Subdued Northeast Monsoon

Rainfall over the southern peninsular region has been deficient so far, indicating that the northeast monsoon has remained subdued this year.

Significance:

Pattern of Rainfall in India: India receives rainfall during two seasons:

• About 75% of the country’s annual rainfall is received from the Southwest monsoon between June and September.
• The Northeast monsoon occurs during October to December, and is a comparatively small-scale monsoon, which is confined to the Southern peninsula. It is called the winter monsoon.

Northeast Monsoon and Rainfall:

• After the complete withdrawal of the Southwest monsoon from the country takes place by mid-October, the wind pattern rapidly changes from the south-westerly to the north-easterly direction.
• The period after the Southwest monsoon season, from October to December, is the peak time for cyclonic activity in the North Indian Ocean region covering the Arabian Sea and the Bay of Bengal.

The winds associated with the formation of low pressure systems, depressions, or cyclones influence this monsoon, and therefore, the rainfall.

Regions associated with Northeast Monsoon:

• The rainfall associated with the Northeast monsoon is important for Tamil Nadu, Puducherry, Karaikal, Yanam, coastal Andhra Pradesh, Kerala, north interior Karnataka, Mahe and Lakshadweep.
• Tamil Nadu records about 48% of its annual rainfall during these months, making it the key factor for undertaking agricultural activities and reservoir management in the state.
• Some South Asian countries such as Maldives, Sri Lanka and Myanmar, too, record rainfall during October to December.

Reasons for deficient rainfall this Northeast monsoon:

Prevailing La Niña conditions in the Pacific Ocean:

• La Niña conditions enhance the rainfall associated with the Southwest monsoon, but has a negative impact on rainfall associated with the Northeast monsoon.
• La Niña (Spanish for ‘little girl’) refers to the large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall.
• It usually has the opposite impacts on weather and climate as El Niño, which is the warm phase of the so-called El Niño Southern Oscillation (ENSO).

• El Niño (Spanish for ‘little boy’) is the abnormal surface warming observed along the eastern and central regions of the Pacific Ocean (region between Peru and Papua New Guinea).
• La Nina and El Nino are large-scale ocean phenomena which influence the global weather winds, temperature and rainfall.
• They have the ability to trigger extreme weather events like droughts, floods, hot and cold conditions, globally.
• Each cycle can last anywhere between 9 to 12 months, at times extendable to 18 months and re-occur after every three to five years.

Inter Tropical Convective Zone (ITCZ):

• The current position of the ITCZ has also contributed to the poor rainfall during the ongoing monsoon season.
• The ITCZ is a low-pressure belt, whose northward and southward movements along the equator determine the precipitation in the tropics.
• Currently, the ITCZ is located to the north of its normal position.

Other Important Atmospheric Circulation

Madden-Julian Oscillation (MJO): The MJO can be defined as an eastward moving 'pulse' of clouds, rainfall, winds and pressure near the equator that typically recurs every 30 to 60 days.

New Monsoon Forecast Model

• The India Meteorological Department (IMD) may introduce new monsoon models this year to better forecast changes in rainfall.
• The monsoon that concluded in 2020 was unique, in that with monsoon 2019, it was only the third time in a century that India saw back-to-back years of above normal rainfall. In both years — and monsoon 2019 was a 25-year high — the IMD failed to forecast the magnitude of the excess and only indicated that the monsoon would be “above normal”.
• D.S. Pai, who heads the IMD’s Climate Research Services, Pune, said in his talk there were three different models that could be tested this year. Two of them were dynamical models and one a statistical model.

Computer simulation

• In the former, the climate on any particular day is simulated on supercomputers and meteorologists observe the changing daily output. The other is the traditional statistical model that equates relationships of physical parameters, such as for instance sea surface temperatures, snowfall, the temperature of landmass etc, with the actual observed rainfall in the past.
• The three models under consideration are:

1. 12 global circulation models (dynamical) whose outputs would be combined into a single one;
2. a model that gauges rainfall based on the sea surface temperature in the tropics (developed by Professor Sumant Nigam, University of Maryland, U.S.) and
3. the statistical model based on climate variables observed during the pre-monsoon. All of them are ‘ensembles’ meaning smaller models are combined to arrive at an average value.

• M. Mohapatra, Director General, IMD, told The Hindu that the traditional statistical model would continue to be used this year. “However, we will continue to have discussions and will decide later on what new approach can be added.”

What is Madden Julian Oscillation?

• It is an oceanic-atmospheric phenomenon which affects weather activities across the globe. It brings major fluctuation in tropical weather on weekly to monthly timescales.
• 2. The MJO can be defined as an eastward moving 'pulse' of clouds, rainfall, winds and pressure near the equator that typically recurs every 30 to 60 days.
• 3. It’s a traversing phenomenon and is most prominent over the Indian and Pacific Oceans.

Phases of MJO (Madden Julian Oscillation)

The MJO consists of two parts or phases. Strong MJO activity often dissects the planet into halves. One half within the enhanced convective phase and the other half in the suppressed convective phase.

1. Enhanced rainfall (or convective) phase:

Winds at the surface converge, and the air is pushed up throughout the atmosphere. At the top of the atmosphere, the winds reverse (i.e., diverge). Such rising air motion in the atmosphere tends to increase condensation and rainfall.

2. Suppressed rainfall phase:

• Winds converge at the top of the atmosphere, forcing air to sink and, later, to diverge at the surface. As air sinks from high altitudes, it warms and dries, which suppresses rainfall.
• It is this entire dipole structure, that moves west to east with time in the Tropics, causing more cloudiness, rainfall, and even storminess in the enhanced convective phase, and more sunshine and dryness in the suppressed convective phase.

How Does MJO Affect Indian Monsoon?

• The Indian Ocean Dipole (IOD), El Nino and MJO are all oceanic and atmospheric phenomena, which affect weather on a large scale. IOD only pertains to the Indian Ocean, but the other two affect weather on a global scale-up to the mid-latitudes.
• IOD and El Nino remain over their respective positions, while MJO is a traversing phenomenon.
• The journey of MJO goes through eight phases.
• When it is over the Indian Ocean during the Monsoon season, it brings good rainfall over the Indian subcontinent.
• On the other hand, when it witnesses a longer cycle and stays over the Pacific Ocean, MJO brings bad news for the Indian Monsoon.
• It is linked with enhanced and suppressed rainfall activity in the tropics and is very important for the Indian monsoonal rainfall.

Periodicity of MJO:

• If it is nearly 30 days then it brings good rainfall during the Monsoon season.
• If it is above 40 days then MJO doesn't give good showers and could even lead to a dry Monsoon.
• Shorter the cycle of MJO, better the Indian Monsoon. Simply because it then visits the Indian Ocean more often during the four-month-long period.
• Presence of MJO over the Pacific Ocean along with an El Nino is detrimental for Monsoon rains.

Monsoon decided the history of the Indian subcontinent

• A recent study by researchers at the Indian Institute of Technology, Kharagpur (IIT­KGP) has revealed that abrupt changes in the Indian monsoon in the last 900 years decided the course of human history in the sub-continent.
• A paper titled “Abrupt changes in Indian summer monsoon strength during the last 900 years and their linkages to socio-economic conditions in the Indian subcontinent” by Anil K. Gupta, professor at the geology and geophysics department of IIT­KGP, highlights that decline of Indian dynasties was linked to weak monsoon and reduced food production.
• Several dynasties, such as the Sena in Bengal, Solanki in Gujarat in the mid­13th century and Paramara and Yadav in the early to mid­14th century – all of which flourished during abundant rainfall — declined during the dry phases of Indian summer monsoon (ISM), suggesting role of the climate in the socio­political crisis, the study revealed.
• Deficient rainfall led to the collapse of the Mansabdari system, started by Mughal emperor Akbar, in the late 17th century. Similarly, drought interspersed with violent monsoon rains sounded the death knell for the Khmer empire of southeast Asia in the 15th century.
• The paper published in the international journal PALEO 3 highlights three phases in the 900­year stretch — Medieval Climate Anomaly from 950 CE to 1350 CE, Little Ice Age from 1350 CE to 1800 CE and Current Warm Period from 1800 CE till today. The paper highlights strong monsoons during the Medieval Climate Anomaly and Current Warm Period and phases of weak and strong monsoon in Little Ice Age.

• For the study on long-term spatio-temporal variability of the ISM, a group of researchers, which also included experts from Wadia Institute of Himalayan Geology, looked at palaeoclimatic records using oxygen isotope proxy record from speleothems (a structure formed in a cave by deposition of minerals from water) at the Wah Shikar cave in Meghalaya.

SCO Exercise Peaceful Mission: 2021

• The 6th Edition of Shanghai Cooperation Organisation (SCO) Exercise Peaceful Mission: 2021 was held today.
• It was hosted by Russia in the Orenburg Region of South West Russia.
• The aim of the exercise is to foster close relations between the SCO Member States and to enhance the abilities of the military leaders to command multinational military contingents.
• An Indian military contingent comprising of an all-arms combined force of 200 personnel from the Indian Army and Indian Air force is participating in the exercise.
• The Exercise Peaceful Mission: 2021 is based on joint counter-terrorism operations at the operational and tactical level in an urban environment in which Armies and Air Forces of all SCO member states are participating.
• Over the next few days, troops will train, share and rehearse tactical drills which will culminate in a final validation exercise, where-in troops from all Armies and Air Forces will jointly undertake operations in a controlled and simulated environment.

Plant Discoveries 2020

• The Botanical Survey of India, in its new publication Plant Discoveries 2020 has added 267 new taxa/ species to the country’s flora.
• The 267 new discoveries include 119 angiosperms; 3 pteridophytes; 5 bryophytes, 44 lichens; 57 fungi, 21 algae and 18 microbes.
• In 2020, 202 new plant species were discovered across the country and 65 new records were added.
• With these new discoveries, the latest estimate of plant diversity in India stands at 54,733 taxa including 21,849 angiosperms, 82 gymnosperms, 1,310 pteridophytes, 2,791 bryophytes, 2,961 lichens, 15,504 fungi, 8,979 algae and 1,257 microbes.
• “The year 2020 will remain marked in global history for the outbreak of the COVID-19 pandemic, with the havoc it caused and still continues in 2021. This overwhelming addition of 267 plant taxa to Indian flora, which was discovered as either new species or as new distributional records for India, is nowhere less than the average number of new plant discoveries made from India during the past one-and-half decade,” said A.A. Mao, Director of the Botanical Survey of India.
• Among the new discoveries this year, nine new species of balsam (Impatiens) and one species of wild banana (Musa pradhan) were discovered from Darjeeling and one species each of wild Jamun (Syzygium anamalaianum) from Coimbatore in Tamil Nadu and fern (Selaginella odishana) were recorded from Kandhamal in Odisha. There are 14 new macro and 31 new micro fungi species recorded from various parts of India.
• An assessment of the geographical distribution of these newly discovered plants reveals that 22% of the discoveries were made from the Western Ghats followed by the Western Himalayas (15%), the Eastern Himalayas (14%) and the Northeast ranges (12%). The west coast contributed 10% while the east coast contributed (9%) in total discoveries; the Eastern Ghats and south Deccan contribute 4% each while the central highland and north Deccan added 3% each.

South West Monsoon in India (Jun, July, Aug, Sep):

• Definition: It is the seasonal reversal in the wind direction. It is experienced in tropical areas (20°N to 20°S). In India, it is experienced in June, July, August and September.

5 Key players in Monsoon:

1) Differential heating and cooling of land (LP) and Sea (HP) in summer.

2) Northward shifting of ITCZ in July over Ganga plains

• ITCZ is a low pressure zone located at the equator where trade winds converge.
• This leads to the formation of thermal low over North and Northwest India.
• Southeast Trade winds from S hemisphere cross the Equator (40°E to 60°E of longitude) and turn right due to Coriolis force in the SW direction.
• It is also called as Monsoon trough during monsoon year.
• The location and shift of ITCZ governs the breaks and pulsating nature of Monsoons.

3) The presence of High Pressure (HP) area over Madagascar (20°S of Indian Ocean). The intensity and position of this HP area affects Indian monsoon.

4) Tibetan plateau gets intensely heated

• This results in strong vertical air currents and the formation of High Pressure over the plateau @ about 9 km above sea surface.
• This is Tropical Easterly Jetstream which is associated closely with the burst of monsoon.
• Tropical Easterly Jetstream is also a major reason why there are no cyclones during Monsoon because the presence of an Easterly jet over the Indian landmass in the upper troposphere prevents vertical circulation of air...which is a pre-condition for formation of cyclones.

5) Northward shifting of Sub Tropical Westerly Jet Stream (STWJ) over North of Himalayas:

6) The presence of Tropical Easterly Jetstream over peninsula in summer. It is associated closely with the burst of Monsoon.

• Role of Tibetan Plateau: It is the highest & largest plateau of the world w avg height of 4000 m and is surrounded by even higher mountain ranges.
• In summer, Tibetan Plateau acts as a heat source. Air above it is heated & is warmer compared to the surrounding air of mountain ranges. Warm air rises above and creates HP belt in the upper air above Tibetan plateau. Air spreads from HP belt and sinks over Indian Ocean around 30°S and 70°E.
• This movement from Tibet to IO is known as Tropical Easterly Jet (TEJ). This jet drives monsoonal winds towards Indian subcontinent and intensifies SW monsoon.
• The area over IO where TEJ sinks is known as Mascarene High.
• It is a major reason why there are no cyclones over the Indian landmass. It is present in the Upper troposphere & prevents d vertical circulation of air which is a pre-condition for formation of cyclone.

7) . S-E trade winds from Southern hemisphere cross the equator and turn right due to coriolis force.

Retreating Southwest (SW) Monsoon Season (Oct, Nov): Retreat of Monsoon and onset of the NE Monsoon:

• Unlike sudden burst of SW monsoon, retreat is gradual.
• It starts retreating from North India by the 2nd week of Sep. By the end of Sep, SW monsoon becomes weak as the LP trough (ITCZ) of Ganga plain starts moving southwards in response to southwards in response march of the Sun.
• The monsoon retreats from the Western Rajasthan by the 1st week of Sep. It withdraws from Rajasthan, Gujarat, Western Ganga plains and Central Highlands by the end of month.
• By beginning of Oct, LP covers Northern Bay of Bengal and by early Nov, it moves over KN and TN. By mid Dec the ITCZ is completely removed from the Peninsula.
• It is marked by clear skies and rise in temp. The land is still moist. Owing to the conditions of HT and humidity, the weather becomes rather oppressive. This is commonly known as ‘October Heat’. In the 2nd ½ f Oct, temp ↓ esp in NI.

What is the difference between Retreating Southwest Monsoon and Northeast Monsoon?

Mechanism of Monsoon

It can be understood in 4 different points:

1) Onset and Landward advance of Monsoon from Kerala:

• Differential heating of land (LP) and sea (HP) in summer.
• Northward shift of ITCZ.
• Northward shifting of STWJ.
• Heating of Tibetan plateau and inducement of TEJ.

2) Rain bearing systems:

• Monsoon rainfall enters in India thr’ 2 branches, Arabian Sea branch of SW Monsoon brings rain to W coast and further interiors of Peninsula (It is related to offshore meteorological conditions along eastern coast of Africa) and BoB Bengal branch gives rainfall over Eastern, NE and NI. Tropical depressions originating in BoB cause rainfall in Northern plains.
• Arabian Sea Branch of SW Monsoon: The total volume is 3 times more than BoB branch. It approaches Southwest coast of India by the 1st week of June. They further split into 3 branches:

1. Western Ghats (WG):

• The high WG leads the moist air to rise quite high. Heavy rain (200-250 cms) falls on the windward slopes of WG.
• The leeward side receives only 60 cm rainfall & rain shadow areas receive much less.
• The winds progressively deposit less rainfall from W to E Ex. Mumbai-200 cm & Chennai-40 cm and from S to N Ex. Thiruvan-puram-325 cm, GA-300 cm & Mumbai-200 cm.
• The intensity of rainfall over the west coast of India is related to 2 factors: The offshore meteorological conditions and The position of equatorial jet stream along the eastern coast of Africa.

2. North of Mumbai: After crossing the Ghats the monsoon winds cross the plateau & reach MP, JH and OD giving rainfall. Part of this wind enters through Narmada valley & causes good rainfall in the Chota Nagpur plateau. Further North, a part of these winds passes over West and Southeast Rajasthan without depositing much rainfall and goes straight to the sub-Himalayan region giving rainfall to the foothills of the W Himalayas, eastern PN, HR and NE RJ where it meets the BoB branch.

3. Saurashtra Peninsula and Kachch: Aravallis runs almost parallel to the direction of this branch and do not form a barrier to intercept the winds, and hence do not receive much rainfall from this branch.

• Bay of Bengal branch of SW Monsoon:

1. These winds pass over Ganga and Brahmaputra delta strike against Shiwalik ranges, Himalayas, Chittagong and Assam hills, then tise and cause heavy rainfall in WB, SK, AR and southern slopes of Khasi-Jaintia hills. Cherrapunji (1250 cm) receives the highest rainfall in the World.
2. 1 branch of BoB branch is deflected in Western direction, blowing from SE, between Himalayas and Peninsula. The winds move up in the Indo gangetic plain parallel to Himalayas.
3. The rainfall decreases up to Kashmir from Ganga plains (250 cm). A small amount of ppt is received on the E Aravalli slopes as they form a barrier, but Thar on the West side lies in the rain shadow for the SW monsoon BoB branch and receives little or no rainfall.
4. During monsoon, strong winds and wet spells last for a few days followed by a period of weak winds or monsoon lull. This leads to dry spells between two wet spells.
5. The normal duration of monsoon in most parts of India is 100 days to 120 days but in RJ it is 45-50 days, while it is max of 6 months in Kerala. Temp are < in d wet season due to cloudiness & rainfall.

3) Break in the Monsoon:

• During SW monsoon, if rain fails to occur for 1 or more weeks, it is known as break in the monsoon. These breaks are quite common during rainy season. The location and shift of ITCZ governs the breaks and pulsating nature of Monsoons. These breaks in diff regions are due to different reasons:

1. In NI, rains are likely to fail if the rain-bearing storms are not very frequent along monsoon trough or the ITCZ over this region.
2. Over the W coast, the dry spells are associated with the days when winds blow parallel to the coast.
3. Dry spells occur over Western RJ due to thermal conditions in the lower atmosphere, such as Inversion of temp.
4. Cyclonic originating head of BoB and their crossing into the mainland may result in BoB branch of SW monsoon winds getting drawn into cyclonic depression resulting in dry spells in SE parts of Ganga plains.

• Position of ITCZ over Northern plain, called as monsoon trough determines tracks of tropical depressions. As the axis of monsoon trough oscillates, the tracks of these depressions also vary. This cause wide fluctuation in the direction and path these depressions take intensity of rainfall a.w.a variations in the amount of rainfall from year to year. If the trough lies close to Himalayas. It causes heavy rainfall in the foothills which results in widespread floods and prolonged dry spells in the plains. If the LP trough lies over Ganga plain, it results in heavy rains in the plains and dry spells in the foothills

4) Retreat of Monsoon and onset of the NE Monsoon.

Completed above.

What are the characteristics of Monsoonal Rainfall?

• Rainfalls received from southwest monsoon is seasonal in character. June - Sep.
• Governed by Relief or Topography.
• Monsoon rainfall decreases from the increasing distance from the sea.
• Monsoon rains occur in wet spells of few days. Then Breaks related to cyclonic disturbances over BoB and their crossing into mainland.
• Heavy downpour leads to runoff and soil erosion.
• 75% of the total rain received during SW monsoon. Agri dependent on it.
• Uneven spatial distribution.
• Rain sometimes end considerably earlier than usual causing great damage to standing crops and making the sowing of winter crops difficult.