Seasonal gyres in shelf seas

Attention is drawn to the existence of stably located, seasonal, baroclinic gyres in shelf seas. During summer each year surface heating over isolated regions of weak tidal stirring, such as topographic depressions, can lead to the formation of domes of cold, dense bottom water trapped beneath the thermocline. The dense water dome is isolated horizontally from adjacent water by bottom fronts which drive a cyclonic, near-surface circulation around the dome. There is clear evidence for the existence of such a seasonal cyclonic gyre in the western Irish Sea and there are at least five other locations on the northwest European shelf where similar structures may be found. Although the existence of summer cyclonic gyres can be understood in terms of relatively simple dynamics, their significance is not widely appreciated. They form a component of the organised, mesoscale circulation on continental shelves, they may be important retention sites for marine plankton and their simulation represents a crucial test for the performance of three-dimensional shelf circulation models