Simulating the heat storage in the North Sea with a three-dimensional circulation model

Abstract

The heat storage in the North Sea has been investigated with the help of a three-dimensional baroclinic circulation model forced by observed SST data. In order to obtain an appropriate representation of all thermal parameters in space and time it is essential to use a model that treats temperature as a prognostic quantity and to ensure a thermal forcing which is as realistic as possible. In particular, the processes connected with the existence of the summer thermocline require special attention because they decisively determine vertical heat flux rates. This is guaranteed by choosing adequate time- and space-dependent vertical eddy viscosity A v and diffusivity coefficients, respectively. The A v -approach applied takes into account the temperature distribution of the stratified water column, that in general shows a separation into a turbulent surface and a turbulent bottom layer, both divided by a laminar intermediate zone. As a first step, the annual variability of the heat storage in the North Sea is demonstrated by presenting climatological monthly means (average over the period 1983–1992) of the heat content. Secondly, the inter-annual variability is analysed by calculating the corresponding standard deviations for selected months. By comparing these standard deviations with actual monthly means of two ‘extreme’ months, it turned out that the variation of the summer thermocline depth considerably reduces the variability found in the sea surface temperature (SST), which mainly reflects the thermal forcing. This in turn means that, due to its influence on the thermocline depth, the wind forcing also has a decisive impact on the heat content. This hypothesis is supported by the result that basin-wide the maximum summer heat content is neither directly correlated to the summer SST nor the summer wind stress. A much more striking phenomenon, related to the basin-wide integrated heat content, is the result that in the 10 year period under consideration, two distinctly different summer heat content levels are dominating in the North Sea system. The heat content level observed in the first part of the period is found to be significantly lower than that in the following period. This indicates that in the mid-eighties a marked change in the heat balance of the North Sea took place. It is noteworthy that the coldest winter marks the transition between both periods. Finally, the annual development of the overall heat content of the North Sea and its corresponding standard deviation is investigated in order to determine heat gain and loss rates, respectively, which renders possible a deeper insight into the heat balance of the North Sea.