‘The Little Girl’ — La Nina — has had an outsized impact across globe since manifesting itself in mid-2010. La Nina and its equally rumbustious sibling, El Nino, come about when the waters of the eastern and central Pacific Ocean along the equator become unusually cold or warm. These changes in the Pacific produce swings in atmospheric pressure, winds, temperature, and rainfall that have a global impact. These coupled with changes in the ocean and atmosphere are collectively called the El Nino/Southern Oscillation (ENSO). For India, an El Nino is often a cause for concern because of its adverse impact on the south-west monsoon; this happened in 2009. A La Nina, on the other hand, is often beneficial for the monsoon, especially in the latter half. The La Nina that appeared in the Pacific in 2010 probably helped last year’s south-west monsoon end on a favourable note. But then, it also contributed to the deluge in Australia, which resulted in one of that country’s worst natural disasters with large parts of the north-east under water. It wreaked similar havoc in south-eastern Brazil and played a part in the heavy rains and consequent flooding that have affected Sri Lanka.
It is becoming increasingly clear that global warming is contributing to the impact that ENSO has. The Indian Ocean is warming rapidly. There are already indications that this warming along with the growing temperature of the western Pacific is influencing the effect of a La Nina. A paper, ominously titled ‘The Perfect Ocean for Drought,’ which was published in the journal Science in 2003, linked the prolonged droughts from 1998 to 2002 that afflicted the United States, southern Europe, and south-west Asia to the warmth of these ocean waters during a protracted La Nina. Such heightened ocean temperatures may well have played a crucial part in weather-related events in recent months. While the La Nina that developed in mid-2010 lent a helping hand to the south-west monsoon, the warmth of the tropical Indian Ocean may have prevented a more equitable distribution of rainfall — eastern Uttar Pradesh, Bihar, Jharkhand, and much of West Bengal received far too little of it. The warming of the Indian Ocean and western Pacific probably provided the extra moisture and energy for the exceptionally heavy rains that Australia and Sri Lanka experienced. It could be one reason why in India the north-east monsoon, which is usually retarded by a La Nina, has this time seen a surfeit of rain. The writing on the wall is clear enough: global warming will worsen the swings of climate variability brought about by factors like ENSO, making extreme weather events such as droughts and floods more frequent. The world needs to pay heed.