I: The Monsoon
About Monsoon
What is a Monsoon?
The term “monsoon” is derived from the Arabic word “Mausin” or the Malayan word “Monsen,” both meaning “season.” Monsoons refer to seasonal winds that are a prominent feature in certain regions of the world. These winds reverse their direction with the change of season, bringing significant changes in weather patterns.
Monsoon Winds
Monsoon winds are classified into two main types: the south-west monsoon winds and the north-east monsoon winds.
- South-west Monsoon Winds: These winds occur during summer and are formed due to an intense low-pressure system over the Tibetan plateau. They bring intense rainfall to most of the regions in India, Indonesia, Bangladesh, Myanmar, and other nearby areas.
- North-east Monsoon Winds: These winds occur during winter and are associated with high-pressure cells over the Tibetan and Siberian plateaus. They bring rainfall mainly to the south-eastern coast of India, the southern coast of Seemandhra, the coast of Tamil Nadu, South East China, Japan, and other regions.
Factors Responsible for South-west Monsoon
Several factors contribute to the formation of the south-west monsoon. These include:
- Intense heating of the Tibetan plateau during the summer months.
- The presence of a permanent high-pressure cell in the South Indian Ocean, east to north-east of Madagascar during summer.
Factors Influencing the Onset of South-west Monsoons
The onset of the south-west monsoon is influenced by several factors, including:
- Intense heating of the Tibetan plateau.
- Subtropical Jet Streams.
- Tropical Easterly Jet Streams.
- Inter Tropical Convergence Zone (ITCZ) – a region of converging winds and associated rainfall.
Factors Influencing the Intensity of South-west Monsoons
The intensity of the south-west monsoons is influenced by various factors, such as:
- The strength of the low-pressure system over Tibet.
- The high-pressure system over the southern Indian Ocean.
- The Somali Jet (Findlater Jet).
- The Somali Current.
- The Indian Ocean branch of the Walker Cell.
- The Indian Ocean Dipole.
Factors Responsible for North-east Monsoon Formation
The formation of the north-east monsoon is influenced by the following factors:
- Strengthening and formation of high-pressure cells over the Tibetan plateau and Siberian Plateau in winter.
- Westward migration and subsequent weakening of high-pressure cells in the southern Indian Ocean.
- Migration of the Inter Tropical Convergence Zone (ITCZ) to the south of India.
Mechanism of Indian Monsoon
The mechanism of the Indian monsoon is not fully understood, and various theories have been proposed to explain its occurrence. These theories can be divided into two categories: the Classical Theory and the Modern Theories.
Classical Theory
The Classical Theory of monsoons is based on historical accounts and early scientific studies. It includes:
- References to monsoons in ancient scriptures like the Rig Veda, which describe the phenomenon without specific mechanisms.
- The first scientific study of monsoon winds was conducted by Arab traders who recognized the importance of understanding monsoon patterns for their trading activities.
- Arab explorer Al Masudi provided an account of the reversal of ocean currents and monsoon winds over the North Indian Ocean in the 10th century.
- In the 17th century, Sir Edmund Halley explained the monsoon as resulting from thermal contrasts between continents and oceans due to their differential heating.
The Classical Theory describes two types of monsoons:
- Summer Monsoon: During summer, the sun’s apparent path is vertically over the Tropic of Cancer, resulting in high temperatures and low pressure in Central Asia. The pressure is sufficiently high over the Arabian Sea and Bay of Bengal, causing winds to flow from the ocean towards the landmass, bringing heavy rainfall to the Indian subcontinent.
- Winter Monsoon: During winter, the sun’s apparent path is vertically over the Tropic of Capricorn. The northwestern part of India grows colder than the Arabian Sea and Bay of Bengal, leading to a reversal of the monsoon flow.
While the Classical Theory provides a basic understanding of the monsoon phenomenon, it has limitations in explaining its intricacies, such as sudden bursts of monsoons and delays in onset.
Modern Theory
Modern theories of monsoons take into account air masses, jet streams, and upper tropospheric circulation. They explain the development of monsoons by considering factors such as the shape of continents, orography, and air circulation conditions in the upper troposphere.
Modern Theory: Air Mass Theory
The Air Mass Theory explains monsoons as a modification of the planetary winds of the tropics. It is based on the migration of the Inter Tropical Convergence Zone (ITCZ) with seasons and its interaction with trade winds. The theory can be summarized as follows:
- The southeast trade winds in the southern hemisphere cross the equator and start blowing in a southwest to northeast direction under the influence of the Coriolis force. These displaced winds are called the southwest monsoon winds when they blow over the Indian subcontinent. When they meet the northeast trade winds, a front called the Monsoon Front (ITCZ) is formed, causing rainfall.
- The northeast trade winds in the northern hemisphere meet the southeast trade winds at the equator, forming the Inter Tropical Convergence Zone (ITCZ). This region experiences ascending air, maximum clouds, and heavy rainfall. The location of the ITCZ shifts north in summer (sun vertically over the Tropic of Cancer) and south with the change of season.
- In July, the ITCZ shifts to around 20-25 degrees North, located in the Indo-Gangetic plain. The south-west monsoon winds blow from the Arabian Sea and Bay of Bengal, bringing significant rainfall.
The seasonal shift of the ITCZ gives rise to the concept of the Northern Inter-Tropical Convergence Zone (NITCZ) during summer (rainy season) and the Southern Inter-Tropical Convergence Zone (SITCZ) during winter (dry season).
Jet Stream Theory
The Jet Stream Theory, proposed by P. Koteswaram, explains the influence of upper-air circulation on monsoons. It focuses on the role of jet streams in the origin of monsoons. The key points of this theory are as follows:
- Jet streams are narrow bands of fast-moving air flowing from west to east (westerlies) at high altitudes. The Sub-Tropical Jet Stream (STZ) plays a significant role in hindering or facilitating the onset and intensity of monsoons.
- The STZ is a narrow band of fast-moving air flowing from west to east between 25-35 degrees North in the upper troposphere, at a height of about 12-14 km.
- During winter, the STZ flows along the southern slopes of the Himalayas. However, in summer, it shifts northwards and flows along the northern edge of the Himalayas.
- The periodic movement of jet streams is often an indicator of the onset and subsequent withdrawal of the monsoon. The northward shift of the STZ is the first indication of the monsoon onset over India.
- The STZ in winter creates strong divergence and high-pressure regions over northwestern India, leading to dry weather conditions.
- In summer, with the emergence of an easterly jet over peninsular India, associated with the northward migration of the STZ, upper-air circulations reverse, becoming active in the upper troposphere and associated with the south-west monsoon winds.
The Jet Stream Theory explains how the mechanism of jet streams influences Indian monsoons by affecting upper-level circulation patterns and the convergence or divergence of winds at different levels of the atmosphere.
Indian Monsoon Mechanism: Role of Sub-Tropical Jet Streams
The Sub-Tropical Jet Stream (STZ) plays a significant role in both hindering and facilitating the onset and burst of monsoons. Its seasonal migration and associated weather patterns are crucial factors. Let’s explore this in more detail.
Sub-Tropical Jet Stream (STZ)
The STZ is a narrow band of fast-moving air flowing from west to east (westerlies) in the upper troposphere. It is positioned between 25-35 degrees North at a height of about 12-14 km. The STZ is a key player in influencing monsoon dynamics.
Seasonal Migration of Sub-Tropical Jet Streams
During winter, the STZ flows along the southern slopes of the Himalayas. However, in summer, it shifts northwards and flows along the northern edge of the Himalayas and the Tibetan Plateau. The periodic movement of jet streams often indicates the onset and subsequent withdrawal of the monsoon.
- In early June, the STZ shifts to the north of the Himalayas, indicating the onset of the monsoon over India.
- In July and August, the STZ flows along the northern edge of the Tibetan Plateau.
- The ridge of the STZ moves northwards into Central Asia, resulting in a high-pressure system over north-west India, which creates conditions for the south-west monsoon.
The movement of the STZ is closely linked to the seasonal variations in monsoon patterns.
Sub-Tropical Jet Stream (STZ) in Winter
During winter, the westerly jet stream blows at high speeds over the sub-tropical zone. It bifurcates due to the Himalayan ranges and the Tibetan Plateau, reuniting off the east coast of China. The northern branch blows along the northern edge of the Tibetan Plateau, while the southern branch flows to the south of the Himalayan ranges, along the 25 degrees North latitude. The western disturbances, or winter monsoons, are associated with the southern branch of the jet stream.
Meteorologists believe that the southern branch of the jet stream influences the winter weather conditions of India. It creates a strong upper jet responsible for steering western disturbances from the Mediterranean Sea. This is followed by cold waves in the northern plains and occasional heavy snowfall in hilly regions.
The dynamics of the STZ and its interaction with other atmospheric features contribute to winter weather patterns in India.
Why No South-west Monsoons During Winter?
Several factors contribute to the absence of south-west monsoons during winter:
- During winter, the Inter Tropical Convergence Zone (ITCZ) moves away from India. The winds blowing over India are mostly offshore, carrying no moisture.
- The southern branch of the STZ, which is strong and positioned south of the Himalayas, creates a ridge that causes strong divergence and high-pressure conditions in north-west India. This blocks the incoming winds and prevents strong convergence along the ITCZ, resulting in dry and warm weather.
- The presence of a strong high-pressure system over Tibet, combined with the high pressure from the southern branch of the STZ, leads to strong divergence and no rainfall.
These factors contribute to the absence of south-west monsoons during the winter months, despite favorable insolation and high temperatures.
II: Indian Monsoons
Tropical Easterly Jet (TEZ)
What is the Tropical Easterly Jet (TEJ)?
- The Tropical Easterly Jet (TEJ) refers to major high-velocity winds in the lower troposphere known as low-level jets (LLJs).
- The most prominent TEJ is the Somali jet and the African Easterly Jet, which are unique and dominant features during the Northern hemispheric summer over southern Asia and northern Africa.
- These jets are found near 5 and 20 degrees North and exhibit fairly persistent direction and intensity from June through the beginning of October.
- The TEJ comes into existence quickly after the Sub-Tropical Jet Stream (STZ) shifts north of the Himalayas in early June.
- It flows from east to west over peninsular India and the northern African region.
Formation and Role of the Tropical Easterly Jet (TEJ)
- The exact cause of the establishment and maintenance of the TEJ is not fully understood, but it is believed to be caused by the uniquely high temperature and heights over the Tibetan Plateau during summer.
- The TEJ plays an important role in kick-starting the southwest monsoon.
- This jet descends over the Indian Ocean (near Madagascar) and intensifies the high-pressure cell, resulting in its movement as the southwest monsoon.
- The easterly jet does not come into existence if the snow over the Tibetan Plateau does not melt, which hampers rainfall occurrence in India.
- Therefore, a year with thick and widespread snow over Tibet will be followed by a year of weak monsoon and less rainfall.
Role of Tibet in Monsoons
- The Tibet Plateau acts as a formidable barrier and receives 2-3 degrees more insolation due to its protruded height, making it 2-3 degrees Celsius warmer than the air over adjoining areas.
- Tibet affects the atmosphere as a mechanical barrier and a high-level heat source.
- At the beginning of June, the Subtropical jet stream completely withdraws from India and occupies a position along 40 degrees North (to the north of the Tibetan Plateau).
- The plateau accentuates the northward displacement of the jet stream, leading to the burst of monsoon by the Himalayas, not thermally induced low-pressure cells over Tibet, and is responsible for the southwest monsoons.
- However, the STZ facilitates the sudden outburst of the monsoon with its sudden northward migration.
- In the middle of October, the plateau causes the advancement of the jet south of the Himalayas and bifurcates it into two parts.
- The winter Tibetan plateau cools rapidly and produces a high-pressure cell, strengthening the northeast monsoons. The cyclonic conditions over Tibet cease, and anticyclonic conditions are established.
Role of Somali Jet
- Permanent jet streams, such as the polar and subtropical jet streams, greatly influence the weather of temperate regions.
- Temporary jet streams, like the Somali jet, aid the progress of the southwest monsoon towards India as it transits Kenya, Somalia, and Sahel.
- The Somali jet flows from Mauritius and the northern part of the island of Madagascar, reaching the coast of Kenya at 3 degrees South.
- It strengthens the permanent high-pressure system near Madagascar and helps drive the southwest monsoons towards India at a greater pace and intensity.
- The path of the Somali jet around 9 degrees North coincides with a zone of coastal upwelling, driving away surface coastal water towards the east. This creates extremely cold water that rises upwards to preserve the continuity of mass.
- A peculiar feature of the Somali jet is its reversal in direction with the onset of the summer monsoon. In winter, it flows from north to south, running southwards from the coast of Arabia to the East African coastline, while in summer, it flows from south to north.
- The African Easterly jet, also known as the Tropical Easterly Jet, and the Somali jet both play important roles in the formation and progression of Indian Monsoons.
Role of the Indian Ocean Dipole (IOD)
- The Indian Ocean Dipole (IOD) is a recently discovered phenomenon that influences Indian monsoons.
- It refers to the sea surface temperature anomaly that occurs occasionally in the northern or equatorial Indian Ocean region.
- The IOD is defined by the difference in sea surface temperature between two areas, namely the western pole in the Arabian Sea (western Indian Ocean) and the eastern pole in the eastern Indian Ocean south of Indonesia.
- The IOD develops in the equatorial region of the Indian Ocean from April to May, peaking in October.
- In a positive IOD year, winds over the Indian Ocean blow from east to west, resulting in the Arabian Sea being much warmer and the eastern Indian Ocean around Indonesia being colder and drier. This leads to good Indian monsoons due to more evaporation in the warm water.
- In a negative IOD year, the reverse happens, making Indonesia much warmer and rainier.
- The atmospheric component of the IOD, known as the Equatorial Indian Ocean Oscillation (EQUINOO), exhibits two phases: positive and negative. It oscillates the pressure cells between the Bay of Bengal and the Arabian Sea. During the positive phase of EQUINOO, enhanced cloud formation and rainfall occur in the western part of the equatorial ocean, while it is suppressed near Sumatra. EQUINOO and IOD go in step during strong positive IOD events, but not always.
The interplay between the Tropical Easterly Jet (TEJ), Somali jet, Tibetan Plateau, and Indian Ocean Dipole (IOD) contributes to the complex dynamics of Indian monsoons. These factors and their interactions are vital for understanding and predicting monsoon patterns, which have significant implications for agriculture, water resources, and the overall socio-economic well-being of the regions affected by the monsoon.
III: Indian Monsoon-Seasonal Variations
The Indian monsoon is a unique weather phenomenon that brings the majority of rainfall to the Indian subcontinent. It is characterized by distinct seasonal variations, with periods of intense rainfall alternating with dry spells. Understanding these variations is crucial for predicting agricultural productivity, water resource management, and overall climate patterns in the region.
In May-Dry Seasons
In the month of May, the Indian subcontinent experiences dry conditions as the monsoon season has not yet arrived. Several factors contribute to the absence of rainfall during this period:
- High Pressure in North-Western India: A high-pressure system forms over northwestern India, which creates strong divergence in the atmosphere. This divergence inhibits the arrival of the south-west monsoon winds, which are responsible for bringing rainfall to the region.
- Low Pressure in Tibet: While high temperatures prevail over most parts of India, low-pressure conditions persist in Tibet and the rest of India. The absence of rainfall can be attributed to the strong divergence caused by the ridge region of the subtropical jet stream (STZ).
- Occasional Thunderstorms: Despite the overall dry conditions, some parts of South India experience occasional thunderstorms during this period, providing temporary relief from the heat.
- Absence of Somali Jet and Tropical Easterly Jet: The absence of these low-level jets also contributes to the dry conditions in May.
In June-Onset of Monsoons (June 1st – June 10th)
June marks the onset of the monsoon season in India, with the south-west monsoon winds making their way into the subcontinent. Several atmospheric changes occur during this period:
- Apparent Movement of the Sun: The onset of the monsoon season is closely linked to the apparent movement of the sun. The summer solstice occurs, resulting in a change in the position of the sun, which triggers the migration of the subtropical jet stream (STZ) to the north of the Tibetan Plateau.
- Migration of High Pressure Cells: As the STZ migrates northward within 1-2 weeks, it leads to a northward migration of high-pressure cells. This sudden outburst of monsoon rainfall usually occurs in the first week of June.
- Tropical Easterly Jet: The reversal of upper air circulation by the tropical easterly jet creates high pressure in the upper troposphere and low pressure in the lower troposphere, facilitating the onset of the monsoon.
- Somali Jet: The Somali jet, a low-level jet, plays a crucial role in the monsoon dynamics. Its lower branch drives the monsoon winds, bringing moisture-laden air to the subcontinent.
- Mascarene High: At the Mascarene High, a strong walker cell creates stronger high-pressure cells, which is favorable for the monsoons. The subsidence of dry air also contributes to the onset of the monsoon.
- South-West Monsoon: The overall atmospheric changes in June result in the establishment of a strong south-west monsoon, bringing widespread rainfall to the region.
In July
July is characterized by the further advancement of the monsoon winds, reaching northwestern India. However, the rainfall distribution during this period varies across different regions:
- Monsoon Winds in North-West India: While the monsoon winds reach northwestern India, the rainfall is relatively low in this region due to the continentality effect, where the absence of a nearby large water body results in decreased moisture availability.
- Severe Rainfall in North-East States and Central India: On the other hand, the north-eastern states and central India receive good rainfall during this period. The discontinuous and relatively low Eastern Ghats contribute to the enhancement of rainfall in these regions.
- Rainfall from the Bay of Bengal Branch: The monsoon winds from the Bay of Bengal branch bring intense rainfall to the Western Ghats and other parts of southern India.
- ITCZ at its Peak: The Intertropical Convergence Zone (ITCZ) is at its peak during late June and early July, resulting in heavy rainfall along its path.
- Rainshadow Effect: Telangana, parts of Karnataka, and Andhra Pradesh receive less rainfall during July due to the rainshadow effect caused by the Western Ghats.
- No Rainfall in Rajasthan and Gujarat: Rajasthan experiences no rainfall during this period as the monsoon winds blow parallel to the Aravalli Mountains. Similarly, Gujarat also receives minimal rainfall due to the absence of an orographic barrier.
In August
August marks the retreat of the monsoon from northwestern India and the strengthening of high-pressure systems in Tibet and Central Asia. Key features of this period include:
- Monsoon Retreat: The monsoon winds start to retreat from northwestern India, resulting in reduced rainfall in this region.
- Building High Pressure: High-pressure systems begin to build up in Tibet and Central Asia, influencing the monsoon dynamics.
- Weakening South-West Monsoons: The south-west monsoon winds gradually weaken during this period.
- Rainfall Picking Up in Parts of Telangana, Andhra, and Karnataka: While the overall monsoon activity decreases, some parts of Telangana, Andhra Pradesh, and Karnataka continue to receive rainfall.
- Retreating ITCZ: The ITCZ starts to retreat from its peak position, affecting the rainfall patterns in the subcontinent.
- Peak Typhoon Season: The Western Pacific experiences its peak typhoon season during August.
- Movement of High Pressure: High-pressure systems begin moving eastwards during this period.
- Weakening Somali Jet: The Somali jet, which plays a crucial role in driving the monsoon winds, starts to weaken.
- Low-Level Disturbances in the Bay of Bengal: The Bay of Bengal experiences low-level disturbances, which can lead to localized rainfall events.
In September
September is characterized by specific weather patterns, including:
- Apparent Movement of the Sun: The equinox occurs in September, marking a change in the apparent movement of the sun.
- Maximum Rainfall in Parts of South India: Parts of South India experience the highest rainfall during this period.
- Occasional Depressions in the Arabian Sea: The Arabian Sea may witness occasional depressions, which can lead to cyclone formation.
- Cyclones in Northern Bay of Bengal: The northern Bay of Bengal is prone to cyclone formation during September, although these cyclones are generally less destructive due to quick landfall.
- Strengthening of High Pressure: High-pressure systems strengthen in Tibet and Central Asia.
- North East Monsoons: The north-east monsoons bring good rainfall to regions like southern China and Vietnam.
- Retreating STZ: The subtropical jet stream starts to retreat during this period.
- Minimal Rainfall Along the Equator: Very little rainfall is observed along the equator.
- Highest Rainfall Along ITCZ: The ITCZ continues to bring the highest rainfall, particularly in regions like Karnataka, Telangana, and Andhra Pradesh.
- ITCZ Leaving India: The ITCZ gradually moves away from the Indian subcontinent.
In October
October marks the retreating phase of the monsoon season. Key features of this period include:
- Retreating Monsoons: The monsoon winds start to retreat from the Indian subcontinent, leading to a decrease in rainfall.
- Cyclones in Late October: Late October sees cyclone formation, which can cause rainfall in regions such as the Bay of Bengal and the Arabian Sea.
- Weakening of Somali Jet and Easterly Tropical Jet: By the end of October, both the Somali jet and the easterly tropical jet diminish in strength.
- STZ in North India: The subtropical jet stream moves back to North India during this period.
- Intense High Pressure: Intense high-pressure systems develop in the Siberian plateaus and Central Asia.
- ITCZ Leaving India: The ITCZ continues its eastward movement, gradually leaving India.
- High Pressure Moving Eastward: High-pressure systems continue to move eastward.
In November
November brings specific weather conditions and changes in monsoon patterns, including:
- North-East Monsoons: The north-east monsoons bring rainfall to regions such as Tamil Nadu, southern Seemandhra, Japan, and Southeast China.
- Peak Cyclone Season: Early November marks the peak cyclone season in the Indian Ocean.
- Winter Season: Winter starts to set in, with southern India experiencing cooler temperatures from late November and northern India from early November.
- Winter Disturbances: Winter disturbances affect north-western India, bringing cold waves and weather disruptions.
- Strong North-East Monsoons: The strong north-east monsoons result in good rainfall in Tamil Nadu, southern Seemandhra, Japan, and Southeast China.
In December
December brings specific weather patterns and climatic events, including:
- Maximum Rainfall Month: Tamil Nadu and the southern Andhra Coast experience the maximum rainfall during this month.
- Cold Waves: North India experiences cold waves during December.
- Southern Branch of STZ: The southern branch of the subtropical jet stream weakens and splits into two branches due to the orographic barrier of the Himalayas.
- Winter Solstice: The Northern Hemisphere experiences the winter solstice, marking the shortest day and longest night of the year.
- Western Disturbances: Western disturbances, originating from the Mediterranean Sea, bring frontal storms to north-western India. These disturbances pick up moisture from the Caspian Sea and the Black Sea, which is important for wheat crops.
In January
January brings changes in monsoon conditions and weather patterns, including:
- Weakening of North-East Monsoons: The intensity of the north-east monsoons weakens during this period.
- Continued Conditions from December: Weather conditions during January are similar to those in December, with cold waves affecting the entire country and the ITCZ in its peak position.
In February
February signifies the transition to the dry season, with conditions similar to those in December and January. The retreat of the ITCZ continues during this period.
In March
March marks the beginning of summer in India, with several weather changes occurring:
- Weakening of Southern STZ: The southern subtropical jet stream starts to weaken and splits into two branches due to the orographic barrier of the Himalayas. The southern branch remains stronger.
In April
April is still part of the dry season, but occasional thunderstorms occur in certain regions of South and Central India, along with the ITCZ. These thunderstorms can cause damage to crops in regions like Karnataka, Andhra Pradesh, and Telangana. The STZ remains intact in the south but starts moving northwards, while high-pressure systems persist over the plains and northwestern India.
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