Intensified atmosphere–sea ice coupling reshapes Arctic sea ice decline and downstream climate extremes

Despite continued global warming and progressive thinning of Arctic sea ice, the rate of September sea ice decline has slowed markedly since the 2012 record low. This apparent stabilization challenges expectations of monotonic ice loss and remains poorly understood. Here we show that the slowdown reflects a regime shift in atmosphere–sea ice coupling, rather than a weakening of atmospheric influence. Around 2004, the relationship between the June Greenland High (GH) and September Arctic sea ice extent intensified dramatically, with the correlation strengthening from r = −0.17 (1979–2003) to r = −0.80 (2004–2025). This shift coincides with the transition to a thinner, younger, and more mobile ice cover that is more responsive to atmospheric forcing, alongside enhanced intra-seasonal persistence of summer Arctic circulation. Within this strengthened coupling regime, post-2012 GH weakening accounts for approximately 60–70% of the observed slowdown in September sea ice decline. Crucially, this apparent stabilization entails pronounced downstream impacts, including intensified marine heatwaves in the Norwegian–Barents Seas, accelerated rainfall transition in the Atlantic Arctic, and amplified summer heat extremes over northern Europe. Our findings demonstrate that atmosphere–sea ice coupling is non-stationary and has fundamentally strengthened in the New Arctic, amplifying the influence of circulation variability on both sea ice trajectories and midlatitude climate extremes, with important implications for prediction and climate projections.