The Oceanographic and Biological Tidal Cycle Succession in Shallow Sea Fronts in the North Sea and the English Channel

Two major discrepancies between existing theories concerning the positioning and development of shallow shelf sea fronts and available data are that these models (1) do not adequately describe the neap-spring adjustment of frontal position and (2) these theories fail to explain the relatively small changes in the location of the front despite the rapid seasonal decay of the stabilizing surface heat flux. In the model presented here, these weaknesses in earlier attempts to describe frontal position are overcome. This is achieved partly by incorporating wind stirring in the heat and mixing balance for the frontal area and partly by allowing for a cross-frontal circulation. This new model defines two types of shallow sea fronts: tidal 'surface' and tidal 'bottom' fronts. Surface fronts are dominated by tidal and bottom fronts by wind stirring. The front type determines the circulation pattern on the stratified side of the front. The circulation pattern, in turn, governs the biological activity. The model describes the persistent but temporally varying redistribution of water from the bottom and surface layers to the thermocline layer which supplies nutrients from the deep (light limited) to the shallow regions where phytoplankton activity is stimulated. This new interpretation of the spring-neap adjustment also implies that, during the heating season, an increasing part of the potential energy to be overcome during a tidal cycle stems from the mixing of nutrient-rich dense bottom water and light surface water imported from offshore. The existence of this mechanism is supported by field observations.