The last time India's seasonal rainfall fell below 90 per cent of LPA was in 2015, which, like this year, was affected by a strong El Nino in the equatorial Pacific Ocean. (File Photo)India recorded its driest monsoon in over a decade, with the seasonal rainfall ending up 13 per cent deficient compared to normal.The four-month June-September monsoon season finished on Wednesday, and India Meteorological Department said the country as a whole had received 87 per cent of normal rainfall. In quantitative terms, India received 759.4 mm of rainfall against the normal of 868.6 mm. Normal is the average seasonal rainfall over the 50-year period between 1971 and 2020, and is also called the long period average, or LPA.The last time India’s seasonal rainfall fell below 90 per cent of LPA was in 2015, which, like this year, was affected by a strong El Nino in the equatorial Pacific Ocean. El Nino, which refers to an abnormal warming of surface waters in the equatorial Pacific Ocean, is known to suppress rainfall over India. The opposite phase, La Nina, when the surface waters are cooler than normal, generally favours good rainfall. This year, the developing El Nino is expected to turn into the strongest ever.Because of this, the IMD in its very first long-range forecast in April, had forecast a below normal rainfall. It had predicted that rainfall was likely to be 90 per cent of the LPA, with an error margin of 4 per cent.In the last ten years, there have been only two occasions when the monsoon rainfall has dropped below the normal range (96 to 104 per cent of LPA). In 2017, India had received 95 per cent rainfall while the following year it was 91 per cent.In June this year, after the first monsoon forecast had been made, the World Meteorological Organization (WMO) had declared the onset of El Nino, and later predicted that it was likely to be develop into one of the strongest ever, with the peak expected to reach between October and December. The periodic warming and cooling of the equatorial Pacific Ocean, called El Nino Southern Oscillation, or ENSO, is a very large-scale ocean-atmosphere interaction that affects weather patterns globally. It is the single biggest contributor to year-on-year variations in global climate.The Indian monsoon remained largely unaffected by the El Nino till about August this year. However, three of the four months of the season still witnessed large deficits in their normal rainfall. June had a deficit of more than 35 per cent, while August and September had shortfalls of 16.3 per cent and 7.6 per cent respectively. Only July received normal rainfall, about 1 per cent more than the LPA.