CALCULATING THE ANNUAL CYCLE OF THE VERTICAL EDDY VISCOSITY IN THE NORTH-SEA WITH A 3-DIMENSIONAL BAROCLINIC SHELF SEA CIRCULATION MODEL

The vertical eddy viscosity (A(n)u) is estimated using a three-dimensi onal baroclinic shelf sea model that treats the temperature as a progn ostic quantity. A(n)u is calculated by means of a turbulent closure ap proach proposed by Kochergin [(1987) Three-dimensional coastal ocean m odels, American Geophysical Union, pp. 201-208] which is closely relat ed to a Mellor and Yamada [(1974) Journal of Atmospheric Science, 31, pp. 1791-1806] level-2-model that has been used very successfully in a large number of applications. The annual cycle of the vertical eddy v iscosity is discussed by looking at horizontal and vertical A(n)u-dist ributions for the year 1988. These examples show that the vertical edd y viscosity is subject to a pronounced annual cycle which can be relat ed to heating and cooling processes as well as to mixing induced by wi nd and bottom friction. A comparison of these results with A(n)u-distr ibutions calculated for the year 1987 additionally demonstrates a stro ng inter-annual variability.