Cross‐Basin Synergistic Effects Stall Summer Sea Ice Decline in the Chukchi Sea

Abstract Although global mean surface temperatures have risen rapidly, summer sea ice loss in the Chukchi Sea has exhibited an anomalous deceleration since the early 2000s. Using observations and numerical experiments, we show that this trend deceleration is closely linked to the synchronized sea surface temperature (SST) between the Kuroshio Extension (KE) and Gulf Stream (GS), known as boundary current synchronization (BCS). The BCS index is significantly correlated with Chukchi sea ice concentration (SIC) ( r = 0.58, p < 0.01), and the trend slowdown disappears after removing the BCS signal. Critically, concurrent basin warming nearly triples the SIC response relative to single‐basin warming, revealing a pronounced synergistic effect. Synchronous SST warming excites a zonally symmetric Arctic Oscillation (AO)‐like circulation pattern, producing a low‐pressure anomaly over the Chukchi Sea that drives anomalous northerly winds and sea ice convergence. Concurrently, the associated increase in cloud cover reduces downward shortwave radiation, suppressing melt and reinforcing near‐surface cooling through the ice–albedo feedback. Atmospheric general circulation model (AGCM) experiments confirm that the synchronous SST forcing drives a poleward shift and intensification of the westerly jet, which in turn excites the summer AO‐like circulation response. These mechanisms are further corroborated by CMIP6 piControl simulations, which successfully reproduce both the BCS–SIC linkage and the synergistic effects, confirming that the relationship can arise from internal variability. Our findings highlight the critical role of cross‐basin SST synchronization in modulating Arctic sea ice variability by generating synergistic atmospheric responses through hemispheric‐scale teleconnections.