Bjerknes found that the key to the southern oscillation lay in the periodic warming of sea surface temperatures in the eastern Pacific Ocean, and he dubbed it El Niño in keeping with the term that fishers had given the phenomenon. This warming reverberates throughout the world’s climate. Most of the time, the waters of the western Pacific, off Indonesia, are warmer than in the eastern Pacific—this drives the easterly “trade winds,” but Bjerknes saw that these were a surface manifestation of an overturning circulation in the upper atmosphere at higher latitudes. He named this the Walker Circulation in honor of Sir Gilbert. During an El Niño episode the contrast narrows as the waters of the eastern Pacific warm up; in response, the Walker Circulation weakens, since it is driven by that difference in temperature and pressure. Less intensive surface winds reduce the ocean’s churn, leading to less of the colder water from the depths welling up to the surface; this sustains the abnormal warmth in the eastern Pacific, and so flattens the usual temperature contrast across the ocean—attenuating further the Walker Circulation.⁶³

This disruption in circulation has consequences for rainfall in the western Pacific and the Indian Ocean—and even the North Atlantic. El Niño years tend to be associated with weak monsoons in Asia, and with excessive rainfall in South America. Bjerknes dubbed the overall oceanic-atmospheric system the El Niño Southern Oscillation (known as ENSO), of which El Niño was the phase of ocean surface warming and La Niña (girl child) of cooling; years that exhibit neither extreme are known as “neutral.” La Niña has the opposite effect of El Niño, strengthening the temperature contrast between the eastern and western Pacific, strengthening the Walker Circulation, and bringing more rain than usual to Asian shores.

The discovery of ENSO marked a breakthrough for understanding the Asian monsoon. Once it had been identified, historical climatologists showed that many of the worst droughts in Asian history—including the droughts that brought famine in the 1870s and the 1890s, and also the Maharashtra drought of 1972–1973, discussed earlier in this chapter—coincided with El Niño events. But the causal relationship between ENSO and the monsoon is complex. There is some evidence to suggest that an especially strong or weak monsoon might foreshadow rather than follow the corresponding phase in the ENSO cycle. Tropical meteorologist Peter Webster argues that there may be truth in Charles Normand’s suggestion—made in the 1950s after his retirement as head of India’s meteorological service and before Bjerknes had discovered El Niño—that India’s weather was more use in predicting what was in store for other parts of the world than it was itself amenable to prediction.⁶⁴

Knowledge of ENSO raised new questions about the periodicity of drought in Asia—a question that, as we have seen, had provoked much discussion in the 1870s. It reinforced the sense, drawn from the Indian Ocean Expedition, that Asia’s climate was fiendishly complex, associated with many other parts of the planet’s climate. ENSO is quasiperiodic; it recurs but the intervals between events vary and are not easy to predict. The early 1970s also brought new knowledge of internal climatic variability on shorter timescales. In 1971, Roland Madden and Paul Julian, based at the US National Center for Atmospheric Research, discovered what came to be known as the Madden-Julian Oscillation (MJO): an oscillation in surface pressure and wind direction over large areas, with consequences on a planetary scale.⁶⁵ The MJO has a clear periodicity; it is, as meteorologist Adam Sobel has put it, “a signal that emerges above the meteorological noise.” Migrating from west to east, from the Indian Ocean to the Pacific, the MJO lasts between thirty and sixty days, and its intensity varies from year to year. In its “active” phase, the MJO brings heavy rain, and a heightened chance of tropical cyclones; in its “suppressed” phase, it interrupts the monsoon flow, even reversing the wind direction, bringing clear skies. The MJO is associated particularly with the northern winter; but scientists also discovered another intraseasonal oscillation in the northern summer months, which propagates northward rather than eastward. Known as the Boreal Summer Intraseasonal Oscillation, its connection with or independence from the MJO has been the subject of debate, but it, too, is thought to play a vital role in the fluctuations of rainfall over Asia each summer.⁶⁶ In the 1980s further research was done to uncover the mechanisms at work behind these intraseasonal oscillations, though some uncertainty remains.⁶⁷ These intraseasonal oscillations might well explain the alternation between active and break periods in any monsoon season, which has such vital and direct effects on agriculture.

Advances in technology and understanding did little to revise Colin Ramage’s verdict, at the end of the Indian Ocean Expedition, that little headway had been made in forecasting the monsoon in a practical sense. Progress had been made in understanding the system on a large scale. But what mattered most to Asian farmers were the rhythms of rainfall within a given monsoon season—the relationship between what meteorologists call “active” and “break” periods of the monsoon. So finely attuned is Asian agriculture to the monsoon that the most devastating effects on cultivation often come from unexpected breaks in the midst of the summer rains, even when rainfall overall is plentiful—the skies brighten suddenly, and crops do not receive the water they need to thrive at a critical phase in their life cycle.

A further push to crack the monsoon’s code came in 1979, as part of a worldwide effort called the Global Weather Experiment—the Indian Ocean component of that came to be known as the Monsoon Experiment, or MONEX. The scale of the operation was vast, even larger than the Indian Ocean Expedition of the 1960s, and more fully equipped with satellite technology. It encompassed 3,400 land stations, 800 upper air observatories, 9 weather ships, 7,000 merchant ships, and 1,000 commercial aircraft drawn in to record observations, 100 dedicated research aircraft, 50 research ships, 5 weather satellites, and 300 balloons. Despite this scale, despite the dazzling advances in equipment, Webster noted that much older ways of knowing the monsoon—the instinctive knowledge of mariners—were still in evidence in 1979; he saw that dhows, the traditional sailing vessels of the northwestern Indian Ocean that had for centuries harnessed the monsoon currents, were still widely used.⁶⁸

COLIN RAMAGE, DIRECTOR OF THE METEOROLOGICAL COMPONENT of the Indian Ocean Expedition, returned to India in the 1970s. In his spare time while there on assignment, he turned amateur historian and wrote a short and provocative essay on how his predecessor John Eliot—the second director of Indian meteorology, after Henry Blanford—had failed spectacularly to forecast the crushing drought of 1899–1900. Ramage delved into the archives of the Times of India and wrote a powerful account of the famine that followed. “The government refused with religious fervor to modify the holy writ of laissez-faire,” he declared. Like many critics of imperial policy at the time, he saw that the railways had done as much to worsen as to alleviate the famine, by making it easier for speculators to ship grain out to areas where purchasing power was higher. He praised the Indian government’s efforts to protect its citizens against the threat of famine, but noted that, despite best efforts, India remained dependent on food imports, as it had since the 1920s. His conclusion was ominous. “Can we be sure that such a devastating famine will not recur?” he asked; not since Indian independence had there been a drought as severe as the drought of 1899. He ended his essay on that note, leaving implicit the underlying question: what would happen if another drought of that magnitude were to materialize?⁶⁹

But another threat was now on the horizon. In 1979, the same year as MONEX, the World Meteorological Organization held its first World Climate Conference. The conference declaration recognized the need to “foresee and prevent potential man-made changes in climate that might be adverse to the well-being of humanity.”⁷⁰ From the 1980s, the combination of climate change and other environmental threats compounded the water-related risks faced by billions of people in Asia.