For nearly a month, the India Meteorological Department’s daily bulletin has looked almost identical, with the same cold wave and cold day forecasts for the North Indian states highlighted in bold. While the regional temperatures plunging during this of the year is nothing out of the ordinary, mercury levels in many regions dipped several notches below what is considered normal on multiple occasions.
The obvious dearth of western disturbances over Northwest India, the ongoing El Niño conditions and an active jet stream have all shouldered the blame for the dry and persistent cold. However, an IISER study has identified new underdogs as the key culprits: a powerful Siberian High and atmospheric blocking.
The Siberian High’s Chilling Presence
Imagine a giant dome of cold air hovering over Siberia. That's the Siberian High, a vast high-pressure system that packs a punch. When it intensifies, as it has this winter, it pushes frigid air southward, sending shivers across North India.
The Siberian High accumulates in the northeastern part of Eurasia between September and April. This year, the system has been unusually strong and persistent, contributing significantly to the bone-chilling temperatures.
Atmospheric Blocking of the Warmth
The Siberian High isn't the only player in this chilly game, however. Atmospheric blocking, a weather phenomenon where high-latitude systems act as roadblocks, is further intensifying the impact of the Siberian High.
A high-pressure system over Central Asia has effectively blocked the movement of warmer westerlies, preventing them from moderating the cold temperatures in North India. Think of it as an invisible wall, stopping the warm air from reaching the region.
This year, the winter atmospheric blocking intensified cold waves and prolonged their persistence from the end of December to January, the study revealed.
Changing Weather Patterns
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The study, spanning from 1982 to 2020, identified 509 cold wave days, which included 45 days of severe cold wave events. For reference, a cold wave event was recognised when the normalised minimum temperature anomaly registered a standard deviation of less than -1 for four or more consecutive days, while a severe cold wave event was identified when this anomaly dropped below a standard deviation of -2 for four or more consecutive days.
Over the study period, researchers observed a decline in the number, duration, and intensity of cold wave events. This reduction has been linked to rising winter minimum temperatures owing to global warming and a decreasing number of western disturbances (WDs).
So what caused the rise in cold waves this year? While the Siberian High and atmospheric blocking are the main drivers of the cold wave, other factors are also playing a role.
Western disturbances—low-pressure systems that typically bring rain and snowfall to North India—have been weak recently. This has allowed the cold wave to persist without interruption. It's important to remember that global warming, while causing a long-term decrease in the number and intensity of cold wave events, can still allow for extreme weather events like this to occur.
More than just an inconvenience, cold waves can significantly impact the lives of people in North India. Crop damage has led to concerns regarding food security while transportation has also been rendered dangerous due to the icy roads. Further, vulnerable groups have been put at more risk from exposure to the cold weather.
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