Observed and simulated seasonal halocline formation in the Canada Basin: 1975 vs 2006-2012
The surface waters of the Arctic Ocean have changed dramatically over the past several decades as a result of the diminishing sea ice cover that once shielded much of the ocean from wind and sunlight across all seasons. Observations collected in the Canada Basin over the past several decades indicate a significantly fresher ocean surface layer in response to perennial sea ice loss, changes to river runoff, and atmospheric circulation. The addition of this relatively light, freshwater at the surface has stabilized the upper-ocean, altering ice-ocean processes, including wind-driven mixing, the vertical transport of heat and nutrients, and sea ice basal melt. Here, we explore the extent to which two climate models accurately simulate the observed surface freshening and seasonal surface processes that contribute to the freshening. We examine 30 simulations from the Community Earth System Model (CESM) 1.1, a climate model used in the IPCC AR5, 3 simulations from CESM 2, a climate model that will be used in the upcoming IPCC AR6, and observations from the 2006-2012 Ice Tethered Profilers (ITPs) and the 1975 Arctic Ice Dynamics Joint Experiment (AIDJEX). We interpret differences between the two time periods in the models and observations using a one-dimensional framework that quantifies differences between the seasonal halocline formation in terms of freshwater input and vertical mixing. In stark contrast to the observations, the model simulates salinity profiles that show relatively little variation between 1975 and 2012. We demonstrate that this bias can be partly attributed to the model’s tendency to simulate unrealistically deep vertical mixing rather than biases related to the seasonal freshwater input.