The secondary monsoon: On rainfall behaviour

There is little understanding of the behaviour of the Indian Ocean and monsoon impact

October 18, 2019 12:02 am | Updated 01:27 am IST

India’s most torrential monsoon in a quarter century officially ended on Wednesday. This has been the most delayed withdrawal of the monsoon since 1961 but both the quantity and the timing have had no effect on the onset of the northeast monsoon, which officially commenced on Thursday. The NE monsoon rains contribute about 20% of India’s annual rainfall and span October-December. While the southwest monsoon has been obsessively studied for centuries and there are well established correlations — for instance, temperatures in the Central Pacific, or land surface air temperature in north-western Europe — between them as well as the quantity and distribution of monsoon rainfall, no such determining parameters exist for the NE monsoon. At best, meteorologists have now progressed to giving a broad outlook of how the rains could pan out over the next few months. This year, however, is particularly significant. Monsoon rains in south India have been 15% above normal. In Andhra Pradesh, Telangana, Karnataka, Kerala and Tamil Nadu, where the Central Water Commission monitors over 30 reservoirs, their water levels were 44.2 billion cubic metres, or 84% of their total live capacity, and much higher than the 10-year average of 66%. This means that excessive rains in the coming months could contribute to the saga of urban inundation.

Among the signatures of global warming is intense rainfall being concentrated over short spells and pockets and long periods of drought. The El Niño phenomenon, which has been linked to the abnormal warming of the equatorial waters off the central and eastern Pacific, has been connected with the failure of the southwest monsoon. However, researchers over the years have noted that this had an opposite effect on the NE monsoon leading to more voluminous showers in the winter and particularly over South India. This summer, the IMD, along with other meteorological agencies around the world, bet that monsoon rains would be on the lower side due to the possible emergence of an El Niño. Even after the threat of El Niño had waned, it didn’t indicate that rains would be torrential in August and September. Conditions in the Indian Ocean turned favourable and led to the excessive monsoon activity this year. This shows that there is a paucity in understanding the behaviour of the Indian Ocean and its influence on the monsoons. India is moving to a system where dynamical models that run on powerful computers will become the mainstay of monsoon forecasting. However these too are heavily reliant on the behaviour of the Pacific Ocean and El Niño-related swings. India needs to step up research to improve the performance of these models. With climate change set to inescapably alter the ocean temperatures around the Indian neighbourhood, giving more importance to understanding the vagaries of the NE monsoon ought to be among India’s key prongs to adapting to climate change.

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