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 changing general circulation and critical to
the future of mid-latitude surface weather. Previous studies suggest
that CMIP5-6 model projections fall into two groups reflecting different
characteristics of climate change, especially in the North Atlantic.
Here we present distinct warming patterns emerging by the late 21st
century from 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 (UTW). Uniquely, in LENS2 during winter, the North
Atlantic sector projects less warming in the Arctic than 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 slower decline in cold extremes by late century compared to LENS1.