What role does Arctic sea ice play in the Indian monsoon?

Incessant rains have wreaked havoc in India’s south-east, affecting at least 1,000 people. 17 people died in Andhra Pradesh And Thousands homeless And Telangana is facing the brunt of floods.

The Indian Meteorological Department has also heavy rain warning In parts of Gujarat, Himachal Pradesh and Delhi.

Unpredictable, erratic rainfall is becoming the norm for the Indian monsoon. What once brought relief and hope to the subcontinent has now become a harbinger of drought and floods.

Climate change is a key factor. However, a closer look at climate models shows a complex play of surface temperatures, pressure gradients, air currents and even sea ice that is responsible for bringing rain to the Indian landmass.

one of Study Published in 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 found that seasonal changes in Arctic sea ice also influence the Indian monsoon.

What is Indian summer monsoon rainfall?

The Indian summer monsoon rainfall (ISMR), which occurs from July to September in the Indian subcontinent and most of the rainfall is recorded in July and August, is one of the most prominent monsoon systems in the world.

In the summer months, sunlight heats the Central Asian and Indian landmass more and more rapidly than the surrounding ocean. This creates a low-pressure belt along the Tropic of Cancer called the Intertropical Convergence Zone.

The trade winds blowing from the south-east turn towards the Indian landmass after crossing the equator due to Coriolis force and low pressure. The winds blowing over the Arabian Sea collect moisture and deposit it in the form of rain in India.

This southwest monsoon splits into two parts in the land area itself. One branch from the Arabian Sea brings rain to the western coast, while the other branch goes to the Bay of Bengal and brings rain to the eastern and northeastern parts of India.

These arms finally converge over Punjab and Himachal Pradesh, while the Arabian Sea arm moves inland and the Bay of Bengal arm moves along the Himalayas.

The ISMR is more complex than scientists previously estimated. Over the past two decades, climate models have shown that surface temperatures in the Indian, Atlantic, and Pacific oceans influence the ISMR. The Circum-Global Teleconnection (CGT), a large-scale atmospheric wave flowing across the mid-latitudes, also significantly influences the monsoon.

How does Arctic sea ice affect the Indian monsoon?

In recent years, scientists have also suggested that declining Arctic sea ice due to climate change may also affect the nature of the monsoon.

In the new study, researchers used observational data from 1980 to 2020 and climate models (specifically the Coupled Model Intercomparison Project Phases 5 and 6) to investigate how Arctic sea ice extent affects atmospheric circulation, which in turn affects ISMR.

The results revealed different and sometimes opposite patterns. According to the paper, reduced sea ice in the central Arctic leads to less rainfall over western and peninsular India, but more rainfall over central and northern India.

On the other hand, in upper latitudes, particularly in the Barents-Kara Sea region encompassing Hudson Bay, the Gulf of St. Lawrence and the Sea of ​​Okhotsk, decreasing sea ice extent delays the arrival of the monsoon and makes it more unpredictable.

Several atmospheric systems also influence this pattern. The scientists found that when sea ice levels increase in the central Arctic, the heat transferred from the ocean to the atmosphere activates cyclonic circulation at slightly lower latitudes, such as the North Atlantic.

This strengthens Rossby waves, which are fast-flowing air currents high in the atmosphere that move from west to east due to the Earth’s rotation and differences in temperature and weather systems.

“Simply put, imagine giant circles in a river of air above us. These circles can push warm or cool air across the planet and move storms around, changing weather patterns,” said Avinash Kumar, a scientist at NCPOR and one of the study’s co-authors. the hindu,

The growing Rossby waves lead to high pressure over northwest India and low pressure over the Mediterranean Sea region. This in turn strengthens a narrow, concentrated band of air called the Asian jet stream over the Caspian Sea, causing the subtropical easterly jet – a jet stream that flows over the Indian subcontinent during summer – to shift northward.

As a result, an abnormal high pressure area is formed over Central Asia, which disrupts the atmospheric stability over the Indian landmass and causes excessive rainfall over western and peninsular India.

On the other hand, low sea ice over the Barents-Kara Sea region generates a series of air currents, which produce abnormal high pressure over southwest China.

This is related to the positive Arctic Oscillation – high pressure over the North Atlantic and Pacific Oceans – as well as the weakening of the CGT, which links weather events in different parts of the world.

As sea ice extent in the region decreases, heat from the Barents-Kara Sea increases, creating an anticyclonic circulation (calm, clear skies) over northwestern Europe. This disturbs the upper atmospheric region over subtropical Asia and India.

This instability, combined with high surface temperatures over the Arabian Sea and moisture from surrounding water bodies, leads to heavy rainfall over Northeast India, while the central and northwestern regions of the country do not receive much rainfall.

Does climate change play a role in this?

Now that scientists know a little more about the forces that conspire to bring or prevent rainfall in India, what can we say about the role of climate change?

“Climate change increases the variability and unpredictability of ISMR by accelerating the loss of Arctic sea ice,” said Dr. Kumar. “The loss of Arctic sea ice may lead to more frequent and severe droughts in some regions, while increasing extreme precipitation and flooding in other regions.”

Apart from shedding light on the physical pathways through which Arctic sea ice influences the Indian monsoon, the current study highlights the urgent need to expand research on climate dynamics and for scientists to prepare more accurate forecasts about the constantly changing monsoon.