The summer monsoon season lasts from June to September and provides about 80% of India’s annual rainfall. Over the past few decades, rainfall has increased and the concentration of precipitation has moved to the west of the continent. Now, researchers at the Indian Institute of Tropical Meteorology have found that the intensity and location of late-season rainfall is strongly influenced by the melting of Arctic sea ice that occurs at the beginning of summer.

They published their work on January 29th. Ocean, land, and atmosphere research.

“Rising global temperatures and uneven distribution across the globe are driving the melting of sea ice,” said lead author Hemantkumar S. Chaudhary, a scientist at the Indian Institute of Tropical Meteorology in the Ministry of Earth Sciences. “The melting of polar sea ice can have significant and far-reaching effects on the Earth’s climate system, primarily through changes in the surface energy balance and energy transport from the equator to the poles.”

In this study, researchers aimed to determine the existence, strength, and physical mechanisms of the link between Arctic sea ice and Indian Summer Monsoon Rainfall (ISMR). They were particularly interested in whether changes in Arctic sea ice were contributing to westward shifts in monsoon rainfall patterns in India.

Using publicly available observational data, the researchers found a strong inverse correlation between Arctic sea ice extent and ISMR on an interannual timescale. This means that in a given year or years, the less sea ice there is in the Arctic, the higher the ISMR. “Variations in Arctic sea ice in June-July had the most severe impact during the late monsoon rains in August-September,” said corresponding author Subodh Kumar Saha, a scientist at the Indian Institute of Tropical Meteorology. “Analysis of data from 1979 to 2022 links decreased Arctic sea ice with increased rainfall in western and northwestern India, indicating a westward shift in the monsoon season.”

To better understand the mechanisms underlying these correlations, the researchers conducted coupled experiments with climate models. These simulations incorporate all components of the climate system (land, atmosphere, ocean, and sea ice) to explore how changes in Arctic sea ice affect ISMR.

“The model results reproduced important aspects of the observed rainfall response, including a westward shift and increased rainfall over northwest India from August to September,” said co-author Sameer Pokhrel, also from the Indian Institute of Tropical Meteorology. “These findings indicate that a long-term decline in Arctic sea ice could cause systematic changes in the South Asian monsoon through an upper-level dynamical, or atmospheric, pathway. As Arctic sea ice continues to decline, it could cause the South Asian summer monsoon to become stronger and more westwardly expanding.”

Next, the team plans to evaluate and extend the study using longer-term data and a range of climate models.

“Our ultimate goal is to advance our understanding of the variability and predictability of summer monsoon precipitation in South Asia in a warming world, especially in the context of rapidly declining Arctic sea ice,” Saha said. Archana Rai of the Indian Institute of Tropical Meteorology is also a co-author of the study.