A Numerical Study of the Long- and Short-Term Temperature Variability and Thermal Circulation in the North Sea

A three-dimensional numerical study is presented of the seasonal, semimonthly, and tidal-inertial cycles of temperature and density-driven circulation within the North Sea. The simulations are conducted using realistic forcing data and are compared with the 1989 data of the North Sea Project. Sensitivity experiments are performed to test the physical and numerical impact of the heat flux parameterizations, turbulence scheme, and advective transport. Parameterizations of the surface fluxes with the Monin-Obukhov similarity theory provide a relaxation mechanism and can partially explain the previously obtained overestimate of the depth mean temperatures in summer. Temperature stratification and thermocline depth are reasonably predicted using a variant of the Mellor-Yamada turbulence closure with limiting conditions for turbulence variables. The results question the common view to adopt a tuned background scheme for internal wave mixing. Two mechanisms are discussed that describe the feedback of the turbulence scheme on the surface forcing and the baroclinic circulation, generated at the tidal mixing fronts. First, an increased vertical mixing increases the depth mean temperature in summer through the surface heat flux, with a restoring mechanism acting during autumn. Second, the magnitude and horizontal shear of the density flow are reduced in response to a higher mixing rate. Thermal and salinity fronts generate a seasonal circulation pattern in the North Sea. Their impact on the horizontal temperature distributions is found to be in good agreement with the observations. It is shown that, in the absence of strong wind forcing, both the vertical temperature distribution and the thermal circulation experience semimonthly variations in response to the spring-neap cycle in tidal mixing. At spring tides, the surface mixed layers are shallower, in agreement with observations at two mooring stations, and the baroclinic circulation intensifies, whereas the opposite occurs at neaps.