Using the Florida Strait as a prototype, it is shown that lateral boundaries can have a significant dynamic influence on mesoscale variability, provided the distance between the lateral boundaries is very close to the critical width for baroclinic instability (Dc=&pgr;Lr/√2, Lr is the internal Rossby radius of deformation). As a result, the close relation between strong background currents and mesoscale variability found in open-ocean settings breaks down. In addition, straits that are narrow enough can very effectively block through-strait movement of strong mesoscale variability. ¿ 2000 American Geophysical Union