Arctic sea ice and freshwater changes driven by the atmospheric leading mode in a coupled sea ice-ocean model

Abstract

Observational and modeling studies have indicated recent large changes of sea ice and hydrographic properties in the Arctic Ocean. However, the observational database is sufficiently sparse that the mechanisms responsible for the recent changes are not fully understood. A coupled Arctic ocean-sea ice model forced by output from the NCEP-NCAR reanalysis is employed to investigate the role that the leading atmospheric mode has played in the recent changes of the Arctic Ocean. A modified Arctic Oscillation (AO) index is derived for the region poleward of 62.5°N in order to avoid ambiguities in the distinction between the conventional AO and the North Atlantic Oscillation index. The model results indicate that the AO is the driver of many of the changes manifested in the recent observations. The model shows reductions of Arctic sea ice area and volume by 3.2% and 8.8%, respectively, when the AO changes from its negative to its positive phase. Concurrently, freshwater storage decreases by about 2%, while the sea ice and freshwater exports via Fram Strait increase substantially. The changes of sea ice and freshwater storage are strikingly asymmetric between the east and the west Arctic. Notable new findings include 1) the interaction of the dynamic and thermodynamic responses in the sense that changes of sea ice growth and melt are driven by, and feed back negatively to, the dynamically (transport) driven changes of sea ice volume; and 2) the compatibility of the associated freshwater changes with recently observed changes in the salinity of the upper Arctic Ocean, thereby explaining the observed salinity variations by a mechanism that is distinct from, but complementary to, the altered circulation of Siberian river water. In addition, the enhanced freshwater export could be a contributing factor to the increased salinity in the Arctic Ocean. The results of the simulations indicate that Arctic sea ice and freshwater distributions change substantially if one phase of the AO predominates over a decadal timescale. However, such results are based on an idealization of the real-world situation, in which the pattern of forcing varies interannually and the number of positive-AO years varies among decades.