Why India’s monsoon is difficult to forecast | The Economist

METEOROLOGISTS are forecasting a bumper monsoon for India this year. This is good news for the more than 600m people—about half of India’s population—who depend on the rains it brings. Knowing when and where the monsoon will arrive is especially important for farmers; even now, two-thirds of India’s fields lack irrigation. But forecasting the monsoon remains fantastically difficult, especially as four in every ten monsoons are classified as abnormal anyway. What makes India’s monsoon so unpredictable?

The word monsoon derives from mausam in Hindi (and originally from Arabic), meaning “weather”. Monsoon climates typically have two very distinct seasons: wet and dry. In India, the onslaught of the rains begins when moist air is carried northwards from the Indian ocean during the summer. The winds transporting the main or “south-west” monsoon come from an area south of the equator which is characterised by high atmospheric pressure. As the air gathers moisture during the journey, atmospheric convection forms huge storm clouds which arrive first in southern India around early June (as they did this year). The monsoon creeps north and west, showering Pakistan and north India about a month later. By September it is in retreat, and has normally withdrawn from the south of the country by December. Many factors seem to affect the duration and intensity of the monsoon. One is El Niño, a climatic phenomenon associated with warmer temperatures in the tropical Pacific ocean. Last year the monsoon proved disappointing while El Niño was in full swing: total rainfall between June and September was 14% below the 50-year average. How exactly the phenomenon interacts with the monsoon is not well understood, however, as even large Niños in the past have coincided with normal monsoons.

Anthropogenic emissions also seem to affect rain patterns. India is the world’s fourth-biggest emitter of greenhouse gases. The insulating effect of such emissions helped make last year the hottest on record; this year looks set to be even more scorching. A warmer atmosphere probably means even greater variability in the monsoon. Rainfall extremes are expected to increase, thanks in part to the fact that a warmer atmosphere can hold more moisture (about 7% more, for every degree Celsius of warming). Air pollution complicates matters further. It is a terrible problem in India, contributing to more than 600,000 early deaths a year. Cooking at home, and the smoke it releases, accounts for much of the trouble. Aerosols such as black carbon interact with sunlight. Some of these tiny particles—many less than a tenth the width of a human hair—scatter light, while others absorb it. In the former case, this prevents the light from warming the earth’s surface. In the latter, absorbing the light causes the particles to warm the air around them. Both alter the heating of the atmosphere, and therefore the heating of the land relative to the ocean—the process which drives the monsoon.

Scientists are using a variety of techniques to better forecast the monsoon, from monitoring changes in land use (because vegetation stores more moisture) to sending underwater robots into the Bay of Bengal (to learn more about the salinity and temperature of the ocean). Their research could improve climate models and farming practices—but improved water-storage facilities, better irrigation and more access to insurance schemes might have to make up for the gaps in knowledge that will persist.

Source: The Economist explains: Why India’s monsoon is difficult to forecast | The Economist

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