A persistent area of unusually warm tropical ocean helped intensify snowfall over East Antarctica, contributing to a net temporary increase in ice sheet mass of about 695 billion tons, according to a new study published in Nature on August 19. The researchers found that sustained warming in the tropical warm pool during 2021-23 triggered a chain of atmospheric changes that ultimately affected the climate thousands of kilometers away in Antarctica. The warming generated a train of Rossby waves that traveled toward the continent, helped establish a north-south circulation pattern over East Antarctica, altered the movement of moisture, and increased regional snowfall. Together, these changes temporarily slowed the overall mass loss of the Antarctic ice sheet. A notable rise in ice in Antarctica The Antarctic ice sheet is a major source of uncertainty when scientists estimate how much global sea levels could rise in the future. Over the past two decades, Antarctica has lost ice at an average rate of about 140.5 billion tons per year. However, between 2021 and 2023, that pattern briefly changed. The ice sheet gained about 695 billion tons of mass, making it the largest Antarctic mass gain observed by the GRACE satellite missions. To understand what caused the unusual increase, researchers led by the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) gathered various types of evidence. They analyzed satellite gravity measurements, records of snow accumulation preserved in ice cores, and simulations of atmospheric circulation. Their goal was to determine where the extra moisture was coming from and how weather patterns were bringing it to East Antarctica. A climate signal from the tropics The team identified sustained warming in the tropical warm pool during 2021-23. This region is located where the tropical western Pacific meets the eastern Indian Ocean and contains some of the warmest ocean waters on the planet. That warming triggered what scientists call a Rossby wave train, a large-scale pattern of atmospheric waves capable of transmitting changes in climate and circulation over enormous distances. The disturbance traveled to high southern latitudes and helped reorganize atmospheric conditions around Antarctica. Feedbacks from the mean eddy flow strengthened and prolonged the resulting circulation pattern. This produced a north-south dipole, with unusually low pressure south of Australia and unusually high pressure along the east coast of Antarctica. The resulting pressure pattern changed the paths taken by moisture moving through the atmosphere. In particular, it enhanced the transport of water vapor from the mid-latitudes of the Indian Ocean to East Antarctica via atmospheric rivers. Atmospheric rivers caused heavy snowfall Atmospheric rivers are relatively narrow corridors in the atmosphere that can transport enormous amounts of water vapor over long distances. When they reach cold regions like Antarctica, that moisture can fall in the form of heavy snowfall. Water vapor tracking simulations showed that the dipole circulation directed moist air from the mid-latitude Indian Ocean toward East Antarctica and allowed more atmospheric rivers to reach the continent. This produced sustained heavy snowfall in the Queen Mary Land-Wilkes Land region, adding substantial mass to the ice sheet. Experiments with atmospheric circulation models provided further evidence that warming of the tropical warm pool directly drove both circulation changes and increased snowfall. The researchers also examined how much of the increase in snowfall could be attributed to anthropogenic forcing. They found that this contribution was equivalent to only 9% of the observed snowfall anomaly. That result suggests that the general increase in atmospheric humidity associated with global warming was not the primary explanation for this particular event. A remote Antarctic ‘regulator’ Additional observations and simulations indicate that comparable periods of sustained warming in the tropical warm pool occur about once every decade. The researchers therefore describe the tropical warm pool as a remote “regulator” that can influence snowfall and ice mass in East Antarctica over periods lasting several years. Changes in tropical ocean temperatures can alter atmospheric circulation in ways that ultimately affect the amount of snow that falls on the distant Antarctic continent. The findings reveal a long-distance climate connection in which conditions in the tropics can have important consequences for Antarctica. Antarctica continues to lose ice in the long term Despite the surprising increase of 695 billion tons, researchers emphasize that the event was temporary and does not reverse the long-term decline of the Antarctic ice sheet. The West Antarctic Ice Sheet continues to lose mass. Some outlet glaciers in East Antarctica also remain vulnerable as warm ocean water melts ice shelves from below and contributes to faster ice flow. The study shows how sustained warming in the tropical warm pool can temporarily increase the mass of the Antarctic ice sheet through changes in atmospheric circulation and snowfall. It also identifies the north-south dipole circulation over East Antarctica as an important link connecting tropical climate conditions to changes in Antarctic ice mass. “We found a previously underrecognized ‘warm tropical pool and East Antarctic Ice Sheet’ teleconnection pathway,” said Yunhe Wang of IOCAS, first author of the study. “Our research provides a theoretical basis for understanding the mass changes of the Antarctic ice sheet and conducting future research on the climate of East Antarctica.”
