Factors Controlling Indian Climate

The climate of India is highly diverse, ranging from tropical wet in the south to alpine in the Himalayan north. It is broadly classified as a tropical monsoon climate. The unique meteorological patterns of the Indian subcontinent are dictated by a complex interplay of geographical, physiographic, and atmospheric factors. These factors can be categorized into two primary groups: factors related to location and relief, and factors related to air pressure and winds.

Factors Related to Location and Relief

Latitude

The Tropic of Cancer ($23.5^\circ \text{ N}) passes through the middle of India, dividing the country into two distinct climatic zones: </p> <ul> <li> <b>Subtropical and Temperate Zone:</b> The northern part experiences high annual ranges of temperature with cold winters and hot summers. </li> <li> <b>Tropical Zone:</b> The southern part, lying closer to the equator, experiences high temperatures throughout the year with a low annual range of temperature and no distinct winter season. </li> </ul> <h5>The Himalayan Mountains</h5> <p> The Himalayas act as an effective climatic divide for the subcontinent in two major ways: </p> <ul> <li> <b>Shielding Effect:</b> They form an impenetrable physical barrier that protects the Indian subcontinent from the freezing, frigid continental air masses blowing southward from Siberia and Central Asia. </li> <li> <b>Monsoon Trap:</b> They intercept the moisture-laden Southwest Monsoon winds, forcing them to shed their moisture across the Indo-Gangetic plains, preventing them from escaping northward into Tibet. </li> </ul> <h5>Distribution of Land and Water</h5> <p> India is flanked by the Indian Ocean to the south, the Arabian Sea to the west, and the Bay of Bengal to the east. </p> <ul> <li> <b>Differential Heating:</b> Land heats up and cools down much faster than water. This differential heating creates intense low pressure over the Afro-Asian landmass during summer and high pressure over the Indian Ocean, driving the monsoon winds. </li> <li> <b>Maritime Influence:</b> The long coastline imparts a moderate, equable climate to the peninsular region, keeping seasonal temperature variations minimal. </li> </ul> <h5>Distance from the Sea (Continentality)</h5> <p> As distance from the sea increases, the moderating influence of the ocean diminishes. This gives rise to extreme climatic conditions known as <b>continentality</b>. Regions like Delhi, Punjab, and Rajasthan experience very high temperatures in summer and low temperatures in winter, alongside low annual rainfall. </p> <h5>Altitude and Relief</h5> <p> Temperature decreases with an increase in altitude due to the normal lapse rate. </p> <ul> <li> <b>Temperature Variation:</b> Hill stations like Ooty and Shimla are significantly cooler than nearby plains at the same latitude. </li> <li> <b>Rain-shadow Effect:</b> The Western Ghats act as a barrier to the Southwest Monsoon winds. The windward side (Western Coastal Plains) receives heavy rainfall (exceeding 250 cm), while the leeward side (Deccan Plateau) lies in the rain-shadow zone and remains relatively dry. </li> </ul> <h4>Factors Related to Air Pressure and Winds</h4> <h5>Surface Pressure and Wind Systems</h5> <p> The seasonal reversal of wind directions defines the Indian monsoon system: </p> <ul> <li> <b>Winter Months:</b> High pressure develops over Central Asia and the northern Indian plains. Winds blow southward from the land toward the low-pressure oceanic regions to the south as dry Northeast Monsoons. </li> <li> <b>Summer Months:</b> Intense heating creates a low-pressure trough over northern and northwestern India. This causes a complete reversal of wind direction, pulling moisture-laden Southeast Trade winds from the Southern Hemisphere across the equator, turning them into the Southwest Monsoons. </li> </ul> <h5>Jet Streams and Upper Air Circulation</h5> <p> Upper air circulation over India is dominated by westerly and easterly jet streams: </p> <ul> <li> <b>Subtropical Westerly Jet Stream:</b> During winter, this jet stream flows south of the Himalayas over northern India. It plays a critical role in bringing Western Disturbances into the subcontinent. In summer, it shifts north of the Himalayas, allowing tropical monsoons to establish. </li> <li> <b>Tropical Easterly Jet Stream:</b> It develops during the summer over the peninsular region (around14^\circ \text{ N}). It is responsible for steering tropical depressions into India, which cause widespread rainfall over the eastern coast and Gangetic plains. </li> </ul> <h5>Western Cyclonic Disturbances and Tropical Cyclones</h5> <ul> <li> <b>Western Disturbances:</b> Shallow cyclonic depressions originating over the Mediterranean Sea travel eastward via the Subtropical Westerly Jet Stream. They bring crucial winter rainfall to the plains of Punjab and Haryana and snowfall to the Western Himalayas, benefiting Rabi crops (especially wheat). </li> <li> <b>Tropical Cyclones:</b> Originating over the Bay of Bengal and the Arabian Sea during the transition months (October-November) and pre-monsoon months (May), these cyclones affect the coastal regions of Odisha, Andhra Pradesh, West Bengal, and Gujarat, causing heavy rainfall and severe damage. </li> </ul> <h5>The Inter-Tropical Convergence Zone (ITCZ)</h5> <p> The ITCZ is a low-pressure zone located at the equator where trade winds converge. </p> <ul> <li> <b>Summer Shift:</b> In summer, the ITCZ shifts northward to around20^\circ \text{ N} – 25^\circ \text{ N}$, positioning itself over the Indo-Gangetic plains. This shift is referred to as the Monsoon Trough. This intense low-pressure zone acts as the main pull factor for the Southwest Monsoon winds.

Global Teleconnections Influencing Indian Climate

The Indian monsoon is not an isolated phenomenon; it is deeply intertwined with several global atmospheric and oceanic phenomena.

El Niño-Southern Oscillation (ENSO)

• El Niño: The anomalous warming of surface waters in the eastern tropical Pacific Ocean. It weakens the Walker Circulation, which usually results in a weakened Southwest Monsoon, causing droughts or deficit rainfall in India.
• La Niña: The anomalous cooling of the eastern tropical Pacific waters. It strengthens the monsoon winds, leading to above-normal rainfall and widespread flooding across India.

Indian Ocean Dipole (IOD)

Often called the “Indian Niño,” the IOD is the difference in sea surface temperatures between the western and eastern Indian Ocean.

• Positive IOD: The western Indian Ocean (near Africa) becomes warmer than the eastern part (near Indonesia). This enhances the Indian monsoon, often neutralizing the negative impacts of an El Niño year.
• Negative IOD: The eastern Indian Ocean becomes warmer, hindering the monsoon wind flow and leading to deficient rainfall in India.

Madden-Julian Oscillation (MJO)

The MJO is a moving band of rain-bearing clouds that travels eastward along the equator over a cycle of 30 to 60 days.

• If the active phase of the MJO resides over the Indian Ocean during the monsoon season, it triggers an intensification of rainfall (a “break-free” active period). If it is in the Pacific Ocean, it can cause a temporary lull or “break” in the Indian monsoon.

Comparative Summary: Key Climatic Drivers