Momentum balances on the North Carolina inner shelf

Abstract

Four months of moored current, pressure, temperature, conductivity, wave, and wind observations on the North Carolina shelf indicate three dynamically distinct regions: the surf zone, the inner shelf between the surf zone and the 13‐m isobath, and the midshelf. In the surf zone the along‐shelf momentum balance is between the cross‐shelf gradient of the wave radiation stress and the bottom stress. The linear drag coefficient in the surf zone is about 10 times larger than seaward of the surf zone. On the inner shelf the along‐shelf momentum balance is also frictional; the along‐shelf wind stress and pressure gradient are balanced by bottom stress. In the cross‐shelf momentum balance the pressure gradient is the superposition of roughly equal contributions from the Coriolis force (geostrophy) and wave setdown from shoaling, unbroken surface gravity waves. At midshelf the along‐shelf momentum balance is less frictional and hence flow accelerations are important. The cross‐shelf momentum balance is predominantly geostrophic because the greater depth and smaller bottom slope at midshelf reduce the importance of wave setdown. The cross‐shelf density gradient is in thermal wind balance with the vertical shear in the along‐shelf flow in depths as shallow as 10 m. The dominant along‐shelf momentum balances provide a simple estimate of the depth‐averaged, along‐shelf current in terms of the measured forcing (i.e., wind stress, wave radiation stress divergence, and along‐shelf pressure gradient) that reproduces accurately the observed cross‐shelf variation of the depth‐averaged, along‐shelf current between the surf zone and midshelf.