Arctic sea ice has undergone persistent shrinkage since the 1980s, including an abrupt regime shift around 2007 from thick, deformed ice to thinner and more spatially uniform ice. However, the implications of this transition for Arctic Ocean stability, and the ability of state-of-the-art climate models to reproduce such abrupt changes, remain poorly understood. Here, we integrate observations and climate model simulations to investigate this Arctic climatic transition and its underlying mechanisms.We identify a pivotal oceanic shift in 2007, characterized by an abrupt 0.8 °C increase in sea surface temperature (SST) and an approximately twofold increase in SST variability within the marginal ice zone. The coincident shifts in SST mean state, variability, and marine heatwave (MHW) activity are consistent with a noise-induced transition of the coupled sea-ice–ocean system, in which MHWs act as internal perturbations on a progressively thinning and structurally weakened sea-ice background. As the ice cover became increasingly vulnerable, MHWs became more frequent and intense. These MHWs accelerated sea-ice melt and enhanced solar heat accumulation in the upper ocean, reinforcing a positive feedback among MHWs, sea-ice loss, and Arctic Ocean warming in short term. We further evaluate the capability of CMIP6 models to reproduce this coupled SST–MHW–sea-ice transition. Most models fail to capture the synchronous abrupt shifts in SST mean state, SST variability, and MHW intensity. Only a small subset of models, including EC-Earth3 and EC-Earth3-Veg, simulates comparable abrupt transitions during the historical period. This capability is associated with their relatively high transient climate response and stronger surface-albedo feedback, which favour the amplification of sea-ice–ocean energy imbalances. In addition, high-resolution simulations tend to exhibit an earlier onset of the Arctic abrupt transition than their low-resolution counterparts. Under continued greenhouse warming, further destabilization of Arctic sea ice is likely to promote stronger MHWs and accelerate upper-ocean warming. Improving the representation of internal ocean–atmosphere variability and feedback coupling involving marine extremes is therefore critical for constraining the timing and risks of future Arctic climatic transitions. Our results highlight the important role of marine heatwaves in triggering and amplifying Arctic regime shifts, with implications for Arctic climate prediction and future risk projections.