A research team led by Dr. Juan Dou and Professor Xiangzhou Song (Hohai University), with collaboration from Professor Renhe Zhang (Fudan University), found that the Southern Annular Mode (SAM) in spring underwent a significant structural shift around 1998. This change strengthened its lagged (delayed) influence on Antarctic summer sea ice, particularly in the Weddell Sea and Ross Sea regions.

Southern Annular Mode (SAM), also called the Antarctic Oscillation (AAO), is the dominant mode of atmospheric variability in the Southern Hemisphere extratropics (south of about 20°S). It explains a large portion (~30%) of climate variability in this region and primarily describes changes in the strength and north-south position of the circumpolar westerly winds (the polar jet stream).

SAM is quantified as the normalized difference in zonal-mean sea-level pressure (SLP) between mid-latitudes (~40°S) and high latitudes (~65°S, around Antarctica):

• Positive SAM: Lower pressure over Antarctica + higher pressure in mid-latitudes → stronger, more poleward-shifted westerlies.
• Negative SAM: Higher pressure over Antarctica + lower pressure in mid-latitudes → weaker, more equatorward westerlies.

It is often measured using:

• Station-based indices (e.g., Marshall index from observations at ~40°S and 65°S).
• Reanalysis-based principal component (PC1) of SLP or geopotential height anomalies.

The SAM index is currently positive (as of 13 June 2026) but is forecast to return toward neutral by late June.

Researchers used techniques like running empirical orthogonal function (EOF) analysis and k-means clustering on reanalysis data to detect this structural regime shift around 1998.

This 1998 shift is a subtle but important change in how the SAM operates in spring, rather than just its overall strength (though the SAM has also shown long-term positive trends linked to ozone depletion and greenhouse gases).

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Interdecadal shift in the spring Southern Annular Mode intensifies its lagged influence on Antarctic summer sea ice

Paper Title: Interdecadal shift in the spring Southern Annular Mode intensifies its lagged influence on Antarctic summer sea ice

Authors: Juan Dou,  Xiangzhou Song (Hohai University), Renhe Zhang (Fudan University), et al.

Journal: National Science Review (Advance Article) Peer-Reviewed Publication

DOI: 10.1093/nsr/nwag314 (Published online ~May/June 2026)

Open Access? Likely (common for NSR advance articles)

Abstract

Antarctic sea ice has recently shifted to a new low-extent regime, yet the underlying mechanisms remain uncertain.

This study reveals an interdecadal shift in the spring Southern Annular Mode (SAM) pattern around 1998, after which its lagged influence on summer Antarctic sea ice intensified, particularly in the Weddell and Ross Seas.

After 1998, the SAM exhibits stronger wave-like structures, featuring an intensified Pacific–South American (PSA)-like pattern that drives regional sea-ice loss.

In the Weddell Sea, the positive SAM‑related anticyclonic circulation induces upper-ocean warming that promotes summer ice melt. However, in the Ross Sea, a deepened Amundsen Sea low enhances offshore ice export and positive ice‑albedo feedback, increasing summer ice loss.

This interdecadal SAM shift is likely modulated by tropical variability, as the increased co-occurrence of a positive SAM with La Niña strengthens PSA-mediated processes.

These findings highlight the nonstationary nature of tropical–polar linkages in recent Antarctic sea-ice lows.

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This study reveals an interdecadal shift in the spring Southern Annular Mode (SAM) pattern around 1998. After this shift, the spring SAM’s lagged influence on Antarctic summer sea ice became significantly stronger, especially in the Weddell Sea and Ross Sea sectors.

Main Findings

Pre-1998 vs. Post-1998:

Before ~1998, the connection between spring SAM and the following summer sea-ice extent was weak. After 1998, the SAM pattern developed stronger wave-like (zonal wave-3) features, making it more effective at influencing summer sea ice.

The strengthened SAM became more tightly coupled with the Pacific-South American (PSA) teleconnection pattern and the Amundsen Sea Low (ASL).

This change enhanced preconditioning effects from spring into summer through both dynamic (ice transport) and thermodynamic (heat exchange, albedo feedback) processes.

Regional Mechanisms (Post-1998)

Weddell Sea:

Positive spring SAM alters circulation, leading to persistent upper-ocean warming that reduces sea ice the following summer.

Ross Sea:

Deeper ASL strengthens offshore winds/ice transport in spring → more open water → increased solar absorption in summer → strong ice-albedo feedback amplifying melt.

Role of ENSO

The shift coincides with changes in SAM–ENSO interactions. Positive SAM events have more often aligned with La Niña conditions since the late 1990s, amplifying tropical-extratropical teleconnections. This helps explain extreme low sea-ice events (e.g., record lows in Feb 2022 and 2023) following strong positive spring SAM + La Niña.

Implications

The SAM–sea ice relationship is non-stationary (changes over decades), which has important consequences for predictability and climate model performance.

Improved representation of SAM’s structural changes, ASL dynamics, and SAM–ENSO coupling is needed in models.

Helps explain recent dramatic Antarctic sea-ice declines beyond simple linear trends.

The ASL has shown a notable deepening trend in recent decades (especially since the 1970s/1980s), linked to positive SAM trends, ozone changes, and greenhouse gases. This has been implicated in regional warming, sea ice changes, and accelerated ice loss in West Antarctica. However, the relationship is non-stationary and modulated by natural variability.

The ASL is a critical “control knob” for West Antarctic climate. Its dynamics illustrate the complex interplay between global modes (SAM, ENSO), anthropogenic forcing, and regional feedbacks. In the specific context of the spring SAM shift you asked about earlier, the strengthened coupling to the ASL post-1998 helps explain amplified lagged effects on summer sea ice loss through enhanced dynamic and thermodynamic processes.