Vertical mixing in the marginal ice zone of the northern Barents Sea-Results from numerical model experiments

Numerical ocean model simulations of the marginal ice zone (MIZ) of the Barents Sea have been made for the years 2003-2005. As part of a project studying carbon cycling in the northern Barents Sea MIZ, the model simulations provide a pre-history and context for interpretation of physical, biological and chemical field data collected during the annual project cruises in this period. Large-scale features as well as the temporal evolution of stratification and vertical mixing, from well-mixed winter conditions to the end of the ice-free season, are described. Modelled ice concentration at the times of the annual project cruises is in agreement with that inferred from satellite data. The simulated seasonal development of mixing and stratification in the MIZ, from winter via the melting period and through the productive summer season, is described. Turbulent mixing forced by tidal currents and wind episodes is examined, and resulting hydrographical conditions and diffusivities are compared with previously published measurements from the project cruises. The vertical and temporal extent to which such variable mixing influences the water column is realistically modelled, but the strength of mixing appears to be inaccurately distributed. Most importantly, differences in modelled and observed water-column stratification are identified. Enhanced near-surface mixing appears to protrude too deeply in the model, and the water column is excessively homogenized below the pycnocline. Experiments with the Mellor-Yamada Level 2.5 turbulence scheme are compared with those from the Richardson number scheme routinely used in the model. Some important differences between the schemes are identified, but both have similar problems with respect to resulting hydrography. Simulations with horizontal grid resolution increased from 4 x 4 km to 800 x 800 m allows for processes inducing significantly more energetic frontal mixing at the MIZ edge to be resolved. (C) 2008 Elsevier Ltd. All rights reserved.