Impacts of Surface Fluxes on the Mixed Layer Depth Evolution in the Southern Beaufort Sea and Amundsen Gulf

The mixed layer (ML) is an oceanic surface layer in which physical properties such as density, salinity and temperature are constant due to the strong mixing at the surface. This layer plays a crucial role in the oceanic ecosystem since its variations has a considerable effects on the upper ocean characteristics and marine biology. The objective of this study is to characterize and model the seasonal evolution of the surface mixed layer on the Mackenzie Shelf (in southern Beaufort Sea) and the nearby Amundsen Gulf. The data we used was obtained during the CASES program (Canadian Arctic Shelf Exchange Study), the CFL program (Circumpolar Flaw Lead) and the Malina program. Most of the CASES data was obtained at a fixed station in Franklin Bay between 3 December 2003 and 30 May 2004. The CFL data was obtained while drifting in central Amundsen Gulf between October 2007 and July 2008. We have a unique data set covering two winters in Amundsen Gulf and seven years (2002-2009) on the Mackenzie Shelf.

All the methods to estimate the MLD have been developed for the mid- and low latitudes. We compared several methods of estimating the MLD (Holton and Talley, 2009; Thompson and Fine, 2003; Sereeze et al., 2000). We found that a modified version of the Holton and Talley (2009) gives the best results for the polar region. The Franklin Bay data provided us with a time series of over 500 CTD profiles (and MLDs) in the winter of 2003-2004 at a single location.

The relationship between the under ice oceanic fluxes and the variations of the MLD was examined for the region. A one-dimensional model developed by Prieur et al. 2010 is employed in order to calculate the oceanic fluxes (i.e. mass and buoyancy) using the mass balance in the ML. Emery (1976) examined the relationship between heat content of the surface layer in mid-latitude and vertical motion deduced from temperature fluctuations. He developed a so-called “divergent heat budget equation” (his eq. 8). Prieur et al. (2010) developed a similar approach for the Mediterranean Sea, but based on the mass budget of the ML instead of the heat budget. We are developing a similar approach for the Polar Mixed Layer based on the mass content (density) of the surface layer because the salt content controls the density at the temperatures found in the Arctic. The advantage of this simple model is that it can be used to characterize the ML using drifting and fixed profiler data. The model results at the fixed station during CASES show that the MLD deepens in the winter and early spring, the maximum is reached in April. Estimated surface flux by the model between days 20-100 has the same trend as the MLD (i.e. while buoyancy fluxes decreases, the MLD decreases). In general, as in the CASES 2003-2004 data, the CFL and the Malina cases, the estimated surface mass flux can explain the MLD variations: the MLD deepens when the estimated surface flux has decreasing (negative) trend (cooling) and shallows when the trend of surface flux is increasing (heating).