The South Asian monsoon, past, present and future

From The Economist:

1. A gamble on the rains

With rheumy eyes and a face wizened by the sun, Narayanappa looks down to the ground and then, slowly, up to the skies. After weeks of harsh heat his land, one and a half hectares (four acres) of peanuts, chillies and mulberry bushes, has turned to dust. At the beginning of June, a dozen families local to Kuppam, a village in the Chittoor district of the south-eastern state of Andhra Pradesh, came together, as they do every year, to sacrifice a goat as a divine downpayment on a good monsoon. By mid-June the monsoon rains should be quenching the parched ground. Yet there is no sign of the livid clouds running up from the south-eastern horizon which serve as its evening harbingers, rising and roiling, filling the sky with their rumbling and the night with veiled lightning. The sky is as blank as the ground is dry. Narayanappa has his sacks of nuts ready to sow. But time is running out.

In his office at the India Meteorological Department in New Delhi, Madhavan Nair Rajeevan, the department’s boss, looks at portents which are dry in a different way—figures and lines on paper and screens. Where once the oncoming monsoon was spotted through telescopes on the veranda of the observatory built by the Maharajah of Travancore on a hill above Thiruvananthapuram (formerly Trivandrum) in Kerala, now the signs of its coming are looked for through tracked radar and satellites. But they are still of intense interest to the country’s rulers, and its people. The monsoon’s arrival in Thiruvananthapuram at the beginning of June marks the official beginning of India’s rainy season. The rains’ subsequent movement is tracked on a daily basis by national television stations, rather like the advance of the spring cherry blossom in Japan but with far greater human consequence.

A century of meteorological progress means that Mr Rajeevan can say with much more confidence than his predecessors how fast the summer monsoon will sweep up the nation and how much rain, overall, it will bring. When the monsoon started late this year he could give a convincing non-goat-related reason; Cyclone Vayu, in the Arabian Sea, upset the flows on which the monsoon depends. But though meteorology has improved, it has a long way to go. On average the monsoon is a regular wave of rain, rising and falling over the months from June to September. In any given year, though, the smooth wave is overwritten by spikes and troughs, bursts of intense precipitation and weeks of odd dryness, variations known as “vagaries” which science still struggles to grasp.

There is a complex structure in space, as well as time. Some places may be almost completely skirted by the rains. Others see deluges violent enough to destroy crops and carry away soil, the water running off the land before it can be caught and stored. The flooding that goes with such rains is expected to become worse and wider-spread as the global climate warms. Agriculture remains the Indian economy’s largest source of jobs, directly accounting for a sixth of its GDP and employing almost half of its working people. A bad monsoon can knock Indian economic growth by a third. The effects in Bangladesh, Bhutan, Nepal, Pakistan and Sri Lanka are on a similar scale. Almost a quarter of the world—1.76bn souls—lives with the South Asian monsoon.

As Guy Fleetwood Wilson, a finance minister, put it in 1909, the “budget of India is a gamble in rain.” Thanks to Mr Rajeevan and his colleagues, the odds of each year’s gamble are now better known. But obvious steps that might lower the stakes being played for are still not taken. Storage systems in cities have fallen into disuse; aquifers under farmland are depleted year by year faster than the monsoons can refill them. In a country where more people will face the risks of climate change in the decades to come than any other, the problems of the current climate are being ducked.

The metamorphosis brought by the burst of the monsoon is profound. Brown landscapes turn green, dusts become muds, cracks turn into mouths through which the earth slakes its thirst. The Ganges and the other great rivers fill then overflow, spreading silt-rich fertility across their floodplains. In the countryside the air takes up the petrichor aroma of fresh earth. In gardens, the scent of frangipani carries on the damp breeze; in cities, that unmistakably Indian blend of ordure, asphalt and spice.

It’s a sea breeze

The people respond. The rains bring a sense of relief and a new sensuality. In “The Cloud Messenger” by Kalidasa, one of the greatest Sanskrit poets of north India, the meeting of earth and clouds is nothing less than a kind of lovemaking. In the Sangam literature of the deep south, the heroine waits for her lover, who is away seeking war, wealth and adventure, to return with the rains. People still tell stories of inhibitions cast aside and new lovers taken. The heart takes on the driving, unpredictable rhythms of the rain.

For all its complexity and importance, on every scale from that of smallholders to empires, at its heart the monsoon is something fairly simple: a season-long version of the sea breezes familiar to all those who live by coasts. Because land absorbs heat faster than water does, on a sunny day the land, and the air above it, warm faster than adjoining seas. The hot air rises; the cooler air from above the sea blows in to take its place.

A monsoon is the same sort of phenomenon on a continental scale. As winter turns to summer, the Indian subcontinent warms faster than the waters around it. Rising hot air means low pressure; moist maritime water is drawn in to fill the partial void. This moist water, too, rises, and as it does, its water vapour condenses, releasing both water, to fall as rain, and energy to drive further convection, pulling up yet more moist air from below.

The heroine waits for her lover, who is away seeking war, wealth and adventure, to return with the monsoon rains

There are other monsoonal circulations around the world—in Mexico and the American south-west and in west Africa, as well as in East Asia, to the circulation of which the South Asian monsoon is conjoined. But geography makes the South Asian monsoon particular in a number of ways. The Indian Ocean, unlike the Pacific and the Atlantic, does not stretch up into the Arctic. This means that water warmed in the tropical regions cannot just flow north, taking its heat with it. It stays in the Arabian Sea and the Bay of Bengal, lapping at India from the west and the east. And to the subcontinent’s north sits the Tibetan plateau, the highest on the planet. The summer heat there draws the monsoon’s moisture far higher into the atmosphere than it would otherwise be able to go, adding mountains of cloud to the Himalayan peaks.

The monsoon is thus a mixture of necessity and chance. Given the arrangement of sea and land and the flow of heat from equator to pole, such a season has to exist; given the vagaries of weather from year to year, and within the seasons themselves, it springs surprises for good and ill. It is also, and increasingly, a mixture of the natural and the human—as ever more humans depend on it, as humans learn new ways of anticipating it, and as humans face up to the climate change which will reshape it.

2. The winds that made Asia

The rains for which Narayanappa waits are not the whole story. The word “monsoon” blew into English from Portuguese in the late 16th century not because European sailors cared about the rain on alien plains, but because when they followed Vasco da Gama around the tip of Africa they came across a type of wind they had never encountered, and for which they had no name.

The Portuguese monção comes in its turn from the Arabic, mawsim, which means “season”. In the Atlantic Ocean, the only one to which the Portuguese were accustomed, winds in any given place tend to blow in pretty much the same direction throughout the year, though their intensities change with the season and their prevailing direction changes with the latitude. In the Indian Ocean, the prevailing winds flip back and forth.

This is because of the role played in the monsoon by the “intertropical convergence zone” (ITCZ) which encircles the world close to the equator. The ITCZ is a zone of low pressure over the warmest water. In all the oceans, this low pressure draws in steady winds from the south-east known as the southern trade winds.

During the northern hemisphere’s winter, the ITCZ sits south of the equator in the Indian Ocean. As warmth creeps north, so does the ITCZ, becoming a dynamic part of the monsoon. It ends up nestled against the Himalayas, bringing the southern trades with it. But their move from the southern hemisphere to the northern, and the constraining effect of high pressure over Africa, sees them twisted from south-easterlies to south-westerlies. When these south-westerly trades pick up in late spring—wind speeds in the Arabian Sea can double over a few weeks—the rains are on their way to Thiruvananthapuram.

Link to the rest at The Economist

Not about books, but PG was taken by the quality of the writing in the OP.

2 thoughts on “The South Asian monsoon, past, present and future”

    • Yep.

      You can always tell amateurs in any field based in reality — they’ve never read enough within that field (or any other) to recall all the (often notorious to educated people) failed analyses of causes.

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