A persistent patch of unusually warm tropical ocean helped intensify snowfall over East Antarctica, contributing to a temporary net ice sheet mass gain of about 695 billion tons, according to a study published in Nature on 19 August.
Over the past two decades Antarctica has lost ice at an average rate of roughly 140.5 billion tons a year, and the ice sheet remains a major source of uncertainty in estimates of future sea level rise. Between 2021 and 2023 that pattern briefly reversed, producing the largest Antarctic mass gain observed by the GRACE satellite missions.
To work out what caused it, researchers led by the Institute of Oceanology of the Chinese Academy of Sciences combined gravity satellite measurements, snow accumulation records preserved in ice cores and simulations of atmospheric circulation, with the aim of establishing where the extra moisture came from and how it reached the continent.
The team identified sustained warming during 2021-23 in the tropical warm pool, the region where the tropical western Pacific meets the eastern Indian Ocean and which contains some of the warmest ocean water on the planet.
That warming triggered a Rossby wave train, a large-scale pattern of atmospheric waves capable of transmitting changes in circulation across enormous distances. The disturbance travelled towards the high southern latitudes and helped reorganise conditions around Antarctica, with eddy mean flow feedbacks strengthening and prolonging the resulting pattern.
The result was a north-south dipole: unusually low pressure south of Australia and unusually high pressure along the East Antarctic coast. That arrangement changed the routes moisture takes through the atmosphere, strengthening the transport of water vapour from the midlatitude Indian Ocean towards East Antarctica through atmospheric rivers.
Atmospheric rivers are relatively narrow corridors in the atmosphere that carry very large quantities of water vapour over long distances. When they reach cold regions, the moisture falls as heavy snow. Water vapour tracking simulations showed the dipole circulation directing moist air from the midlatitude Indian Ocean towards the continent and allowing more atmospheric rivers to arrive, producing sustained heavy snowfall across the Queen Mary Land to Wilkes Land region.
The researchers also examined how much of the increase could be attributed to human-driven warming, and found that contribution equivalent to only 9 per cent of the observed snowfall anomaly, suggesting the general rise in atmospheric moisture was not the main explanation for this particular event.
Comparable periods of sustained warming in the tropical warm pool occur roughly once a decade, which leads the authors to describe the region as a remote "regulator" of snowfall and ice mass in East Antarctica over periods lasting several years.
"We found a previously underrecognized 'tropical warm pool-East Antarctic Ice Sheet' teleconnection pathway," said Yunhe Wang of IOCAS, the study's first author. "Our research provides a theoretical basis for understanding Antarctic ice-sheet mass changes and conducting future research on the East Antarctic climate."
The gain was temporary and does not reverse the long-term decline. The West Antarctic Ice Sheet continued to lose mass throughout, and some East Antarctic outlet glaciers remain vulnerable to warm ocean water melting ice shelves from below.
Stripped of the physics, the sequence reads as a straightforward logistics operation. A consignment of 695 billion tons was dispatched from the tropical warm pool, routed via the midlatitude Indian Ocean, and carried south along a corridor the researchers themselves call a river. It arrived, in bulk, in Queen Mary Land.
The service is not frequent. On the evidence presented, the tropical warm pool makes a delivery of this kind roughly once every ten years, which is a schedule most freight operators would struggle to sell.
Nor was the order placed by the recipient. The paperwork, such as it is, attributes 9 per cent of the load to human activity, leaving the remaining 91 per cent as a matter between the Indian Ocean and the weather.
The Department of Unnecessary Research wishes to be clear about the account overall. Antarctica has not started gaining ice. It has taken in a single large delivery it did not order, from a supplier it cannot contact, against a standing withdrawal of about 140.5 billion tons a year.