The word is a traveller’s word. Mausim — Arabic for “season” — entered the world’s languages on the decks of trading ships, because for two thousand years the sailors of the Indian Ocean built their entire commercial and cultural world around a single natural phenomenon no other sea on Earth offered so reliably: a wind that kept a calendar. One direction all summer, the exact opposite direction all winter, switching with a regularity precise enough to plan a career, a fortune, or a marriage around. A ship’s captain in Muscat or Aden could leave harbour riding the winter wind, arrive in Calicut or Cochin, sell an entire cargo of horses or dates, wait out the intervening months for the reversal, and ride the summer wind directly home. To those mariners the monsoon was never experienced as weather in the way a storm or a drought is weather. It was infrastructure — as dependable, in its way, as a modern shipping lane or a scheduled airline route, and every major port city of the medieval Indian Ocean, from Hormuz to Malacca, built its entire commercial calendar around the wind’s own timetable.
The Romans, once they learned of this arrangement, wanted access to it badly. Tradition credits the Greek navigator Hippalus, writing around the first century of the common era, with revealing to the Mediterranean world the possibility of a direct open-ocean crossing: strike boldly out across the open Arabian Sea, riding the summer monsoon, straight to the pepper ports of the Malabar coast, rather than hugging the coastline the whole way as earlier traders had done. Historians today treat the individual credit given to Hippalus with appropriate caution — Arab and Indian sailors had almost certainly worked some version of this open-water route for centuries before any Greek wrote it down — but the historical consequence attributed to the discovery is documented fact regardless of who deserves the original credit: Roman fleets did make the monsoon run in substantial numbers, and Roman gold drained steadily eastward in exchange for pepper and fine cotton, provoking Pliny the Elder’s famous, bitter complaint about the drain on the imperial treasury. The monsoon had become, in effect, India’s first export industry in the modern economic sense. It sold safe, predictable passage, and merchants on three continents paid handsomely for the privilege of using it.
For millennia, then, the wind was used with total confidence by people who could not have explained it. The first person to ask the right scientific question about it in print was Edmond Halley — the same astronomer remembered today chiefly for the comet that bears his name — who in 1686 proposed a genuinely thermal explanation for the monsoon: the sun heats land far faster than it heats the adjoining sea, the warmed air over the land rises, and cooler sea air rushes in to replace it. It was a simple theory, only partly correct by modern standards, and yet it was genuinely revolutionary for its moment, because it was the first attempt to explain the monsoon through physical cause and effect rather than through observed regularity alone. Halley could not explain why the resulting winds bend the way they do — that answer, the Coriolis deflection this cover package’s Article 4 has already described, waited two and a half centuries for a fuller theory of the planet’s rotation to be developed. But the core insight, that the monsoon is fundamentally a heat engine rather than a mystery, was entirely his, and virtually everything discovered about the system since has been elaboration on that single founding idea.
The most consequential elaboration began, tragically, in famine. In 1876, the monsoon failed catastrophically across southern India and kept failing in successive seasons; the Great Famine that followed killed on a scale historians still argue over, with estimates for the broader 1876–78 famine years running into the tens of millions across the affected regions. Out of the shadow of that catastrophe, the India Meteorological Department was founded in 1875, initially with a single dedicated scientist, Henry Francis Blanford, and handed an audacious institutional mandate that no meteorological service anywhere in the world had yet attempted: predict the monsoon’s behaviour before it arrives, so that famine relief and grain movement could be organised in advance rather than improvised afterward. Blanford found his earliest workable signal in Himalayan snow cover, reasoning — correctly, as later science would confirm through an entirely different mechanism — that a heavier winter snowpack tended to precede a weaker following monsoon, and on 4 June 1886 he issued what is generally regarded as the world’s first operational long-range seasonal forecast for any monsoon system anywhere.
The figure who eventually turned this early craft into genuine, generalisable science was Gilbert Walker, a Cambridge-trained mathematician who succeeded Blanford’s successors as head of Indian meteorology and spent nearly two decades, largely working from the hill station of Shimla, correlating the Indian monsoon’s year-to-year behaviour against every other measurable climate variable he could obtain data for anywhere on Earth. His patience extracted the grand prize: a vast, slow oscillation of atmospheric pressure rocking back and forth between the Pacific and Indian Ocean basins, which he named the Southern Oscillation. Walker never lived to learn what physically drove the pattern he had so painstakingly documented; that connecting piece, linking his Southern Oscillation directly to the ocean warming already known to Peruvian fishermen as El Niño, was supplied only in 1969, by the Norwegian-American meteorologist Jacob Bjerknes, who welded the two half-understood phenomena together and gave the combined, unified circulation the name it still carries in every forecast issued today: the El Niño–Southern Oscillation, or ENSO.
It is worth pausing on the geography of that discovery chain, because it is genuinely remarkable and rarely appreciated outside specialist circles. The single most consequential climate phenomenon affecting the modern world, the one responsible for this year’s Indian shortfall as much as for droughts and floods on three other continents, was not first discovered in the Pacific Ocean, where it physically originates. It was discovered in India, pursued specifically because of the Indian monsoon’s own economic and human stakes, by scientists working in the aftermath of an Indian famine. This year’s forecast, tracked across this entire cover package from April’s first long-range outlook to May’s revision to July’s district-by-district reckoning, is, in a direct and traceable intellectual lineage, still Gilbert Walker’s puzzle — being solved today with satellites, supercomputers, and machine-learning tools he could never have imagined, but built on the very same correlation he first noticed from a hill station overlooking the plains his forecasts existed to protect.
– Rashmi M
