Indian Climate | Factors Affecting Indian Climate

Table of Contents

• Indian Climate – What type of Climate Does India Have?
• Features of Indian Climate
  • Rainfall
  • Temperature
• Factors Influencing Indian Climate
  • Latitudinal location
  • Distance from the Sea
  • Himalayas and Indian Climate
    • Why rainfall decreases form east to west in plains region (Indus-Ganga Plains)?
    • Then how come Haryana and Punjab not deserts like Rajasthan?
  • Physiography and Indian Climate
    • Why are some parts in peninsular India semi-arid?
    • Why no significant rainfall in Gujarat and Rajasthan? Explain the formation of Thar Desert?
    • How come Cherrapunji and Mawsynram receive abnormally high rainfall?
  • Monsoon Winds and Indian Climate
  • Upper Air Circulation
    • Westerly Jet Stream
    • Easterly Jet Stream
  • Tropical Cyclones and Western Disturbances
  • El-Nino, La Nina, ENSO and Indian Climate
    • El Nino
    • La Nina
    • ENSOShareWhatsAppTweetEmail

Indian Climate – Climate of India – Features of Indian Climate – Factors Influencing The Climate Of India: Physiography, Monsoon Winds, El Nino, La Nina etc..

Indian Climate – What type of Climate Does India Have?

• India’s climate closely resembles the climate that of a tropical country although its northern part (north of tropic of cancer) is situated in the temperate belt.
• Indian subcontinent is separated from the rest of Asia by the lofty Himalayan ranges which block the cold air masses moving southwards from Central Asia.
• As a result, during winters, the northern half of India is warmer by 3°C to 8°C than other areas located on same latitudes.
• During summer, due to over the head position of the sun, the climate in the southern parts resemble equatorial dry climate.
• The north Indian plains are under the influence of hot dry wind called ‘loo’ blowing from the Thar, Baloch and Iranian Deserts, increasing the temperatures to a level comparable to that of the southern parts of the country.
• Thus the whole of India, south of the Himalayas can be climatically treated as a tropical country.
• The seasonal reversal of winds in Arabian Sea and Bay of Bengal give India a typical tropical monsoon climate.
• So Indian climate, to be precise, is tropical monsoon type (a distinct wet and dry climate) rather than just a tropical or half temperate climate.

Features of Indian Climate

• India has high Regional Climatic Diversity because of its topographical diversity (location, altitude, distance from sea and relief).

Rainfall

• The climate in most of the regions is characterized by distinct wet and dry seasons. Some places like Thar desert, Ladakh have no wet season.
• Mean annual rainfall varies substantially from region to region. Mawsynram and Cherrapunji in Meghalaya receives around 1,000 cm of annual rainfall while at Jaisalmer the annual rainfall rarely exceeds 12 cm.
• The Ganga delta and the coastal plains of Odisha see intense rainfall in July and August while the Coromandel Coast goes dry during these months.
• Places like Goa, Hyderabad and Patna receive south-west monsoon rains by the first quarter of June while the rains are awaited till early July at places in Northwest India.

Temperature

• Diurnal and annual temperature ranges are substantial.
• Highest diurnal temperature ranges occur in the Thar desert and the highest annual temperature ranges are recorded in the Himalayan regions.
• Both diurnal and mean annual temperature ranges are least in coastal regions.
• In December, the temperature may dip to – 40°C at some places in J&K while in many coastal regions average temperature is 20-25°C.
• Winters are moderately cold in most of the regions while the summers are extremely hot.
• Himalayan regions experience brutal winters while the summers are moderate.

Factors Influencing Indian Climate

• Latitudinal location
• Distance from the Sea
• The Himalayas
• Physiography
• Monsoon Winds
• Upper Air Circulation
• El Nino and La Nina
• Tropical Cyclones and Western Disturbances

Latitudinal location

• Indian climate resembles the climate of a tropical country.
• The mainland of India extends between 8°N to 37°N.
• Areas south of the Tropic of Cancer are in tropics and hence receive high solar insolation. The summer temperatures are extreme and winters temperatures are moderate in most of the regions.
• The northern parts on the other hand lie in the warm temperate zone. They receive comparatively less solar insolation. But summer are equally hot in north India because of hot local wind called ‘loo’. Winter are very cold due to cold waves brought by the western disturbances.
• Some places in Himalayas record low temperatures particularly in winter.
• Coastal regions see moderate climatic conditions irrespective of latitudinal position.

Distance from the Sea

• Coastal regions have moderate or equable or maritime climate where as interior locations are deprived of the moderating influence of the sea and experience extreme or continental climate.
• The monsoon winds first reach the coastal regions and hence bring good amount of rainfall.

Himalayas and Indian Climate

• This is the most important factor that influences Indian Climate.
• The Himalayas act as a climatic divide between India and Central Asia.
• During winter, Himalayas protect India from cold and dry air masses of Central Asia.
• During monsoon months these mountain ranges act as an effective physical barrier for rain bearing south-west monsoon winds.
• Himalayas divide the Bay of Bengal branch of monsoon winds into two branches – one branch flowing along the plain regions towards north-west India and the other towards South-East Asia.
• If the Himalayas were not present, the monsoon winds would simply move into China and most of the north India would have been a desert.

Why rainfall decreases form east to west in plains region (Indus-Ganga Plains)?

• In summer, there are many minor low pressure cells that exist all over the plain region.
• As the monsoon winds move from east to west the moisture levels decrease due to successive rainfall at each low pressure regions.
• By the time winds reach western parts of the plains (Delhi, Haryana etc.) all the moisture in the monsoon winds in exhausted.

Then how come Haryana and Punjab not deserts like Rajasthan?

• They receive rainfall due to Western Disturbances in winter. (In summer the rainfall is very low.)

Physiography and Indian Climate

• Physiography is the most important factor that determines the mean annual rainfall received by a region.

Why are some parts in peninsular India semi-arid?

• Places on the windward side of an orographic barrier receive great amount of rainfall where as those on the leeward side remain arid to semi-arid due to rain-shadow effect.
• Example: The south-west monsoon winds from the Arabian sea strike almost perpendicular at the Western Ghats and cause copious rainfall in the Western Coastal plain and the western slopes of the Western Ghats.
• On the contrary, vast areas of Maharashtra, Karnataka, Telangana, Andhra Pradesh and Tamil Nadu lie in rain-shadow or leeward side of the Western Ghats and receive scanty rainfall.

Why no significant rainfall in Gujarat and Rajasthan? Explain the formation of Thar Desert?

• Monsoons winds flowing in Rajasthan and Gujarat are not obstructed by any orographic barrier and hence these regions receive no rainfall.
• [Monsoon winds blow almost parallel to Aravalis and hence there is no orographic rainfall].
• [No convection cell or vertical wind movements arise in Rajasthan and Gujarat: Monsoon winds blow towards low pressure cells in Tibet and hence only horizontal wind movements exist in Gujarat and Rajasthan]
• [Sub-tropical high pressure belt: In winter the region experiences strong divergence because of the STJ – Sub-Tropical Jet.]

How come Cherrapunji and Mawsynram receive abnormally high rainfall?

• Mawsynram and Cherrapunji
  are the wettest places on earth with mean annual rainfall over 1000 cm.
• Copious rainfall in these places is due to funneling effect followed by orographic upliftment. [Funneling effect = clouds are channeled into a narrow region between mountains and hence the cloud density is extraordinary]

Monsoon Winds and Indian Climate

• The most dominating factor of the Indian climate is the ‘monsoon winds’.

Important features of Indian Monsoons are

1. Sudden onset (sudden burst)
2. Gradual progress
3. Gradual retreat
4. Seasonal reversal of winds

• The complete reversal of the monsoon winds brings about a sudden change in the seasons.
• The harsh summer season suddenly giving way to monsoon or rainy season.
• The south-west monsoons from the Arabian sea and the Bay of Bengal bring rainfall to the entire country.
• The north-eastern winter monsoon do not cause much rainfall except along the Caromandel coast (TN coast) after getting moisture from the Bay of Bengal.

Upper Air Circulation

• The changes in the upper air circulation over Indian landmass is brought about by Jet streams. (Explained in detail in Indian Monsoons)

Westerly Jet Stream

• Westerly jet stream blows at a very high speed during winter over the sub-tropical zone.
• Southern branch of the jet stream exercises a significant influence on the winter weather conditions in India.
• This jet stream is responsible for bringing western disturbances from the Mediterranean region in to the Indian sub-continent.
• Winter rain and heat storms in north-western plains and occasional heavy snowfall in hilly regions are caused by these disturbances.
• These are generally followed by cold waves in the whole of northern plains.

Easterly Jet Stream

• Reversal in upper air circulation takes place in summer due to the apparent shift of the sun’s vertical rays in the northern hemisphere.
• The westerly jet stream is replaced by the easterly jet stream which owes its origin to the heating of the Tibet plateau.
• This helps in the sudden onset of the south-west monsoons.

Tropical Cyclones and Western Disturbances

• Tropical cyclones originate in the Bay of Bengal and Arabian Sea and the influence large parts of the peninsular India.
• Majority of the cyclones originate in the Bay of Bengal and influence the weather conditions during the south-west monsoon season (low intensity cyclones).
• Some cyclones are born during the retreating monsoon season, i.e., in October and November (high intensity cyclones) and influence the weather conditions along the eastern coast of India.
• The western disturbances originate over the Mediterranean sea and travel eastward under the influence of westerly jet stream.
• They influence the winter weather conditions over most of Northern-plains and Western Himalayan region.

El-Nino, La Nina, ENSO and Indian Climate

El Nino

• Adversely affects monsoon rainfall and cyclogenesis in Bay of Bengal.
• Good for cyclogenesis in Arabian Sea.
• Droughts are common during El Nino events due to less monsoonal and cyclonic rainfall.

La Nina

• Good for monsoons and cyclogenesis in Bay of Bengal.
  
• Suppressed cyclogenesis in Arabian Sea.
  
• Floods are common.
  

ENSO

• Southern Oscillation is simply the oscillation or alternating positions of low pressure and high pressure cells over eastern and western Pacific.
• Southern Oscillation coinciding with El Nino is called ENSO or El Nino Southern Oscillation. (SO usually coincides with EL Nino. This why El Nino is usually referred to as ENSO)
• ENSO = [warm water in eastern Pacific + low pressure over eastern Pacific] + [cool water in western Pacific + high pressure in western Pacific]
• Climatic conditions same as El Nino
  .

Western Disturbances | Cloudburst

Table of Contents

• Western Disturbances
  • Weather associated with Western Disturbances
  • Importance of Western Disturbances
• Cloudburst in Jammu and Kashmir, Himachal Pradesh, UttarakhandShareWhatsAppTweetEmail

Western Disturbances – Weather associated with Western Disturbances – Importance of Western Disturbances. Cloudburst in Jammu and Kashmir, Himachal Pradesh, Uttarakhand.

Must Read:

• Indian Monsoons |ITCZ|Inter-Tropical Convergence Zone
• Indian Monsoons Mechanism|Jet Stream Theory
• Indian Monsoons – Easterly Jet|Tibet|Somali Jet
• Indian Monsoons – South West|North East Monsoons

Western Disturbances

• In the winter season, the sub-tropical jet (STJ) is bifurcated into two branches due to physical obstruction of the Himalayas and Tibetan Plateau.
• One branch is flows to the south of the Himalayas, while the second branch is positioned to the north of the Tibetan Plateau.

• The ridge of the jet stream creates anticyclonic (with clockwise air circulation) conditions over North-West India.
• Consequently, the winds tend to descend over the north-western parts of India, resulting into the development of atmospheric stability and dry conditions (anticyclonic condition = no rainfall).
• But the sub-tropical jet (STJ) causes periodic changes in general weather conditions.
• The STJ drives the temperate low pressures over Mediterranean Sea towards east across Afghanistan, Pakistan and reach north-west India.
• These storms are residual frontal cyclones which move at the height of 2000 metres from the mean sea level.
• On an average, 4 to 6 cyclonic waves reach north-western India between October and April each year.

Weather associated with Western Disturbances

• The arrival of these temperate storms [remnants of temperate cyclones] [western disturbances] causes precipitation leading to an abrupt decrease in air temperature over North-West India.
• The weather becomes clear after the western disturbances passes away.
• Western Disturbances also bring heavy snowfall in the Himalayan Region and a cold wave to north Indian plains.

Importance of Western Disturbances

• The western disturbances affect weather conditions during the winter season up to Patna (Bihar) and give occasional rainfall which is highly beneficial for the standing rabi crops, (wheat, barley, mustard, gram, lentil, etc.).

Cloudburst in Jammu and Kashmir, Himachal Pradesh, Uttarakhand

• A cloudburst is an intense torrential rainfall brought by a thunderstorm that lasts for a relatively short duration (few minutes to few hours).
• Cloudburst leads to flash floods and causes lot of damage to life and property.
• Every intense rainfall is not a Cloudburst. Cloudburst specifically occurs when an air mass with high humidity is struck at a place due to various reasons.

• In 2010, South-Western strip of Russia (Caucasus Region, Moscow etc.) saw higher than normal temperatures (highest in in the last 100 years) and there were numerous cloudbursts in Jammu and Kashmir.
• A strong upper-atmospheric high was located over European Russia towards the beginning of summer.
• It diverted the jet stream (meandering of Sub-Tropical Jet Stream) and its rain-giving train (trough) of summer storms farther north than usual, giving much of Southern European Russia drought conditions.
• In addition, southern desert heat from central Asia, the Arabian Peninsula and North Africa began to flow northward, which strengthened this ridge of STJ and tightened its hold over the region.

• The stalled system prevented weather systems being drawn across Russia and the obstacle acted as a barrier trapping hot air to the south and cold air to the north.
• The consequence of this static mass of hot air was the heat wave that devastated Russia.
• With the jet stream stalled the Sub-Tropical Jet was unable to transit across the Himalayas as it would do ordinarily, the monsoon cell to the south, fed by warmer waters in the Indian Ocean, had nowhere to go and as a consequence it deposited vast amounts of rain over Pakistan, Himalchal Pradesh and Jammu and Kashmir and this led to extensive flooding
  .

Indian Monsoons: South West Monsoons & North East Monsoons

Table of Contents

• How Jet Streams affect Indian Monsoons? [Indian Monsoon Mechanism]
• Indian Monsoons Seasonal Variations
• Projects to understand monsoonsShareWhatsAppTweetEmail

• Indian Monsoons |ITCZ|Inter-Tropical Convergence Zone
• Indian Monsoons Mechanism|Jet Stream Theory
• Indian Monsoons – Easterly Jet|Tibet|Somali Jet
• Indian Monsoons – South West|North East Monsoons <– You are Here

How Jet Streams affect Indian Monsoons? [Indian Monsoon Mechanism]

Summing up all the points from the previous posts.

• As the summer time approaches, there is increased solar heating of the Indian subcontinent and the Tibetan Plateau.
• In the peak summer months (25th of May – 10th of Jun), with the apparent northward movement of the sun, the southern branch of the SJT, which flows to the south of the Himalayas, shifts to the north of the Himalayas.
• When the sun’s position is about to reach the Tropic of Cancer (June), the SJT shifts to the north of the Tibetan Plateau (1st of Jun – 20th of June). The ITCZ is close to its peak position over the Tibetan Plateau.
• The altitude of the mountains initially disrupts the jet but once it has cleared the summits it is able to reform over central Asia.
• Its movement towards the north is one of the main features associated with the onset of the monsoon over India.
• With the northward shift of SJT, an Easterly Jet is formed over the Indian plains. It generally forms in the first week of June and lasts till late October.
• It can be traced in the upper troposphere right up to the west coast of Africa.
• The northward shift of SJT and ICTZ moves the subtropical high pressure belt to the north of the Tibetan Plateau and the Easterly Jet creates a low pressure region in the Indian plains (Easterly Jet creates anticyclonic conditions in upper troposphere).
• This low pressure in the northern plains coupled with the intense low of the Tibetan Plateau leads to the sudden onset of south-west monsoons (1st of Jun – 20th of June).
• The monsoon cell is situated between the Indian Ocean (North of Madagascar)(High Pressure Cell) and Tibetan plateau (Low Pressure Cell).
• In summer the sub-tropical easterly jet fluctuates between the plains region of India and peninsular India varying the intensity of rainfall from location to location.
• During March to May, the building up of this cell is blocked by the STJ which tends to blow to the south of the Himalayas (Northwest India and Plains region are occupied by Subtropical High Pressure Belt. This high pressure belt undermines the influence of low pressure cell over Tibet).
• As long as the STJ is in this position the development of summer monsoons is inhibited (the high pressure belt stays over north India).
• With the STJ out of the way (high pressure belt migrates to the north of Tibet) the sub continental monsoon cell develops (Somali Jet) very quickly indeed, often in a matter of a few days.
• Warmth and moisture are fed into the cell by a lower level tropical jet stream which brings with it air masses laden with moisture from the Indian Ocean.
• The end of the monsoon season is brought about when the atmosphere over the Tibetan Plateau begins to cool (August – October), this enables the STJ to transition back across the Himalayas.
• With the southward shift of ITCZ, subtropical high pressure belt returns back to the Indian plains and the rainfall ceases.
• This leads to the formation of a anticyclonic winter monsoon cell typified by sinking air masses over India and relatively moisture free winds that blow seaward.
• This gives rise to relatively settled and dry weather over India during the winter months.

Indian Monsoons Seasonal Variations

Indian Monsoons in May – Dry Season 

Indian Monsoons in June – Onset of Monsoons Jun 1st – June 1oth 

Indian Monsoons in July – Monsoon winds reach North-West India

Indian Monsoons in August – Monsoons Retreat from North-West India

Indian Monsoons in September – Maximum Rainfall in parts of South India 

Indian Monsoons in October – Retreating Monsoons – Cyclones in Late October

Indian Monsoons in November – North-East Monsoons – Peak Cyclone Season in early November

Indian Monsoons in December – Maximum Rainfall Month in Tamil Nadu and Southern Andhra Coast

Indian Monsoons in January – North-East Monsoons weaken

Indian Monsoons in February – Dry Season Begins

Indian Monsoons in March – Dry Season intensifies 

Indian Monsoons in April – Dry Season- Occasional Thunderstorms in South and Central India

Projects to understand monsoons

• First attempt was made during International India Ocean Expedition (HOE) from 1962 to 1965.
• It was organized jointly by the International Council of Scientific Unions (ICSU), Scientific Committee on Ocean Research (SCOR) and UNESCO with World Meteorological Organization (WMO) joining the meteorology programme.

ISMEX

• 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.

MONEX

• Data collection effort was made under the aegis of MONEX-1979.
• It was organised jointly by many researching organizations and the World Meteorological Organisation (WMO) under their World Weather Watch (WWW) programme.
• It is so far the largest scientific effort made to understand monsoons.
• Details are not necessary. Remember the names. They can be asked in prelims. MONEX was asked in previous paper
  s

Indian Monsoons: Role of Tropical Easterly Jet, Tibet, Somali Jet & Indian Ocean Dipole

Table of Contents

• Indian Monsoons – Role of Tropical Easterly Jet (TEJ) [African Easterly Jet]
  • Tropical Easterly Jet (TEJ)
• Indian Monsoons – Role of Tibet
• Indian Monsoons – Role of Somali Jet
• Indian Monsoons – Role of Indian Ocean DipoleShareWhatsAppTweetEmail

Indian Monsoons – Role of Tropical Easterly Jet (TEJ) [African Easterly Jet], Indian Monsoons – Role of Tibet, Indian Monsoons – Role of Somali Jet and Indian Monsoons – Role of Indian Ocean Dipole.

• Indian Monsoons |ITCZ|Inter-Tropical Convergence Zone
• Indian Monsoons Mechanism|Jet Stream Theory
• Indian Monsoons – Easterly Jet|Tibet|Somali Jet <– You are Here
• Indian Monsoons – South West|North East Monsoons

Indian Monsoons – Role of Tropical Easterly Jet (TEJ) [African Easterly Jet]

• The establishment and maintenance of the TEJ is not fully understood but it is believed that the jet may be caused by the uniquely high temperatures and heights over the Tibetan Plateau during summer.
• The TEJ plays an important role in kick starting southwest monsoon.
• This jet descends over the Indian Ocean (near Madagascar) and intensifies its high pressure cell so as to move as south-west monsoon.

Tropical Easterly Jet (TEJ)

• There are major high velocity winds in the lower troposphere called low-level jets (LLJs).
• In the tropics, the most prominent of these are the Somali Jet and the African Easterly Jet [Tropical Easterly Jet].
• The TEJ is a unique and dominant feature of the northern hemispheric summer over southern Asia and northern Africa. The TEJ is found near between 5° and 20°N.
• It is fairly persistent in its direction, and intensity from June through the beginning of October. It’s position fluctuates between 5° and 20°N.
• TEJ comes into existence quickly after the STJ has shifted to the north of the Himalayas (Early June).
• TEJ flows from east to west over peninsular India at 6 – 9 km and over the Northern African region.
• The formation of TEJ results in the reversal of upper air circulation patterns [High pressure switches to low pressure] and leads to the quick onset of monsoons.
• Recent observations have revealed that the intensity and duration of heating of Tibetan 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.

Indian Monsoons – Role of Tibet

• The Tibetan Plateau is an enormous block of highland acting as a formidable barrier.
• Due to its protruded height it receives 2-3°C more insolation than the neighboring areas.
• The plateau affects the atmosphere in two ways: (a) as a mechanical barrier, and (b) as a high- level heat sources.
• At the beginning of June the subtropical jet stream is completely withdrawn from India and occupies a position along 40° N (to the north of Tibetan Plateau).
• The plateau accentuates the northward displacement of the jet stream. Hence the burst of monsoon in June is prompted by the Himalayas and not by the thermally induced low pressure cell over Tibet. (Tibetan plateau is responsible for south-west monsoons. But it is the STJ that facilitates sudden outburst of monsoons with its sudden northward migration)
• In the middle of October the plateau proves to be the most important factor in causing the advance of the jet south of the Himalayas or bifurcating it into two parts.
• The winter Tibetan Plateau cools rapidly and produces a high pressure cell. (Cyclonic condition over Tibet ceases and an anticyclonic condition is established). The high pressure cell over Tibet strengthens N-E monsoons.
• Tibet gets heated in summer and is 2°C to 3°C warmer than the air over the adjoining regions.
• Because the Tibet Plateau is a source of heat for the atmosphere, it generates an area of rising air (convergence)(intense low pressure cell).
• During its ascent the air spreads outwards in upper troposphere (divergence) and gradually sinks (subsidence) over the equatorial part of the Indian Ocean.
• It finally approaches the west coast of India as a return current from a south-westerly direction and is termed as equatorial westerlies.
• It picks up moisture from the Indian Ocean and causes rainfall in India and adjoining countries.

Indian Monsoons – Role of Somali Jet

• Polar and subtropical jet streams are the permanent jet streams which greatly influence the weather of temperate regions.
• Temporary jet streams are narrow winds with speeds more than 94 kph in the upper, middle and sometimes in lower troposphere. They are few. Important ones are Somali Jet and The African Easterly jet or Tropical Easterly Jet.
• These two jet streams play an important role in the formation and progression of Indian Monsoons.
• The progress of the southwest monsoon towards India is greatly aided by the onset of Somali jet that transits Kenya, Somalia and Sahel.
• It was observed to flow from Mauritius and the northern part of the island of Madagascar before reaching the coast of Kenya at about 3º S.
• It strengthens permanent high near Madagascar and also helps to drive S-W monsoons towards India at a greater pace and intensity.
• The importance of the low level jet arises from the fact that its path around 9º N coincides with a zone of coastal upwelling.
• As the strong winds drive away the surface coastal waters towards the east, extremely cold water from the depths of the sea rise upwards to preserve the continuity of mass.
• The peculiar feature of Somali Current is reversal in direction with the onset of the summer monsoon.
• In winter, this current is from north to the south running southwards from the coast of Arabia to the east African coastline; but with the advent of the summer monsoon it reverses its direction and flows from the south to the north.

Indian Monsoons – Role of Indian Ocean Dipole

• Indian ocean Dipole is a recently discovered phenomena that has a significant influence on Indian monsoons.
• Indian ocean Dipole is a SST anomaly (Sea Surface Temperature Anomaly – different from normal) that occurs occasionally in Northern or Equatorial Indian Ocean Region (IOR).
• The Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.
• IOD develops in the equatorial region of Indian Ocean from April to May peaking in October.
• With a positive IOD winds over the Indian Ocean blow from east to west (from Bay of Bengal towards Arabian Sea). This results in the Arabian Sea (western Indian Ocean near African Coast) being much warmer and eastern Indian Ocean around Indonesia becoming colder and dry.
• In the negative dipole year, reverse happens making Indonesia much warmer and rainier.
• Positive IOD is good for Indian Monsoons as more evaporation occurs in warm water.
• Similar to ENSO, the atmospheric component of the IOD is named as Equatorial Indian Ocean Oscillation (EQUINOO)(Oscillation of pressure cells between Bay of Bengal and Arabian Sea).
• During the positive phase of the ‘Equatorial Indian Ocean Oscillation (EQUINOO),’ there is enhanced cloud formation and rainfall in western part of the equatorial ocean near the African coast while such activity is suppressed near Sumatra.
• While EQUINOO and IOD go in step during strong positive IOD events, they do not always do so.