Indian Climate - Weather Mechanisms

Weather Mechanisms

The variations in local climate across India arise from several interacting mechanisms. The principal controls are:

• the surface distribution of pressure and winds,
• upper-air circulation driven by global circulation patterns, air-mass inflow and jet streams, and
• the inflow of western disturbances during winter and tropical depressions during the south-west monsoon season that generate most of India's rainfall events.

Pressure and surface winds

These mechanisms are best understood with reference to the two contrasting seasons when the most striking changes in weather occur: winter and summer.

Winter-time controls (lower and upper levels)

• During winter, the pressure pattern over Central and West Asia strongly influences India's weather. A persistent high-pressure centre north of the Himalaya directs near-surface winds southwards into the Indian subcontinent. These are dry continental north-westerly winds experienced over the north-western plains in winter.
• On weather maps this produces a zone of contact between the dry north-westerly continental air and relatively warmer Indian maritime trades over north-west India. The position of this contact zone is unstable and occasionally shifts eastwards as far as the middle Ganga valley; when it does, much of north and western India comes under the influence of dry north-westerly winds.
• At higher levels the circulation is different. Over West and Central Asia the prevailing winds are westerlies which, encountering the elevated Tibetan highlands, become bifurcated into two streams forming the jet stream. One branch flows north of the Tibetan barrier, while a southern branch flows eastwards south of the Himalaya.

Jet stream affecting Indian climate

• The southern branch of the westerly jet has a mean position near about 25°N in February and is strongest at about the 200-300 millibar level in the lower stratosphere/lower troposphere. This southern jet is important for Indian winter weather because it steers the western disturbances that enter the subcontinent from the west and north-west during winter.
• Western disturbances are extratropical cyclonic storms that bring light rain to the plains and snowfall to the Himalaya; their arrival and tracks are controlled by the upper-air westerly jet.

Summer reversal and the monsoon circulation

• As the sun moves northward in summer, wind circulation over the subcontinent reverses at both surface and upper levels.
• Near the surface a belt of low pressure forms roughly parallel to the Himalaya. This persistent low pressure region is part of the broader Inter-Tropical Convergence Zone (ITCZ), which migrates northward over India in summer.
• By mid-July the ITCZ commonly lies near about 25°N. By this time the westerly jet withdraws away from the Indian region.
• The low pressure at the surface draws in moist maritime air from the southern hemisphere and the equatorial Indian Ocean. This maritime tropical (mT) air, after crossing the Equator, is deflected and flows north-westward into the Indian low, forming the south-west monsoon.
• Some meteorologists view the south-west monsoon as a northward continuation of the equatorial westerlies converging on the ITCZ. Along the ITCZ, an easterly inflow along the north-eastern margins converges, while the north-western margin is a zone where dry continental air subsides.

At the lower troposphere the south-west monsoon brings most of India's seasonal rainfall, whereas at middle and upper tropospheric levels an easterly jet stream forms over the North Indian Plain during the monsoon and plays a role in steering tropical depressions. These tropical depressions greatly influence the spatial distribution and intensity of monsoon rainfall-the heaviest rains occur along the tracks of such depressions, and their frequency, path and intensity determine regional monsoon patterns.

Onset of the Monsoon

The onset of the south-west monsoon is complex and cannot be explained by a single theory, but the principal factors are well established:

• strong differential heating of land and sea in summer which produces a very large thermal low over the north Indian plains and adjoining regions,
• the consequent northward shift of the ITCZ, and
• the deflection of south-easterly trades crossing the Equator so that they approach the Indian coast as south-west winds.

Monsoon in India

• The northward shift of the ITCZ is related to the withdrawal of the westerly jet stream from its winter position south of the Himalaya. After withdrawal, an easterly jet stream develops over northern India in response to intense summer heating of the Himalayan and Tibetan highlands.
• Solar heating of these elevated regions produces a circulation in the middle troposphere with outflows directed both equatorward and poleward. The equatorward outflow manifests as the easterly jet over India while the poleward outflow contributes to the westerly jet over East-Central Asia. The Himalayan-Tibetan topography is therefore a key factor in the monsoon's establishment and structure.
• Typical timing: the south-west monsoon normally sets in over the Kerala coast by 1 June, reaches the vicinity of Bombay (Mumbai) and Kolkata between about 10-13 June, and by about 15 July generally covers the entire subcontinent.

Rain-bearing systems

The Indian peninsula divides the incoming south-west monsoon flow into two principal branches:

• the Arabian Sea branch, and
• the Bay of Bengal branch.
• The Bay of Bengal branch spawns most tropical depressions that track inland and cause heavy rainfall over West Bengal, the sub-Himalayan region and the northern plains.
• The Arabian Sea branch delivers rain to the western coast (Kerala, Karnataka, Maharashtra, Gujarat) and parts of central India (Madhya Pradesh). The Arabian Sea branch is often stronger because it draws moisture from a larger expanse of ocean and, in many cases, its entire flow reaches India; in contrast, a portion of the Bay of Bengal branch continues eastward to Myanmar, Thailand and the Malay Peninsula.
• With the exception of the eastern Coromandel coast of Tamil Nadu, most parts of India receive the bulk of their annual rainfall from the south-west monsoon.

Distribution of rainfall

• Annual rainfall exceeds 300 cm over parts of the western coast (windward Western Ghats) and in north-eastern India.
• Annual rainfall is less than 50 cm in western Rajasthan and adjoining parts of Gujarat, Haryana and Punjab.
• Low rainfall is also observed in the interior of the Deccan plateau east of the Sahyadris.
• A third region of very low precipitation lies around Leh in Ladakh. Snowfall is largely restricted to the Himalayan ranges.
• Relief (orography) strongly governs the distribution of monsoon rainfall. When moisture-laden air is forced to ascend along the windward slopes of mountains or the edge of plateaux, it cools, becomes saturated and releases heavy rain. On the leeward side the descending air warms and becomes drier, creating a rainshadow region. For example, the windward Western Ghats may receive over 250 cm of rain, while the leeward side of the same range may receive less than 50 cm.
• North-eastern India receives heavy rainfall because of its hilly topography and the Eastern Himalayas. The moisture-laden Bay of Bengal branch feeds the Brahmaputra valley and the hills of Meghalaya; Cherrapunji and Mawsynram, located near the head of a funnel-shaped valley and facing perpendicular monsoon winds, record some of the world's highest annual totals: about 1,087 cm at Cherrapunji and 1,141 cm at Mawsynram.
• Rainfall in the northern plains decreases from east to west. Approximate annual totals are: Kolkata (Calcutta) ~120 cm, Patna ~102 cm, Allahabad ~91 cm, and Delhi ~56 cm.
• The monsoon is pulsatile-rainfall occurs in wet spells separated by dry spells. This pulsating behaviour is largely due to the formation and landward movement of cyclonic depressions (tropical lows) at the head of the Bay of Bengal. The passage of these depressions along the axis of the monsoon trough determines where and how much rain falls. When the trough's axis shifts northward toward the Himalaya, the plains often experience longer dry spells while hill catchments receive heavy rainfall that can cause floods.

Important features and significance of monsoon rains

• The monsoon is the most influential climatic factor for India's economy. The amount and distribution of monsoon rainfall determine cropping patterns and agricultural productivity.
• Nearly 80 percent of Indian agriculture depends directly on monsoon rains; irrigation for the remaining agriculture depends substantially on stored monsoon water in reservoirs.
• Animal husbandry and allied rural livelihoods are strongly dependent on adequate and timely rainfall.
• Hydroelectric power generation is also dependent on monsoon inflows; historically, nearly two-fifths of India's power generation has come from hydro sources (subject to variation over time and development).
• Monsoon characteristics-uncertainty in onset and withdrawal, large interannual variability, erratic behaviour and unequal spatial distribution-make agriculture and water resources management challenging.
• Both excess and deficiency of monsoon rainfall produce disasters: floods and droughts, respectively. The timing of a rainfall deficiency or excess during critical crop growth stages often matters more than the absolute deviation from the mean.

Seasons In India

The monsoon-type climate produces distinct seasonality. The India Meteorological Department (IMD) divides the year into four principal seasons:

• hot-dry season (mid-March to end-May),
• hot-wet / south-west monsoon season (June to September),
• retreating south-west monsoon (October to November), and
• cold-dry season (December to February).

(i) Hot-dry season

From mid-March to May there is a progressive rise in temperature across India. Mean temperatures over the northern plains rise from about 26°C in mid-March to as high as 45°C in mid-May at some places. In the peninsular region temperature rises are less extreme and may show a different seasonal pattern.

Wind direction in the summer season

• The hot-dry season is characterised by weak winds and low humidity over most parts of the country, though local severe weather occurs. Examples include tornado-like dust storms in Punjab and Haryana, andhis in Uttar Pradesh and kalbaisakhis (norʼwesters) in West Bengal, which may give short but heavy showers and strong winds.
• Strong hot, dry westerly winds called loos blow frequently during May and early June over the north-western plains; these desiccate soil and vegetation and can cause heat-related casualties.
• Poor rainfall in the preceding monsoon exacerbates drought conditions in the hot-dry season because surface moisture, fodder and water supplies are depleted.

(ii) Hot-wet (south-west monsoon) season

• This is the main rainy season. From early June the south-westerly winds from the Indian Ocean begin to blow towards the low pressure area over north-west India; the low reaches its deepest intensity in July.
• These south-west monsoons bring sudden and sustained rainfall and reduce temperatures; the abrupt beginning of widespread rainfall is often called the monsoon burst. The burst typically occurs in the first week of June along the west coast and may be delayed in the interior until the first week of July.
• Temperatures fall with the onset of reliable rains though a secondary temperature maximum often occurs in September over northern India.

Wind direction in the rainy season

• The south-west monsoon reaches India in two main branches: the Arabian Sea branch and the Bay of Bengal branch.
• The Arabian Sea branch reaches Kerala by about 1 June and Mumbai (Bombay) by about 10 June. The Western Ghats obstruct this branch so the windward slopes receive very heavy rainfall; after crossing the Ghats the branch moves over the Deccan plateau and parts of central India before joining the Bay of Bengal branch over the Ganga plains.
• Portions of the Arabian Sea branch also strike the Saurashtra peninsula and Kutch and then pass over western Rajasthan and the Aravallis, giving small amounts of rain there.
• The Bay of Bengal branch initially strikes the Burmese coast and south-east Bangladesh. The Arakan Hills deflect a large part of this branch westwards towards the Indian subcontinent so that it enters West Bengal and Bangladesh from the south and south-east rather than directly from the south-west.
• On entering India the Bay branch splits under the influence of the Himalaya and the thermal low: one branch moves westward along the Ganga plains towards Punjab, the other moves up the Brahmaputra valley causing heavy rains over north-east India. A sub-branch strikes the Garo and Khasi Hills of Meghalaya.
• Cherrapunji and Mawsynram, situated on windward slopes at the head of funnel-shaped valleys and aligned with prevailing monsoon winds, record extremely high rainfall: approximately 1,087 cm at Cherrapunji and 1,141 cm at Mawsynram.
• By mid-September the south-west monsoon begins to retreat as the sun moves southwards and the continental thermal low weakens. The retreat is gradual and earliest in the extreme north; autumnal clear weather strengthens from late September into October.
• During October-November the rainiest part of the year for the east coast of Tamil Nadu and the south-east peninsula occurs, because low-pressure systems shift southwards over the Bay of Bengal and later into the equatorial belt by December.

(iii) Cold-dry season

• The cold season extends from December to February. January is generally the coldest month; mean temperatures in the northern plains typically range from about 10-15°C, while the southern peninsula may have means around 25°C.
• Cold conditions over the northern plains lead to the formation of a surface high-pressure area. Winds blow outward from this high towards the equatorial low, producing winds from land to sea over much of India and creating a generally dry season.
• Winds over the Ganga plains are commonly north-westerly or north-easterly and become north-easterly over the Bay of Bengal. Most of the country experiences fine weather with clear skies, low humidity and cool breezes.

Wind direction in the winter season

At intervals this fine weather is disturbed by shallow cyclonic depressions-western disturbances-which originate over the eastern Mediterranean region and Pakistan. These systems bring light rains to the plains and snowfall over the Himalayan ranges.

The only large region of India that receives most of its rainfall from the north-east trade winds (the north-east monsoon) is the coast of Tamil Nadu. These winds pick up moisture while crossing the Bay of Bengal and produce the principal rainy season for Tamil Nadu during October-November.

Summary: India's weather mechanisms are the result of interactions between seasonal thermal contrasts, large-scale upper-air circulation (including jet streams), the orography of the Himalaya and peninsular highlands, and oceanic moisture sources. Together these determine the timing, distribution and variability of the monsoon-an element of climate that is central to India's agriculture, water resources and economy.