On the importance of stable stratification to the structure of the bottom boundary layer on the western florida shelf

This is a preliminary report of bottom boundary layer (BBL) observations made with a cyclosonde on a continental shelf in the summer when the water is relatively stably stratified. Vertical profiles of temperature, current direction and speed show persistent and large temperature changes (1.5°-4.0°C), large direction changes (30°-75°) during periods of long-isobath flow and a low speed “jet” in the lowest 6–11 m above the bottom. The BBL is interpreted to be a turbulent Ekman layer of depth ranging from 6–11 m in which stable stratification is very important in determining its depth, Ekman veering and speed profile. The temperature in the BBL is seen to change with time at a faster rate than outside the BBL. This is explained by upwelling (downwelling) of colder (warmer) water in the BBL due to Ekman veering in the boundary layer. Such upwelling (downwelling) is expected when the isopynals are non-parallel to the bottom and the geostrophic current above the boundary layer is aligned predominantly along isobaths with deep water to the right (left).