Missing freshwater found below the mixed layer during the melt season in the Canada Basin of the CESM Climate Model

The Canada Basin is a region of rapid sea ice retreat that has been observed and modeled extensively over the past several decades. Several observational studies have noted that the upper ocean exhibits an apparent trend toward a significantly fresher surface layer and, therefore, a more stratified upper ocean. Here, we explore the extent to which a climate model can accurately simulate these observed changes. Specifically, we examine simulations from the Community Earth System Model (CESM) 1.1, a climate model used in the IPCC AR5, and observations from the Ice Tethered Profilers (ITPs) and the Arctic Ice Dynamics Joint Experiment (AIDJEX). Our results indicate that, in stark contrast to the observations, the model simulates salinity profiles that show relatively little variation between 1975 and 2012. We demonstrate that, during the spring and summer months, this bias can be primarily attributed to the model’s tendency to unrealistically store freshwater from sea ice melt below the mixed layer, deeper than suggested by observations, leading to a significantly weaker than observed seasonal pycnocline. The results may provide insight for climate model improvement that could have wide reaching implications because upper ocean stratification influences the vertical transport of heat, freshwater, and nutrients.