special oceonographic and atmospheric studies were carried out with the aid of research vessels, instrumented aircrafts, rockets as well as special upsonde and dropsonde soundings. Two more experiments were conducted, jointly, by India and the former USSR in 1973 and 1977, with limited participation from other countries.

These experiments are known as the Indo-Soviet Monsoon Experiment (ISMEX) and Monsoon-77 respectively. It was observed from these experiments that there is a specific zone off the coast of Kenya where the monsoons from the southern hemisphere crossed the equator on their way to India.

It was also observed that the fluctuations in the intensity of low-level across the equator resulted in the fluctuations of rainfall over Maharashtra. Upper air observations over the Bay of Bengal were also made in 1977.

More intensive data collection effort was made under the aegis of another international experiment—the Monsoon Experiment in 1979. It is popularly known as MONEX-1979. It was organised jointly by Global Atmospheric Research Programme (GARP) of the International Council of Scientific Unions (ICSU) and the World Meteorological Organisation (WMO) under their World Weather Watch (WWW) programme.

It is so far the largest scientific effort made to extend the frontiers of our knowledge of the monsoons by the international scientific community. As many as 45 countries pooled their talents and resources under the aegis of the United Nations for this great venture.

Some idea of the dimensions of this experiment may be had from the fact that in May 1979 as many as 52 research ships were deployed over the tropical oceans between 10° N and 10° S latitudes. Besides 104 aircraft missions were successfully completed over different parts of the Pacific, the Atlantic and the Indian Ocean.

The great MONEX was designed to have three components considering the seasonal characteristics of the monsoon:

(i) Winter Monex from 1 December 1978 to 5 March 1979 to cover the eastern Indian Ocean and the Pacific along with the land areas adjoining Malaysia and Indonesia.

(ii) Summer Monex from 1 May to 31 August 1979 covering the eastern coast of Africa, the Arabian Sea and the Bay of Bengal along with adjacent landmasses. It also covered the Indian Ocean between 10° N to 10° S latitudes.

(iii) A West African Monsoon Experiment (WAMEX) over western and central parts of Africa from 1 May to 31 August 1979.International MONEX Management Centres (IMMC) was set up in Kuala Lumpur and New Delhi to supervise the winter and slimmer components of the experiment.

MONEX-1979 suffered some setback due to abnormal behaviour of the monsoons in that year. None of the cold surges was intense in China Sea during the winter MONEX. A strong anticyclone developed in the Arabian Sea in summer of 1979. The southwest monsoon was deflected southwards before touching the Kerala coast under the influence of this anticyclone and started blowing parallel to the coast.

Consequently the onset of southwest monsoon over Kerala was delayed by 12 days. Moreover, July was characterized by several weak or break- monsoon occurrences and there was only one monsoon depression.

Therefore, 1979 was not a normal monsoon year and MONEX failed to study the normal behavior of the monsoons. But the vagaries of the monsoon are proverbial and in a scientific and analytical understanding of the monsoons, a study of anomalies is perhaps more important. It is in this context that MONEX-1979 assumes un-paralled significance.

Teleconnections, the Southern Oscillation and the El Nino:
Recent studies have revealed that there seems to be a link between meteorological events which are separated by long distances and large intervals of time. They are called meteorological teleconnections. The one which has aroused considerable
interest among the meteorologists is the difference between an El Nino and the Southern Oscillation. El Nino (EN) is a narrow warm current which appears off the coast of Peru in December. In Spanish, it means The Child Christ because it appears around Christmas. In some years this warm current is more intense than usual.

The El Nino phenomena, which influence the Indian monsoon, reveal that when the surface temperature goes up in the southernPacific Ocean, India receives deficient rainfall. However, there had been some years during which the El Nino phenomena did not occur, but India still got deficient rainfall, and conversely, India received sufficient rainfall during an El Nino year.

A study of the last one hundred years of the Indian monsoons shows that out of 43 deficient monsoon years, 19 were associated with an El Nino. On the other hand, there were 6 El Nino years which were also years of good monsoon rain. Thus, although there is a tendency for poor monsoons to be associated with an El Nino, there is no one-to-one correspondence.

Southern Oscillation (S.O.) is the name ascribed to the curious phenomena of sea-saw pattern of meteorological changes observed between the Pacific and Indian oceans. This great discovery was made by Sir Gilbert Walker in 1920.

While working as the head of the Indian Meteorological service, he noticed that when the pressure was high over equatorial south Pacific, it was low over the equatorial south Indian Ocean and vice versa. The pattern of low and high pressures over the Indian and Pacific Oceans (S.O.) gives rise to vertical circulation along the equator with its rising limb over low pressure area and descending limb over high pressure area.

This is known as Walker Circulation. The location of low pressure and hence the rising limb over Indian Ocean is considered to be conductive to good monsoon rainfall in India. In other words when there is low pressure over the Indian Ocean in winter months, the chances are that the coming monsoon will be good and will bring sufficient rainfall.

Its shifting eastward from its normal position, such as in El Nino years, reduces monsoon rainfall in India. Due to the close association between an El Nino (E.N.) and the Southern Oscillation (S.O.), the two are jointly referred to as an ENSO event. Some of the predictors used by Sir Gilbert Walker are still used in long-range forecasting of the monsoon rainfall.

The main difficulty with the Southern Oscillation is that its periodicity is not fixed and its period varies from two to five years. Different indices have been used to measure the intensity of the Southern Oscillation, but the most frequentlyused is the Southern Oscillation Index (SOI).

This is the difference in pressure between Tahiti (17°45’S, 149°30’W) in French Polynesia, representing the Pacific Ocean and Port Darwin (12°30’S, 131°E), in northern Australia representing the Indian Ocean. The positive and negative values of the SOI i.e. Tahiti minus the Port Darwin pressure are pointers towards good or bad rainfall in India (see the following table)Scientists of India Meteorological Department (IMD) joined an international study programme called the Tropical Oceans and Global Atmosphere (TOGA) in 1985. This is an interesting and ambitious programme.which investigates both teleconnections effects and the internal variability. As a follow up to TOGA, the climate variability (CLIVAR) was set up in January 1995,to develop an internationally operational climate forecasting system.

Positive SOI:

(i) Tahiti pressure greater than that of Port Darwin

(ii) Pressure high over east Pacific and low over Indian Ocean.

(iii) Low rainfall over eastern Pacific and prospects of good monsoon rain over India and Indian Ocean.
Negative SOI:

(i) Port Darwin pressure exceeds that of Tahiti.

(ii) Pressures high over Indian Ocean and low over east Pacific.

(iii) Low rainfall or poor monsoon over Indian Ocean and higher than usual rain over east Pacific.

Another major programme is the Indian Middle Atmospheric Programme (IMAP) initiated by the Department of Space. This programme has been launched to augment the existing weather prediction scheme. This is expected to improve scientific understanding of climatic changes that take place in Indian tropical region and the area along the Tropic of Cancer when the monsoon winds 
descend.

After the severe drought of 1987, parametric and power regression models have been developed to forecast monsoon rainfall by utilising signals from 15 parameters. Some of the parameters are global while others are regional. These parameters are divided into four broad categories, viz. (a) temperature, (b) pressure (c) wind pattern and (d) snow cover and are listed 
below:

(a) Temperature related parameters:

1. El Nino in current year 
2. El Nino in previous year
3. Northern India (March) 
4. East coast of India (March)
5. Central India (May) 
6. Northern hemisphere (Jan. and Feb.)

(b) Wind related parameters:

7. 500 hPa (1 hecta pascal, equals 1 mb) ridge (April)
8. 50 hPa ridge-trough extent (Jan. and Feb.)
9. 10 hPa (30 km) westerly wind (Jan.)

(c) Pressure anomaly (SOI): 
10. Tahiti-Darwin (Spring) 11. Darwin (Spring)
12. South America, Argentina (April) 13. Indian Ocean Equatorial (Jan.-May)

(d) Snow cover related parameters: 
14. Himalayan (Jan.- March) 15. Eurasian (Previous December)

It was observed in late eighties that whenever more than 50% parameters showed favourable signals, the monsoon rainfall in India was normal and when 70% or more parameters were favourable, the monsoon rainfall was above normal.

Somewhat similar set of predictors for monsoon was suggested by H.N. Srivastava and S.S. Singh in 1994 while discussing long range weather forecasting techniques.

One more parameter, viz., surface pressure anomaly of north-eastern hemisphere was also added later on, thus making a total of 16 parameters. These 16 parameters have been used by the IMD to develop the power regression model. Although this model has been accurately forecasting rainfall in India since 1989, it is far from being an elaborate and foolproof model.

A model capable of forecasting area specific rainfall is yet to be built. The study of data flowing from MONEX, TOGA and other experiments is continuing and our meteorologists are hopeful of discovering more parameters which may help in developing better models capable of predicting rainfall more accurately.