The summer monsoon season runs from June through September and delivers roughly 80% of the rain India receives in a year. Over the last few decades, the amount of rain has increased while the concentration of rainfall has shifted west over the continent. Now, researchers at the Indian Institute of Tropical Meteorology have found that the intensity and location of the late-season rainfall is significantly influenced by Arctic sea ice melt that occurs earlier in the summer.
They published their work on Jan. 29 in Ocean-Land-Atmosphere Research.
"Rising global temperatures, with uneven distribution across the planet, are driving the melting of sea ice," said first author Hemantkumar S. Chaudhari, scientist with the Indian Institute of Tropical Meteorology, Ministry of Earth Sciences. "The melting of polar sea ice can have significant, far-reaching effects on the global climate system, primarily through changes in surface energy balance and energy transport from the equator to the poles."
In this study, the researchers aimed to identify the existence, strength and physical mechanism of connections between Arctic sea ice and Indian summer monsoon rainfall (ISMR). They were specifically interested in whether Arctic sea ice changes contributed to the westward shifts in monsoon rainfall patterns over India.
Using publicly available observational data, the researchers found a strong inverse correlation between Arctic sea ice extent and ISMR on interannual timescales. This means, in a given year or year-to-year, the less Arctic sea ice there is, the higher the ISMR. "June to July Arctic sea ice variability exerted the most profound influence on the late phase of monsoon rainfall, which is August to September," said corresponding author Subodh Kumar Saha, scientist with the Indian Institute of Tropical Meteorology. "Analyses of data from 1979 through 2022 linked declining Arctic sea ice with increased rainfall over western and northwestern India, indicating a westward shift in the monsoon season."
To better understand the mechanisms underpinning these correlations, the researchers conducted coupled climate model experiments. These simulations incorporate all components of the climate system — land, atmosphere, ocean, sea-ice — to investigate how changes inArctic sea ice might impact the ISMR.
"The model results replicated key aspects of the observed rainfall response, including the westward shift and enhanced rainfall over northwest India during August to September," said co-author Samir Pokhrel, also with the Indian Institute of Tropical Meteorology. "These findings indicate that the long-term decline in Arctic sea ice can drive systematic changes in the South Asian monsoon through upper-level dynamical pathways, meaning those in the atmosphere. As Arctic sea ice continues to decline, it may contribute to a stronger and westward-expanding South Asian summer monsoon."
Next, the team plans to evaluate and expand their work using data from a longer period of time and a suite of climate models.
"Our ultimate goal is to advance understanding of South Asian summer monsoon rainfall variability and predictability in a warming world, particularly in the context of rapidly declining Arctic sea ice," Saha said. Archana Rai, Indian Institute of Tropical Meteorology, also co-authored this study.
