Abstract
Regional climate variability manifests through distinct atmospheric regimes influencing weather, climate, and predictability. Yet, their response to global warming remains unresolved. Using a hundred realisations from the Community Earth System Model Large Ensemble, we examine shifts in wintertime North Atlantic atmospheric regimes and the North Atlantic Oscillation before and after 1995, highlighting the detectable influence of anthropogenic warming on atmospheric circulation. The large-ensemble framework isolates internal variability by removing the ensemble mean. Under anthropogenic warming, the number of regime states associated with the forced response remains constant, although their spatial circulation patterns undergo substantial reorganisation. In contrast, internal variability alone exhibits a reduction in the number of regime states. Future projections indicate a shift toward more frequent positive phases of the North Atlantic Oscillation, accompanied by low-amplitude negative phases late in the century, alongside a marked decline in its variability and altered mid-tropospheric westerlies.
