Dynamics Of Indian Climate[1]

DYNAMICS OF INDIAN CLIMATE: India’s unique geography and geology procure an extraordinary variety of climate, ranging from tropical in the South to temperate and alpine in the Himalayan north, where elevated regions receive sustained winter snowfall. The Himalayas along with the Hindu Kush Mountains in Pakistan encumber cold Central Asian Katabatic winds to enter into Indian Territory and keep Indian Territory warmer than most location at similar latitudes. On the other hand, Thar Desert in the north-west attracts moisture laden southwest summer monsoon winds that blow between June and October; procure maximum percentage of rainfall to India. Nevertheless, in spite of these variegated climatic system, Indian climate is worldly recognized as ‘Tropical Climate’; and it is not as simple as it seems rather has very distinct characteristic viz… • In summer season, there is very high temperature in the middle part, whereas the southern part experiences mild temperature (due to oceanic influence), and the northern India possess low temperature. • In the winter, the middle part has mild temperature, and the northern India possess very low temperature, while, southern part has mild temperature. • The short rainy season ushers rain on Indian sub-continent, but again like temperature it is not equally distributed. This seasonal pattern is the result of seasonal variation in the temperature and pressure pattern and its resultant effect on the direction of winds, which bring about a marked reversal in the wind direction, and this is popularly described as “Monsoon”. Therefore, Monsoon may be defined as “a radical reversal in the wind direction that accounts for the occurrence or otherwise of precipitation.” During summer, the wind blows primarily from the south-west (sea to land), while, in winter it blows in the opposite direction (land to sea), and it is termed as South-West Monsoon and North-East Monsoon accordingly. Salient Features of Indian Monsoon: • It blows from the south-west direction in the summer and northeast in the winter. • The rainfall brought by the south-west monsoon decreases away from the sea. Therefore, India has Humid, Semi-arid, and Arid climate accordingly. • Monsoons are pulsating in nature. • It varies from year to year. • Within the region, there is high reliability and variability of rainfall. All these variability of Indian weather system are the result of following factors: • India’s territorial geometry (i.e. peninsular). • Occurrence and unique arrangement of mountains. • Circulation of jet streams (westerly in the winter and easterly in the summer). • Inflow of western disturbances (depressions), during winter and tropical depression during southwest monsoon. and • Surface pressure distribution and winds.

MECHANISM OF INDIAN MONSOON: For better understanding, we can study mechanism of Indian monsoon through following heads viz…. WINTER: As the sun’s vertical rays move south of the equator, the temperatures also going down with the sun’s rays. Consequently, most parts of the country experience cool weather; temperatures change by about 0.6 °C per degree of latitude. Besides, sun’s departure, a highpressure centre in Central Asia also influences Indian weather conditions. However, in the upper level or lower troposphere, a different pattern of air circulation is observed. These wind systems by following its path cross the Himalayas, roughly parallel to Tibetan plateau. It had been observed after World War II, and given a specific name i.e. Jet stream, which helps a lot in understanding the Indian weather system. Moreover, recent theory of mechanism of monsoon is based upon the movement of jet stream. Tibetan plateau act as a barrier in the path of these jet streams, consequently, it is bifurcated into two branches viz… 1. One branch blows to the north of the Tibetan plateau, and 2. Other blows to the south of the Tibetan plateau.

The southern branch of jet stream plays important role in the winter season of India. It gives two effects i.e. • It intensifies surface anticyclone, and • It brings strings of surface cyclonic disturbances. The path of jet streams is not straight rather follow a zigzag path (as shown in the figure), and this movement first recognized by Rossby, hence, it is known as Rossby waves. This Rossby movement is very noticeable, inasmuch, at one place it subsides (creates anticyclonic situation),

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and at another up (i.e. cyclonic

place it again goes situation).

The subsiding winds are nothing other than Sub-Tropical anticyclone over North-Western India. This surface winds directed towards north-west in north India and easterly over Peninsular India. Jet streams also steer the western depressions over northern India. The lows which are not usually frontal, appear to penetrate across the Middle East from the Mediterranean and are important sources of rainfall for northern and north-west India. Moreover, the general pattern of dry weather is thus interrupted by some cloud and rain (which turns snow on the Himalayan slopes) and even by cold spells as cP air spills into Indus lowlands. This rain in north-west India is greater than the rain received by south-west monsoon. These climatic disturbances are generally preceded by warm weather or sudden rise in temperatures. And after rains, which are light and spread over a couple of days, followed by clear sky and drop in temperature, with several cold waves. Further extension of these winds pass over the Bay of Bengal, where it pick up moisture and engender a weak Tropical disturbances, which bring rain to southeastern India and the east coast of Sri Lanka. SPRING: This is a transitional phase between winter and summer; because sun’s vertical rays advancing towards Tropic of Cancer fade the winters’s dominancy and alarm the summer’s arrival. Resultantly, the southern branch of jet stream starts to migrate towards northward. Nevertheless, India is still dominated by a subsiding and outward blowing anticyclonic circulation and the clear sky allow maximum and increasing insolation. However, some amount of rainfall still receives north India by western disturbances particularly towards the Ganges delta where low level inflow of warm humid air is overrun by potentially cold air triggering squall lines know as “norwesters.” SUMMER: The northward shift of sun’s vertical rays usher summer season for Indian sub-continent. The wind circulation over the sub-continent undergoes a complete reversal at both, the lower levels as well as the upper levels. By the months of April and May, occasionally some moisture-laden winds are attracted towards the periphery of the trough, results a sudden contact between dry and moist air masses,

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which procure great intensity to local storms. These local storms are associated with violent winds, torrential rains and hail storms, locally known as ‘Norwesters’. In the northeastern region, it is known as ‘Kalbaisakhi’ (calamity of the month of Baisakh). In the plains of Punjab to Bihar a hot, dry, and oppressing winds blow as ‘Loo.’ In the same months (i.e. April and May), Kerala and coastal areas of Karnataka also receive pre-monsoon showers, locally known as “Mango Showers.” However, further extension of this shower is checked by a relatively high air pressure zone lying over the Deccan Plateau.

BURST OF MONSOON: the sudden reach of the moisture laden south-west wind accompanied with violent thunder and lighting is known as the burst of the monsoon

By the middle of July, the low-pressure belt near the surface makes its stronghold, roughly parallel to the Himalayas between 20 ° N and 25 ° N. This low-pressure belt is termed as Inter Tropical Convergence Zone (ITCZ), which attracts inflow of winds from different direction. The shift in the position of the ITCZ is possibly the prime cause of withdrawal of the southern branch of westerly jet stream from its position over the north Indian plain. Hence, withdrawal of southern branch of jet stream and powerfulness of ITCZ, invite winds from different directions. Among them, the most important winds are the South-East Trade Winds, which originate at oceanic surface. It change its direction (shown in the figure) from southeast to southwest after crossing equator (due to Coriolis force), and termed as South-West Monsoon, procuring maximum rainfall to Indian sub-continent.

BREAK IN THE MONSOON: Since, south-west monsoon is of pulsating nature. Hence, Indian sub-continent does not receive continuous rain rather experiences rain after a break (of one or more weeks) which is known as break in the monsoon

By the time, the Easterly jet stream (invited by low-level changes) establishes its course over the southern Asia (at about 15 ° N in August and up to 22 ° N in September). The northwestward movement of the monsoon is apparently related to the extension of the upper tropospheric easterlies over India. The easterly jet stream, which owes its origin to the summer heating of the Himalayan and Tibetan highlands steers the tropical depression into India, which plays a significant role in the distribution of monsoonal rainfall over the Indian sub-continent.

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The south-west monsoons approach peninsula of India through two different ways i.e. viz… 1. Arabian Sea Branch – blows through Arabian Sea. 2. Bay of Bengal Branch – blows through Bay of Bengal.

The Arabian Sea branch causes rainfall all along the west coast of Western Ghats – Maharashtra, Gujarat and parts of Madhya Pradesh. The monsoon winds, which strike the Western Ghats, shed off their moisture on the windward side of the Western Ghats. Trivandrum receives the first showers and thereafter the monsoon gradually proceeds northwards. Ten days later, it reaches Mumbai. Crossing the Western Ghats, it overruns the Deccan plateau and Madhya Pradesh causing fair amount of rainfall that goes on decreasing gradually. Thereupon, it enters the Ganga plains and mingles with the Bay of Bengal branch. Another part of Arabian Sea branch strikes the Saurashtra Peninsula and the Kachchh. It then passes over West Rajasthan and along the Aravallis causing only scanty rainfall. The Bay of Bengal branch moves northwards into the central Bay of Bengal and directed towards the Burmese coast and parts of southeast Bangladesh. The Arakan hills along the Burmese coast deflect a big chunk of this branch and enable it to enter Indian sub-continent. The monsoon, therefore, enter West Bengal and Bangladesh from south and southeast instead of southwest direction. Thereafter, this branch splits into two by the effect of Himalayas and the thermal low in northwestern India. One branch moves westward along the Ganga plains reaching as far as the Punjab. Other branch moves towards the Brahmaputra valley in the north and northwest causing widespread rains in the Northeastern India. AUTUMN: This is a transitional season, arises due to the weakening of monsoon trough and gradual shift of sun’s rays towards the south, and again invites westerly jet stream to establish its southern branch over northern India. This withdrawal of monsoon begins in the last week of September and 1st week of October, known as northeast monsoon or Retreating monsoon.

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While traveling towards the Bay of Bengal the dry cold northeast winds pick up some moisture and pour it over cities like Chennai (which lie almost in the rain shadow region during south-west monsoon). FACTORS RESPONSIBLE FOR SPATIO-TEMPORAL VARIATIONS IN THE SOUTHWEST MONSOONS: •

The north-south shifting of the Mid-Latitude Westerlies accompanied by jet streams. At one point of time it facilitate rainfall (i.e. in winter) and in next season prevent rainfall (summer) on Indian sub-continent. • Sometimes, subtropical high of Arabia extends over Central India, consequently monsoon trough shifts northward. Therefore, the westerly troughs travel along the southern edge of the Himalayas causing profuse rainfall on the Himalayan slopes, whereas, plains experience drought condition. • The unusual cooling (3 to 4 °C) of Somali Current before onset of the monsoons, causing very strong wind in the form of jet flows as westerly (towards Indian peninsula) in Arabian Sea. Since, this is off shore current; hence, it drives away the surface waters from the coastal East Africa towards east, causing upwelling of cool oceanic current (known as Somali Current). Moreover, this cold current when reached near the Indian coast, decreases the temperature by 10 to 15 °C, resultantly, the southwest monsoon, when crosses these region does not pick up moisture (because low temperature hinders evaporations, hence, there is dearth of moisture) nor it develops instability (inevitable for rainfall), therefore, there is no rain on subcontinent. • Another important factor is El Nino effect. El Nino is a periodic appearance of warm water over cold water (or Humboldt Current) off the Peruvian coast. Normally, Humboldt Current is a cold current, consistently flowing along the Peru coast and maintaining this region as High Pressure Zone. Moreover, this High Pressure Zone in East Pacific region espouses the winds to blow towards Australian, Indonesian etc. low-pressure regions. This phenomenon underpins the surge of tropical cyclones westward into Bay of Bengal. This is normal condition and known as La Nina.

La Nina conditions

El Nino conditions

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However, this La Nina condition gets reverse (after 5 to 7 years) due to both atmospheric and oceanic changes (although, there is very obscure knowledge on this account). And the appearance of warm ocean current off the Peruvian coast (now a low pressure zone), simultaneously leads to high pressure zone centered around Australia and south-east Asia. Thus, the low pressure around Peruvian coast attracts winds from SouthEast Asian region. In the course of time, these winds pick moisture and make rainfall in and around Peruvian coast. On the other hand, appearance of high-pressure condition in Southeast Asia restrains any cyclone of depression formation over Indonesia or the Bay of Bengal. Difference between El Nino and La Nina

El Nino Weather • • •

•

La Nina Weather

Rain and flooding along the Pacific coast Warm water disrupts food chain of fish, birds, and sea mammals Tornadoes and thunderstorms in southern US Fewer than normal hurricanes in the Atlantic

• • • • •

Snow and rain on the west coast Unusually cold weather in Alaska Unusually warm weather in the rest of the USA Drought in the southwest Higher than normal number hurricanes in the Atlantic

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However, the appearance of El Nino in 1997, the strongest of the 20th century, did not cause drought in India; rather, the rain that year was slightly above the normal. It is presumed that with the Global Warming such factors are losing their force. THE RHYTHM OF SEASON: The Indian meteorological department (IMD) recognizes following four official seasons in terms of an annual cycle of seasons. WINTER: The Cold Weather Season Once the South-west Monsoon subsides towards south with sun’s vertical rays the average temperatures gradually fall across India. Temperatures change by about 0.6 °C (1.35 ° F). December and January are the coldest months in India. Mean temperatures are higher in the east and the south where they reach 20 °C - 25 °C (68-77 ° F), due to oceanic influence. In northwestern India due to virtually cloudless conditions prevail in October and November; resulting in wide diurnal temperature differences. In much of the Deccan plateau, temperatures range between 16 °C – 20 °C. Western depression with the help jet streams pick-up 7

moisture from Mediterranean Sea and pour rains into northern India. In addition, sometimes, particularly Jammu & Kashmir and Himachal Pradesh experience heavy snowfall with blizzards. Rest of the north India almost never receives snowfall. Although, in the northern plains temperatures occasionally fall below freezing point, but never for more than one or two days. Besides, sometimes, notorious fogs also occur, which frequently disrupt daily life. Eastern India’s climate is quite milder, experiencing relatively warm days and cool nights. High range of temperatures from 23 °C in Patna to 26 °C in Kolkata and lows average from 8 °C in Patna to 14 °C in Kolkata. Moreover, frigid winds from the Himalayas depress temperatures near the Brahmaputra River. The two eastern Himalayan states, Sikkim and Arunachal Pradesh receive substantial snowfall. The extreme north of West Bengal centered on Darjeeling also receives snowfall in little amount. In south India particularly the hinterland of Maharashtra, Madhya Pradesh, parts of Karnataka and Andhra Pradesh, relatively cooler weather occurs. Minimum temperature in western Maharashtra, Madhya Pradesh and Chhattisgarh hovers around 10 °C. In the southern Deccan plateau, temperatures are about 16 °C. Coastal areas with low elevations have temperatures of 30 °C and lows of around 21 °C. In the Western Ghats particularly Nilgiri range, are exceptional, inasmuch, the temperatures fall below the freezing point.

Extremes

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• •

India's lowest recorded temperature reading was −45 °C (−49 °F) in Dras. However, temperatures on the Indian-controlled Siachen Glacier near Bilafond La (5,450 metres (17,881 ft)) and Sia La (5,589 metres (18,337 ft)) have fallen below -55 °C (-67 °F). The highest reliable temperature reading was 50.6 °C (123 °F) in Alwar, Rajasthan in 1955. This mark was also reached at Pachpadra in Rajasthan. Recently, claims have been made of temperatures touching 55 °C (131 °F) in Orissa; (although, it has questioned the methods used in recording such data. The average annual precipitation of 11,871 millimetres (467 in) in the village of Mawsinram, in the hilly northeastern state of Meghalaya, is the highest recorded on Earth. In terms of snowfall, regions of Jammur and Kashmir, such as Baramulla District in the east and the Pir Panjal Range in the southeast, experience exceptionally heavy snowfall.

SUMMER: The Hot Weather Season With the arrival of sun’s vertical rays in the north (towards tropic of cancer), the temperatures rise. The hottest month for the western and southern region of the country is April; for most part of north India, it is May. Temperatures of 50 °C and higher have been recorded in some parts of India. In March, the highest day temperature about 38 °C occurs in the Deccan

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plateau, while in April, temperatures ranging between 38 °C found in Gujarat and Madhya Pradesh. By the month of May, most of the interior parts of India experience mean temperature over 32 °C, while maximum temperature often exceeds 40 °C. In the hot month of April and May, western disturbances, with their cooling influence, may still arrive but rapidly diminish in frequency. Notably a higher frequency of such disturbances in April co-relates with a delayed on set of monsoon. The hot weather season in south India is mild and not so comparatively hot. The peninsular situation of south India with moderative effect of ocean keeps the temperature lower. Hence, temperatures remain between 22 °C to 32 °C. Altitude affects the temperature largely, with higher part of Deccan plateau and other areas being relatively cooler. Hill stations, such as Ooty in the Western Ghats and Kalimpong in the eastern Himalayas have average maximum temperatures around 25 °C. At lower elevations in the parts of northern and western India a strong hot and dry wind centered around ITCZ, locally known as ‘Loo’, blows in the form of the west during the day time, with very high temperature around 45 °C, causes fatal cases of sunstroke. SOUTH-WEST MONSOON SEASON: As result of rapid increase of temperature in the month of May over the northwestern plains, the low-pressure conditions over there get further intensified. By early June, they are powerful enough to attract the trade winds of southern Hemisphere coming from the south Indian oceans. These southeast trade winds after crossing the equator turn as south-west trade winds (due to Coriolis force); enter the Bay of Bengal and the Arabian Sea. The southwest monsoon arrives in India in two branches i.e. the Bay of Bengal branch and the Arabian Sea branch. The second one (Arabian sea branch) extends towards a low pressure area over the Thar desert and is roughly three times stronger than the Bay of Bengal. Usually the south-west monsoon breaks over Indian sub-continent by around 25th of May, when it lashes the Andaman and Nicobar islands in the Bay of Bengal. And it strikes the Indian main land i.e. Malabar coast of Kerala on 1st of June, supplies over 80% of India’s annual rainfall. By 9 June, it reaches Mumbai, but to reach Delhi it takes 20 more days (on 29th of June). On the other hand, the Bay of Bengal branch advances towards India from eastern coast after returning from the mountain ranges of Myanmar, in the form of southeasterly. By July 1, monsoons widespread all over the country. On an average south India receives more rainfall than north India. However, northeast India receives maximum rainfall from the south-west monsoon. POST MONSOON:

(Retreating/North-East Monsoon)

Around September with the sun’s migrating towards south, the northern landmass of the Indian sub-continent begins to cool off rapidly. With this, air pressure begins to build over northern India (sign of re-establishment of southern branch of jet stream). The Indian Ocean and its surrounding atmosphere still hold its heat, which causes the cold winds to sweep down from the Himalayas and the Indo-Gangetic plain towards the vast span of the Indian Ocean. Moreover, this is known as northeast monsoon or retreating monsoon.

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While traveling towards the Indian Ocean the dry cold winds pick-up some amount of moisture from the Bay of Bengal and pour it over cities like Chennai, which fall under the rain shadow of the south-west monsoon. About 50 to 60% of the rain received by the state of Tamil Nadu is from northeast monsoon. In most parts of India, this period marks the transition from wet to dry seasonal conditions. Average daily maximum temperatures range between 28 °C and 34 °C (82–93 °F). The northeast monsoon, which begins in September, lasts through the post-monsoon seasons, and only ends in March, carries winds that have already lost their moisture while crossing central Asia and the vast rain shadow region lying north of the Himalayas. They cross India diagonally from northeast to southwest. However, the large indentation made by the Bay of Bengal into India's eastern coast means that the flows are humidified before reaching Cape Comorin and rest of Tamil Nadu, meaning that the state, and also some parts of Kerala, experience significant precipitation in the post-monsoon and winter periods. However, parts of West Bengal, Orissa, Andhra Pradesh, Karnataka and North-East India also receive minor precipitation from the northeast monsoon. Mukesh Kumar

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