Internal waves and vertical mixing in the Storfjorden Polynya, Svalbard

International audienceLatent heat polynya are preferential sites for dense water formation through brine release. The combined action of winds and heat loss leading to sea ice fracture followed by the reactivation of sea ice formation drives this process of dense water formation. The impact of high-frequency dynamics, and especially internal wave breaking, on this process is investigated here, with the analysis of hydrographic observations collected from a drifting mooring in Storfjorden, Svalbard. The main frequency components of the barotropic tide are first revealed by the analysis of a 23-day time series of pressure data. Interestingly the same frequency peaks are isolated by a spectral analysis of isopycnal displacements, suggesting a tidal forcing for these baroclinic waves. The hypothesis of local generation in the Storfjorden is next examined and numerous sites of potential generation are identified specifically for semi-diurnal and sixth-diurnal frequencies. Diurnal baroclinic waves, however, do not appear to be locally generated, but rather popagate cyclonically along the shelf as internal Kelvin waves. The characteristic of these waves and their energy flux are then inferred. A parameterization of dissipation based on the potential energy of the internal wavefield is next proposed and an upper bound for dissipation rate within overturning regions is provided. The most vigorous mixing is associated with brine release events (i.e. strong vertical mixing through thermohaline convection) with eddy diffusivities of up to 10-3 m2 s-1. The background mixing (likely associated with internal wave breaking), however, displays values within [10-6, 10-4] m2 s-1. This background mixing contributes about O(0.1) W m-2 to the vertical heat flux with a net heating in the bottom layer and cooling in the upper layer. Local heating and cooling rates of the order of 0.025°C month-1 are obtained which is significant with respect to the 0.1°C temperature difference between the bottom and surface layers