A Comparison of Low-Frequency Variability of Summer Arctic Sea Ice Extent in Three Coupled Climate Models

Abstract:

The shrinking summer Arctic sea ice extent (SIE) over the satellite era has challenged our understanding of the climate system. In response to changes in anthropogenic radiative forcings, the multi-model ensemble mean summer Arctic SIE in CMIP5 simulations shows a less dramatic decline than observed during the recent global warming hiatus period, implying that natural variability may have played an important role in the observed decline. Here the mechanisms for low-frequency variability of summer Arctic SIE are studied using long control simulations from three coupled climate models: GFDL CM2.1, GFDL CM3, and NCAR CESM. It is found that Atlantic/Pacific oceanic heat transport into the Arctic and the Arctic Dipole are key players for the low-frequency variability of summer Arctic sea ice concentration in all three models. Compared with satellite observations, the climatological mean summer Arctic SIE appears to be simulated most realistically in GFDL CM3; ice extent is much smaller than observed in GFDL CM2.1 and much larger in NCAR CESM. In all three models, a positive Arctic Dipole phase leads to a reduction of summer Arctic sea ice concentrations on the Pacific side and an increase on the Atlantic side by transporting more ice from the Pacific side to the Atlantic side. In both GFDL CM2.1 and CM3, the increase of sea ice extent on the Atlantic side is less than the reduction on the Pacific side; hence a positive Arctic Dipole leads to a net reduction of the summer Arctic SIE. However, in NCAR CESM due to the excessive climatological sea ice on the Atlantic side, reduction of sea ice on the Pacific side is largely balanced by the expansion on the Atlantic side, resulting in an insignificant net impact of the Arctic Dipole on the summer Arctic SIE. In all three models, variations in the atmospheric heat transport across the Arctic Circle are forced by anti-correlated variations in the Atlantic oceanic heat transport into the Arctic, which provides a negative feedback on summer Arctic SIE variations.