A new publication in the Nature Partner Journal "Climate and Atmospheric Science" is co-authored by NERSC researcher Roshin P. Raj in the Ocean and Sea Ice Remote Sensing Group, as well as professor Ola M. Johannessen from the Nansen Scientific Society. The remaining co-authors are scientists from NCPOR (Goa, India). They investigated the connection between extreme rainfall events during the summer monsoon season in India and the Arctic sea ice melting in the Kara Sea north of Russia, and present interesting findings in their article.

Indian Summer Monsoon Rainfall extreme events: Impacts and causes

You have probably heard about the summer monsoon season in India before. The rainfall during this season is how more than one billion people get their drinking water! But the monsoon does not only bring drinking water, it is also key for the agriculture, the lifeline of the nation’s economy. However, the monsoon rainfall intensity exhibits large spatial and temporal variability. Scientists have already observed the increasing frequency of extreme rainfall events over the Indian landmass during summer monsoon in the last few decades. These extreme events, with widespread fatal consequences in both urban and rural areas, are influenced by several land-ocean-atmospheric processes. With a warmer climate, more extreme events are predicted to occur, making it really important to better understand why extreme monsoon rainfall events occur. Local factors do have an impact, but factors much, much further away also influence the rainfall amount over India.

Would you have guessed that Arctic sea ice melting thousands of kilometers away is likely connected to these extreme events in India?

Why would Arctic sea ice be connected to what happens in India during summer?

All over the globe, climate anomalies occur. They can be caused by local factors, but they can also be related to and caused by anomalies far away. When this is the case, we speak of teleconnections. Extreme events of monsoon rainfall in India is one of these anomalies that cannot only be explained by local factors.

Many studies investigated teleconnections with the northern mid-latitudes, but few explored the Arctic as region with teleconnections to the Indian monsoon. This is why Roshin P. Raj, Ola M. Johannessen, and their co-authors set out to investigate the role that Arctic sea ice might play in extreme monsoon rainfall events in India. They chose the Kara Sea north of Russia as study area due to the fact that it has been exhibiting massive sea ice loss every summer for a long time. Because of this, a lot of heat is leaving the ocean and entering the atmosphere, which can influence regions close by and afar.

Scientists often argue that changes in the Arctic likely impact the atmosphere in the mid latitudes, on short to long timescales. This recent study provides evidence for that a decline in the Arctic sea ice cover also likely has an impact on the atmosphere even further south, in the tropics!

What did the authors do?

A known connection between sea ice loss in the Arctic and strong rainfall in India during the monsoon season is the following: Heat is released during rainfall events, that then travels northward to the Arctic, where it contributes to reducing the sea ice, particularly in the Canadian Arctic. This recent study examined if this connection might be the other way round, meaning that the melting in the Arctic might have an influence on the extreme rainfall in India during the late summer monsoon.

Roshin P. Raj and his colleagues, under the lead of Sourav Chatterjee, set out to analyze observed data from the last 100 years. They compared the sea ice extent in the Kara Sea to amount of rainfall during Indian Summer Monsoon Rainfall events, as well as the frequency of extreme events throughout the entire 20^th century.

Coinciding extreme rainfall and rapid sea ice loss

The researchers show that increased rainfall and declining sea ice coincide! When the sea ice in the Kara Sea melted rapidly for a period during summer, the monsoon in India generally came with exceptionally strong rainfall in the same summer for the observational period of over 100 years. The top left panel in the figure shows the Kara Sea sea ice extent in blue (note that the scale is flipped), and the frequency of extreme monsoon events in red. This pattern is detectable even though the seasonal average monsoon rainfall has not changed much in recent decades.

 Figure 1 from Chatterjee et al. (2021): Figure 1 from Chatterjee et al. 2021: The observed relation between sea ice extent and concentration in the Kara Sea in the Arctic, and the summer monsoon in India! Over the last 100 years, the sea ice extent (blue, in panel a) has varied, and each rapid decline coincides with an increase in the frequency of extreme events in India (red, in panel a). The amount of extreme events happening also varied on a decadal scale, see panel b. The sea ice concentration anomaly over the observational period is strongest in the Kara Sea, compared to the rest of the Arctic, see panel c. Panel d shows that on average the most heat is being released from ocean to atmosphere in the same region. Detailed caption and information can be found in study.

Seasonal atmospheric changes due to melting sea ice

The northwestern part of Europe is often dominated by a high pressure system pattern in the summer months, thanks to the jet stream. Melting sea ice in the Kara Sea and the resulting anomaly in the atmosphere intensifies this high pressure system, according to this recent study.

But even more importantly, what Chatterjee et al. found, indicates that this anomaly in the atmosphere caused by sea ice melting in the summer affects the atmosphere over India, too. They suggest that it contributes to extreme rainfall events over central and western India in the late summer months. This is a nice example of a teleconnection: what happens in the Arctic has impacts on the weather in India!

Roshin P. Raj about the importance of the study's findings:

“It has been known that the extreme events during Indian Summer Monsoon are driven by changes in the westerly winds and associated moisture convergence. Our results show that upper atmospheric circulation changes induced by the sea ice loss in Arctic can facilitate both the enhanced moisture convergence by the westerlies and intensification of convection over central and west India - resulting in extreme rainfall events.”

In short, this study shows that the variability of extreme events during the Indian Summer Monsoon can at least partly result from combined effects of upper-level atmospheric circulation changes due to remote sea ice changes in the Arctic and local sea surface temperature changes in the Arabian Sea! The authors provide an explanation for how this teleconnection possibly works.

Reference:

Chatterjee, S, Ravichandran, M, Murukesh, N, Raj, RP & OM Johannessen. A possible relation between Arctic sea ice and late season Indian Summer Monsoon Rainfall extremes. npj Climate and Atmospheric Science 2021. 4, https://doi.org/10.1038/s41612-021-00193-8