Incessant rainfall has wreaked havoc in India’s southeast, leaving at least 17 dead in Andhra Pradesh and thousands homeless and at the mercy of floods in Telangana.
The Indian Meteorological Department has also warned of heavy rains in parts of Gujarat, Himachal Pradesh, and Delhi.
Unpredictable, erratic rainfall has been becoming the norm for the Indian monsoon. Once a bringer of relief and hope to the subcontinent, these rains are now a herald of drought and floods.
Climate change is an important reason why. However, a closer look into climate models shows the intricate play of surface temperature, pressure gradients, air currents and even sea ice that is responsible for bringing rain to the Indian landmass.
In a study published in the journal Remote Sensing of Environment in June, researchers from India’s National Centre for Polar and Ocean Research (NCPOR), under the Ministry of Earth Sciences, and South Korea’s Korea Polar Research Institute have found that seasonal changes in the Arctic sea ice affect the Indian monsoon as well.
What is the Indian Summer Monsoon Rainfall?
The Indian summer monsoon rainfall (ISMR) over the Indian subcontinent, from July to September, and with most of the rains recorded in July and August, is one of the most prominent monsoon systems in the world.
In summer months, sunlight warms the Central Asian and Indian landmass more and faster than the surrounding ocean. This creates a low pressure band at the Tropic of Cancer called the intertropical convergence zone.
Trade winds blowing from the southeast are subsequently deflected towards the Indian landmass due to the Coriolis force and the low pressure after they cross the equator. As they blow over the Arabian Sea, the winds pick up moisture and deposit that as rain over India.
Over the landmass itself, this southwest monsoon splits into two. The Arabian Sea arm brings rain to the west coast while the other arm travels to the Bay of Bengal and brings rain to India’s eastern and northeastern parts.
The arms finally converge over Punjab and Himachal Pradesh as the Arabian Sea arm moves inward and the Bay of Bengal arm moves along the Himalaya.
The ISMR is much more complex than scientists first anticipated. In the last two decades, climate models have shown that the surface temperatures of the Indian, the Atlantic, and the Pacific Oceans affect the ISMR. The circum-global teleconnection (CGT), a large-scale atmospheric wave flowing at the mid-latitudes, seemed to significantly influence the monsoon as well.
How does Arctic sea ice influence the Indian monsoon?
In recent years, scientists have also suggested that declining levels of Arctic sea ice, due to climate change, could influence the monsoon’s temperament, too.
In the new study, researchers used observational data from 1980 to 2020 and climate models (specifically Coupled Model Intercomparison Project Phases 5 and 6) to check how the Arctic sea ice levels affect atmospheric circulations that in turn influence the ISMR.
The results revealed distinct and at times contrasting patterns. According to the paper, less sea ice in the central Arctic leads to lower rain in western and peninsular India but more rain in central and northern India.
On the other hand, lower sea ice levels in the upper latitudes, particularly in the Barents-Kara Sea region encompassing the Hudson Bay, the Gulf of St. Lawrence and the Sea of Okhotsk, delay the monsoon’s onset and render it more unpredictable.
Several atmospheric systems also influence this pattern. The scientists found that when sea ice levels in the Central Arctic increase, the heat transferred from the ocean to the atmosphere triggers a cyclonic circulation at slightly lower latitudes, like in the North Atlantic.
This bolsters the Rossby waves, fast-flowing streams of air high in the atmosphere created by the earth’s rotation and differences in temperature and weather systems that move west to east.
“To put it simply, imagine giant loops in a river of air high above us. These loops can push warm or cold air across the planet and steer storms around, changing weather patterns as they go,” Avinash Kumar, an NCPOR scientist and one of the study’s coauthors, told The Hindu.
The enhanced Rossby waves result in high pressure over northwest India and low pressure over the Mediterranean region. This in turn strengthens a narrow, concentrated band of wind, called the Asian jet stream, over the Caspian Sea, causing the subtropical easterly jet — a jet stream blowing over the Indian subcontinent during summer — to shift northward.
As a result, an anomalous high pressure region is created over Central Asia, disrupting atmospheric stability over the Indian landmass and bringing more rain over western and peninsular India.
On the other hand, low sea ice over the Barents-Kara Sea region triggers a series of air currents that produce an anomalous high pressure over southwest China.
This correlates with a positive Arctic Oscillation — high pressure over the northern Atlantic and Pacific oceans along with a weakening of the CGT, which connects weather events in different parts of the world.
As sea ice levels decrease in the region, heat rises from the Barents-Kara sea, creating an anticyclonic circulation (calm, clear skies) over northwest Europe. This disturbs the upper atmospheric region over subtropical Asia and India.
This instability, coupled with high surface temperature of the Arabian Sea and the moisture picked up from the surrounding water bodies promotes high rainfall over northeastern India while leaving central and northwest regions of the country without much
Does climate change also play a role?
Now that scientists know a little more about the forces that conspire to bring or withhold rain over India, what can we say about the role of climate change?
“Climate change, by accelerating the reduction of Arctic sea ice, exacerbates the variability and unpredictability of the ISMR,” Dr. Kumar said. “Lower Arctic sea ice can lead to more frequent and severe droughts in some regions while causing excessive rainfall and flooding in others.”
Apart from highlighting the physical pathways in which the Arctic sea ice affects the Indian monsoon, the current study shows the urgent need to expand research on climate dynamics and for scientists to prepare more accurate forecasts of the ever-changing monsoons.
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