Model statistics of inertial shear from multi- year simulations at weathership 'Mike': currents, atmospheric forcing, internal waves, mixed layer, Norwegian Sea)

Abstract

In this memorandum we focus on one of the most important environmental

factors affecting the propagation and dissipation of internal waves:

the presence of strong vertical shear in the mean currents. Using a numerical model for the turbulent surface mixed layer, we have investigated two aspects of inertial shear statistics: the interannual variations due to variability of the atmospheric forcing fluxes, and the sensitivity of the model predicted shear to the grid size employed in the prediction model. To study the interannual variation of shear, we performed a simulation for the month of May for 20 consecutive years from 1960-1979 at the site of weathership 'Mike' in the Norwegian Sea. To examine the dependence of the shear statistics on the vertical grid size, we performed the computations on grid sizes of 1 m, 2 m, 5 m, and a grid approximating the TOPS grid (for the top 100 m). The results show that there are strong interannual variations in the magnitude and predominant depth of inertial shear, with magnitudes varying by a factor of up to three, and the depth of maximum shear moving by 30 m or more. A dependence of the these quantities on the vertical grid size is also found, with the simulation results showing large inaccuracies for 6% greater than 5 m.