What La Nina is and why India should brace itself for a harsh winter

The evolving winter condition during the fall of this year is wielding the threat of harsher than normal winter. Similar La Nina conditions last year, albeit a bit stronger, spoiled the snowy season for the hilly states and corrupted the winter rains for plains of north India. The rainfall deficiency during the winter season, over the states of Punjab and Uttar Pradesh, was in excess of 70 percent. The hilly regions of Jammu and Kashmir and Ladakh, Himachal Pradesh and Uttarakhand witnessed a ‘failed’ snowy season and experienced a shortfall of more than 50 percent. The National Capital observed a record number of seven cold wave days in January 2021, maximum since 2008. This figure was just one day in January 2019 and 2020. The atmospheric envelope around the planet earth is governed by a set of dynamic and thermodynamic forces. The complex nature of processes in the lower layers of the atmosphere yield extensive energy transfer leading to a range of weather phenomena. The atmosphere also has strong coupled interaction with the ocean currents, which lead to warm and cold phases in different sections of the oceans around the globe. The warm phase of the east and the central Pacific Ocean, periodically seen once in two-seven years, is termed as El Nino and the reverse process of cooling in the same area, is labelled as La Nina. This phenomenon impacts the flow of trade winds, prevalent on either side of the equator. During El Nino, warm water on the western boundary of the Pacific Ocean begins to drift eastward. This perturbation decreases the temperature gradient, which in turn slows the trade winds. Conversely, a strong easterly current during La Nina pushes the water towards the west and thereby cools the ocean surface. Scientists have coined the term ‘ENSO’, which is a shared coupling of El Nino and Southern Oscillation Index (SOI). Oceanic Nino Index (ONI) is the principal measure for monitoring, assessing and predicting ENSO. ONI is based on sea surface temperature departures from average in the Nino 3.4 region of central Pacific Ocean. The SOI represents the difference in average air pressure measured at Tahiti (French, Polynesia) and Darwin (Australia), the two big islands in the Pacific Ocean, separated by a distance of over 8,000 km. La Nina is the cooling phase of the El Nino Southern Oscillation cycle in the Equatorial Pacific Ocean, as against the warming phase of El Nino. Current ocean analysis reveals strong trade winds, pushing the surface water from east to west. Cooling of Central and East Pacific had commenced during boreal summers 2020 and is continuing till now. Earlier, there was a slight drop in the ocean surface cooling during Indian Summer Monsoon (ISM) 2021. La Nina is expected to carry on with an 85 percent chance in December 2021-February 2022. It also means that the cooling of the first-year La Nina event of 2020 continues to evolve and may last till early spring of 2022. The continuation of La Nina conditions spanning over two consecutive years is also called ‘Double Dip’ La Nina. While it is uncommon to witness two El Nino events follow each other, back-to-back La Nina episodes are not unusual. Statistically, of the 12 first-year La Nina events, six are followed by La Nina the next winter, four by neutral conditions and two by El Nino. ENSO has a major impact on tropical rainfall as it affects the ocean-atmosphere linkages and also alters the pressure patterns. The influence of ENSO is observed globally, in different seasons over divergent parts of almost all continents. What causes the different phases of ENSO (La Nina/El Nino/Neutral) remains inexplicable and no simple answers can be found. Suffice to say, it is a result of complex dynamics between pressure patterns and winds, predominantly in the tropical region and more so, in the equatorial belt. Commensurate with ENSO state, the resultant change in the trade winds profile churn the sea surface to modify the ocean currents and temperature distribution. ENSO has an impregnable relationship with seasonal progression over the Indian subcontinent. El Nino correlation is observed to be more robust than La Nina, both during the southwest monsoon and winter season. An interesting aspect this time is a continuation of first-year La Nina (2020) to evolve into a second La Nina cold season. Though it is not as strong as last year, it has an indelible presence in the ocean and also in the atmosphere. Also, La Nina is getting colder than expected in the Equatorial Pacific Ocean. La Nina can be viewed as a big influencer of weather conditions during the fall of the year and it could keep responding adversely till early spring. It transports cold air over the Indian subcontinent from Siberia and South China. Occasionally, this cold air may interact with a north-south axis of the deep trough to extend much further south into parts of Maharashtra, Karnataka, Telangana and Andhra Pradesh. La Nina episodes in the winter months feature a wave-like jet stream flow across Iran, Afghanistan and Hindu Kush mountains. The westerly jet stream, a channel of very strong and cold winds circumventing the globe, remains a dominant feature impacting the degree of cold over the Indian region. La Nina features a stronger and extended jet stream across the Atlantic Ocean, penetrating deep over Western Europe and the Mediterranean Sea. The southward shift of this atmospheric tunnel of high-speed winds, due to blocking high-pressure cells, causes an outbreak of severe cold weather over north India, both hills and plains. The freezing winds may endure for a week and sometimes longer, spreading glacial cold over most parts. Though rarely, yet possibly, another weather feature in the form of ‘polar vortex’ may also be factored for the winter season forecast. This cold low-pressure area, also called an Arctic cyclone, strengthens in winter. Generally, the polar vortex remains confined to the extreme northern latitudes but at times, undulations in the wind pattern develop a kink or a crack in the westerly flow and intense cold gets drifted to the mid-latitudes through this gap. La Nina conditions increase the chances of this inclement cold pool getting pushed to the lower latitudes. For the northern parts of India, it is quite likely that the second back-to-back La Nina may result in extreme cold from December 2021 to February 2022. La Nina is expected to solidify its influence and also peak during this period. As ENSO is getting colder than earlier estimates, the confidence in this forecast has grown substantially. At the same time, it is essential to remember that the winter forecast comprises many other factors and connections, from other parts of the globe. La Nina, of course, plays a dominant role in winter outlook, but it is just a part of the sequential activity to the comprehensive forecast. The writer is president, Skymet. Views expressed are personal.​