Analysis of Arctic Sea Ice Anomalies in a Coupled Model Control Simulation

Abstract

In a 600-year control run simulation of the Community Climate System Model, version 3 (CCSM3) of the National Center for Atmospheric Research, three events of extraordinary minimum September sea ice extent are identified, from which the system recovers within a few years. The control simulation is run in a stable climate mode using constant 1990 solar, trace gas, and aerosol forcing. The identified events are all of similar magnitude to the observed 2007 record low Arctic ice cover. In the first event (simulation year 451), the record low ice extent coincides with an extreme, cyclonic phase of atmospheric circulation and follows a 10-year period of steady decline in Northern Hemisphere sea ice volume that is associated with increased winter ice export. The ice extent recovered the following year when both the ice export and summer melt returned to more normal values. However, Arctic ice volume did not fully recover until 20 years after the event. In the second event (model year 490), the system was again preconditioned by a 10-year decline in Arctic sea ice volume because of larger than normal ice export. The event itself was associated with a coincident peak in early summer melt and ice export. During the recovery phase, the ice extent again rapidly returned to normal while the sea ice volume continued to decline for another 10 years, to its lowest value of the entire simulation. The recovery in Northern Hemisphere sea ice volume is associated with periods of reduced wintertime ice export through Fram Strait. The third event (model year 556―558) is characterized by a longer-lived anomaly (3 years). The system was not preconditioned by low ice volume, indicating that redistribution of sea ice (rather than ice mass loss) played an important role. Recovery for the third event is associated with a period of low ice export associated with a rapid return of an expanded Beaufort Gyre, which serves to recirculate sea ice within the Arctic Ocean. Notwithstanding known climate model deficiencies, the control simulation suggests that Arctic sea ice extent anomalies of the magnitude observed in the September 2007 minimum are plausible under 1990 forcing conditions. It is reasonable to conclude that more severe anomalies are achievable with more acute forcing conditions that are in excess of those of 1990.