Are mesoscale eddies in shelf seas formed by baroclinic instability of tidal fronts?

The formation of eddies along tidal fronts in the shelf seas is investigated through both the analysis of high-resolution conductivity-temperature-depth (CTD) data and drifters for the North Sea as well as experiments with an eddy-permitting model. Baroclinic eddies are difficult to observe in the shelf seas due to their small size, and their surface signals are dampened by air-sea interaction and eroded by wind and tidal mixing. High-resolution CTD profiles reveal possible eddy signals along the thermocline where there are intrusions of waters of reduced salinity on the eddy scale. These freshwater intrusions carry low potential vorticity, indicating their origin from the surface mixed layer. An instability analysis of these frontal systems suggests that these eddy signals can be formed by baroclinic instability. Numerical model integrations reveal that mesoscale eddy variability along the thermocline increases in strength in late summer, in accord with the increase in stratification. The stability analysis of the modeled fronts and the scaling of the eddy kinetic energy to the available potential energy are also consistent with baroclinic instability theory. While influencing the eddy variability through the passage of synoptic weather systems, wind energy input is too small to explain the high values of eddy kinetic energy during late summer. These eddy circulations provide localized regions of high vorticity and high stirring, which are important for the transfer of tracers at middepth, such as potential vorticity, salinity, and nutrients.