Modelling the thermal stratification in the North Sea

A one-dimensional model has been used to model the thermal stratification in the North Sea at Station CS (55 °30′N, 00 °55′E). The impact of the choice of the turbulence closure scheme and the description of meteorological forcing on the level of stratification is shown. The model is run with different parameterizations of vertical mixing of heat and momentum: the Mellor-Yamada level 2 turbulence closure scheme used by Sharples and Tett (1994), the quasi-equilibrium Mellor-Yamada-Galperin level 2.5 ‘k’ model (Galperin et al., 1988; Ruddick et al., 1995) and a simple Richardson-number-dependent mixing scheme (Proctor and James, 1996). The resulting thermal structures are compared to the measurements at the North Sea Station CS. The depth of the surface mixed layer and the strength of the thermocline were modelled closest by the quasi-equilibrium Mellor-Yamada-Galperin level 2.5 ‘k’ model. Using this model, the effects of wind stirring and surface heat flux on the thermal stratification have been investigated. Experiments using different wind forcing from predicted hourly values to monthly average are shown. Further tests included averaged solar radiation and dew-point temperature. The depths of the isotherms are fairly sensitive to the description of the wind stress, and the surface layer temperature to the solar radiation. The experiment with hourly predicted wind stress gave the deepest thermocline, because of its high fluctuations. Monthly averages of solar radiation lead to too much heating of the surface layer at Station CS due to a combined effect with the wind speed.