Comparison of Mechanisms for Low-Frequency Variability of Summer Arctic Sea Ice in Three Coupled Climate Models

A more reliable projection of the future changes in summer Arctic sea ice demands a better understanding of the mechanisms driving its low frequency natural variability. In this study, we investigate the mechanisms for low-frequency natural variability of summer Arctic sea ice using long control simulations from three coupled climate models (GFDL CM2.1, GFDL CM3, and NCAR CESM). Despite different Arctic sea ice mean states, there are many robust features in the response of low-frequency summer Arctic sea ice variability to the three key predictors (Atlantic/Pacific oceanic heat transport into the Arctic and the Arctic Dipole) across all three models. For example, in all three models, an enhanced Atlantic (Pacific) heat transport into the Arctic induces summer Arctic sea ice decline and surface warming, especially over the Atlantic (Pacific) sector of the Arctic. A positive phase of the Arctic Dipole induces summer Arctic sea ice decline and surface warming on the Pacific side, and opposite changes on the Atlantic side. There is robust Bjerknes Compensation at low frequency, so that the northward atmospheric heat transport provides a negative feedback to summer Arctic sea ice variations. We also demonstrate that the net influence of the Arctic Dipole on summer Arctic sea ice extent is more (less) effective in simulations with less (excessive) climatological summer sea ice in the Atlantic sector. The response of Arctic sea ice thickness (SIT) and mass to the three key predictors is stronger in models that have thicker climatological Arctic sea ice. We develop a simple theory to show that, at low frequency, changes in SIT induced by ocean-to-ice heat flux anomalies are much larger in regions with thicker climatological sea ice. The results highlight the importance of sustained long-term observations of the key drivers of the low-frequency summer Arctic sea ice variability, such as the Atlantic and Pacific heat transport into the Arctic. If the Atlantic Meridional Overturning Circulation and associated Atlantic heat transport into the Arctic were to weaken in the near future due to multi-decadal natural variability, we may see a hiatus or pause in the decline of summer Arctic sea ice.