Estimation of heat content and variability in the upper Arctic Ocean from the Regional Arctic System Model (RASM) and available observations.

The observed rate of sea ice cover decline in the Arctic for the past decades is faster than those projected by the latest suite of models participating in the Coupled Model Intercomparison Project (CMIP5). We hypothesize that a critical source of energy in the Arctic Ocean, heat content accumulating below the surface mixed layer and above the halocline, has been increasing in magnitude and area and may be contributing to the recent decline in the ice cover.

To address this hypothesis we analyze model output from a subset of the Regional Arctic System Model (RASM), where the atmospheric and land components are replaced with prescribed realistic atmospheric reanalysis data. First, mixed layer and halocline depths are estimated from RASM and compared with available observational estimates. Next, monthly heat content is calculated for the upper Arctic Ocean layer (UAOL), defined by these two hydrographic boundaries, for the period of 1948-2009. Consistent with observations from the Woods Hole Oceanographic Institution’s network of Ice-Tethered Profilers (ITP), model results indicate that heat has been stored in UAOL and that it has increased recently. Analyses of ITP data and RASM show that the total amount and rate of increase of heat content has been largest in the western Arctic and there might be a causal relationship between the accumulation of heat content and the reduction of sea ice volume.