image: Schematic illustration of the lagged influence of spring (SON) positive SAM on February Antarctic sea ice over the Weddell Sea and Ross Sea after 1998. These processes are primarily mediated by the Pacific-South American (PSA) pattern and the Amundsen Sea low (ASL). Solid (dashed) contours indicate anticyclonic (cyclonic) circulation systems, with “H” denoting high-pressure centers. The underlying shadings represent SST anomalies (blue for cold, red for warm anomalies).
Credit: ©Science China Press
A research team led by Dr. Juan Dou and Professor Xiangzhou Song from Hohai University, in collaboration with Professor Renhe Zhang from Fudan University, has revealed that the spring SAM underwent a marked structural shift around 1998. This shift strengthened the delayed influence of spring atmospheric circulation on Antarctic summer sea ice, especially in the Weddell and Ross Seas.
Antarctic sea ice has reached record-low levels in recent years, raising urgent questions about the drivers of its rapid and unusual changes. Its variability is shaped by complex interactions among the atmosphere, ocean, and tropical climate variability. Among these factors, the SAM plays a key role in Antarctic climate and sea-ice variability. Spring SAM is especially important because spring atmospheric anomalies can precondition the following summer sea-ice state through ocean heat storage, sea-ice transport, and ice–ocean feedbacks. Yet SAM is not a fixed or purely zonally symmetric pattern; its wave-like features may change over time and alter its influence on sea ice. This raises a key question: has spring SAM undergone a long-term structural shift, and has this shift altered its delayed impact on Antarctic summer sea ice?
Using running empirical orthogonal function analysis and k-means clustering, the team found that the spring SAM pattern changed significantly around 1998. Before this transition, spring SAM showed only a weak connection with Antarctic summer sea-ice extent. After 1998, however, SAM developed stronger wave-like features and became more closely linked to the Pacific–South American teleconnection (PSA) and the Amundsen Sea Low (ASL). This change greatly enhanced its lagged impact on summer sea ice.
The researchers further identified two regional pathways. In the Weddell Sea, positive SAM-related circulation anomalies alter air-sea heat exchange and promote persistent upper-ocean warming, favoring summer sea-ice melt. In the Ross Sea, a deeper ASL strengthens offshore ice transport in spring, exposing more open water near the coast. As summer sunlight increases, this open water absorbs more solar radiation, triggering an ice–albedo feedback that accelerates sea-ice loss.
The study also suggests that the post-1998 transition may be modulated by ENSO. Since the late 1990s, positive SAM events have occurred more frequently together with La Niña events, which may enhance tropical Pacific teleconnection forcing and amplify SAM’s influence on Antarctic sea ice. The record-low Antarctic sea-ice conditions in February 2022 and 2023 both followed strong positive SAM and persistent La Niña events.
These findings show that the relationship between Southern Hemisphere atmospheric circulation and Antarctic sea ice is nonstationary. They provide new observational evidence for understanding recent Antarctic sea-ice extremes and highlight the need to better represent SAM wave-like structures, the ASL and SAM–ENSO interactions in climate models.