Unraveling the mystery of slowing global warming in Greenland: Hokkaido University, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

A research group made up of Shinji Matsumura, a post-doctoral Researcher at the Graduate School of Environmental Science, Hokkaido University, and Kazuyoshi Suzuki, a senior researcher at the JAMSTEC Institute of Arctic Climate and Environment Research, clarified the mechanism underlying the recent slowing of global warming in Greenland during summer. Analysis of atmospheric and oceanic data revealed that the El Niño Modoki phenomenon, which has occurred frequently in the subtropical Pacific since 2000, is a factor in the deceleration of global warming. It is expected to contribute to future global warming and its impact prediction. The results were published in the December 16, 2021 issue of the international scientific journal Communications Earth & Environment.

The Greenland ice sheet has been melting due to global warming over the past few decades, and there is a concern that the increase in freshwater supply to the ocean will exacerbate sea level rise. However, it has been reported that since the ice sheet melting reached a maximum in 2012, melting and the resulting freshwater supply have decreased. In fact, the temperature rise in Greenland reached a record high in 2012, and since then, a low temperature trend has continued in the summers. This trend has not only been observed in the coastal areas of Greenland but also at observation points on top of the ice sheet at an altitude of 3200 m. However, global warming continues in the Arctic Circle in the summer, except around Greenland; only the areas around Greenland present this peculiar situation.

The research group aimed to clarify the cause of the slowdown of global warming around Greenland in summer, despite the increase in global warming worldwide. Analysis of the atmospheric observation data (JRA55) released by the Japan Meteorological Agency revealed that the atmospheric circulation in the summer Arctic region has changed significantly since the 2000s. From 2000 to 2012, high-pressure circulation was strengthened and high temperatures were observed in Greenland and northern Canada, recording the highest temperature rise ever. However, after 2012, the intensification of cyclone circulation resulted in low temperatures in these areas. Prior to 2000, this remote effect of the atmosphere (teleconnection) was not observed in response to these changes.

The research group analyzed the sea surface temperature observation data (HadISST) based on the satellite observation provided by the UK Met Office at the Hadley Center for Climate Science and Services. Although the frequency of conventional El Niño/La Niña phenomena in the equatorial Pacific has decreased since 2000, the frequency and scale of El Niño Modoki/La Niña Modoki occurring in the subtropical Pacific Ocean remains high. Hence, with the occurrence of El Niño Modoki/La Niña Modoki, changes in sea surface temperature and the precipitation zone shifted northward from the eastern wind region of the equator to the subtropical waters, developed in the westerly wind region, and resulted in increased precipitation. Thus, atmospheric waves were created, and teleconnections were observed even in the summer.

After 2012, the frequent occurrence of El Niño Modoki strengthened the cyclone circulation over Greenland and lowered the on-ground temperature. Meanwhile, from 2000 to 2012, it was assumed that Greenland was warming due to the frequent occurrence of La Niña Modoki. These frequent occurrences of El Niño Modoki/La Niña Modoki were not observed before 2000, and even if they did occur, only small-scale occurrences were observed. Since the cyclone circulation that occurred on Greenland extends over the Arctic Sea at higher latitudes, it may have affected the rate of sea ice decline in recent years. The research group also confirmed that these teleconnections can be reproduced using a simple atmospheric model that reflects the northward movement of the precipitation zone.

According to the post-doctoral researcher Matsumura, "Since La Niña Modoki occurred last summer, there are concerns about further warming of Greenland due to the synergistic effects of global warming of anthropogenic origin. The frequency and scale of recent El Niño Modoki/La Niña Modoki events is unprecedented, and future climate prediction models present poor reproducibility based on past Greenland warming and sea ice reduction; therefore, we will continue to investigate this situation using future prediction models."