Robust Spatial and Seasonal Changes to the Coupled Arctic Energy Budget in a Large Ensemble

Observed declines in both the thickness and spatial extent of Arctic sea ice are almost universally expected to continue over the coming decades in response to greenhouse gas forcing and climate feedbacks. The loss of Arctic sea ice is most prominent in the late summer and early fall, but large changes in the high-latitude energy budget and its component fluxes will be affected throughout the annual cycle. Such fundamental alterations of the surface energy budget can have far-reaching implications on many other systems (e.g., the oceanic and atmospheric circulations), but very large uncertainties remain regarding which processes dominate the seasonal and spatial distribution of Arctic heating and cooling. This study characterizes the robust and the uncertain aspects of the seasonal and spatial distribution of heating and attributes various patterns to specific processes in the coupled ice-ocean-atmosphere Arctic system.

The seasonal cycle of long-term trends in various radiative, thermodynamic, and advective processes are investigated during the 2010-2040 interval of rapid ice loss in a thirty-three member ensemble of the fully-coupled Community Earth System Model–Community Atmosphere Model, version 5 (CESM1-CAM5). CESM1-CAM5 results are compared with other models and observations to the extent possible.  Much of the anomalous heating of the surface ocean during summer from reduced albedo and enhanced shortwave absorption is released only months later in particular spatial distributions of turbulent and longwave fluxes that are largely determined by the spatial distribution of the antecedent ice loss. Some of the robust projections in CESM1-CAM5 are substantially different (both spatially and seasonally) than in other fully-coupled simulations and these differences can be traced to thermodynamic and dynamic processes. Ocean and atmosphere heat fluxes across the subpolar boundary of the Arctic are both modified, but only some of the spatial and seasonal structures in these changes are robust. Robust seasonal and spatial signatures are consistent with only some of the proposed mechanisms of atmosphere and ocean interaction with sea ice loss. All of these results suggest that a structured assessment of the seasonal and spatial distribution of Arctic energy fluxes and their controlling physical processes will enhance our understanding of the intrinsically coupled response to enhanced greenhouse gases.