The climate response to sudden sea ice loss in CCSM4

Abstract:

Arctic sea ice has been disappearing at a rapid rate and the impact this will have on the atmospheric general circulation and weather variability is still highly uncertain. Observational studies are hindered by the short observed record and the difficulty in isolating the dynamical role of sea ice, while modelling studies do not agree on many aspects of the response, particularly the atmospheric circulation. These disagreements may be due to a number of reasons including model differences, different magnitude and spatial patterns of the forcing, different treatment of sea surface temperatures, and natural variability. In this study, we present results from a sea ice perturbation experiment using the coupled Community Climate System Model version 4(CCSM4). By decreasing the albedo the sea ice, we were able to achieve an ice-free Arctic in September, in a simple, energetically self-consistent way that also involves year-round sea ice thinning and oceanic warming. The model was integrated for 800 years to find a robust equilibrium response to sea ice loss. Another 7 realizations of 50 years in length, differing only in their initial conditions, were used to investigate the transient response to rapid sea ice loss. In both the equilibrium and transient response, the general circulation response is very weak and the transient response is highly variable between realizations. We also find a reduction in surface temperature variability on sub-seasonal and interannual time scales over most of the mid and high latitudes with the largest changes being over the Arctic Ocean, associated with the increased maritime influence under low-ice conditions. We quantify this reduced variability, which extends into the midltatitudes and into the free tropospheric geopotential height variability.