Contrasting Arctic Amplification Response in the Community Earth System Model Large Ensembles and Implications for the North Atlantic Region

Abstract

The response of the polar jet to climate warming and rapid Arctic change is a leading uncertainty in climate projections and critical to the future of mid-latitude surface weather. Previous studies suggest that CMIP5-6 model projections fall into two groups of either Arctic- or tropically-driven climate change, especially in the North Atlantic. Here, we present distinct warming patterns emerging by the late 21st century between the first two generations of the Community Earth System Model Large Ensemble (CESM-LE) and use daily diagnostics to assess associated changes in mid-latitude circulation. We show that the subsequent versions of CESM represent categorically different storylines of North Atlantic climate change. The first version of CESM-LE (CESM1-LE, hereafter LENS1) exhibits severe Arctic amplification (AA) along with minor reductions in jet waviness. In contrast, CESM2-LE (hereafter LENS2) presents subdued AA, a more pronounced North Atlantic warming hole, and a late-century climate dominated by upper-tropospheric tropical warming. Uniquely, in LENS2 during winter, the North Atlantic sector projects less warming in the Arctic than in the mid-latitude mid-troposphere. The projected North Atlantic jet is reinforced and poleward-shifted with reduced sinuosity, blocking, and synoptic variability. The surface weather response includes greater precipitation over northern Europe, more intense drying in the eastern Mediterranean, and a lesser decline in cold extremes by late century compared to LENS1.