Momentum balances on the North Carolina inner shelf

Four months of moored current, pressure, temperature, conductivity, wave, a nd 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 momen tum balance is between the cross-shelf gradient of the wave radiation stres s and the bottom stress. The linear drag coefficient in the surf zone is ab out 10 times larger than seaward of the surf zone. On the inner shelf the a long-shelf momentum balance is also frictional; the along-shelf wind stress and pressure gradient are balanced by bottom stress. In the cross-shelf mo mentum balance the pressure gradient is the superposition of roughly equal contributions from the Coriolis force (geostrophy) and wave setdown from sh oaling, unbroken surface gravity waves. At midshelf the along-shelf momentu m balance is less frictional and hence flow accelerations are important. Th e cross-shelf momentum balance is predominantly geostrophic because the gre ater depth and smaller bottom slope at midshelf reduce the importance of wa ve setdown. The cross-shelf density gradient is in thermal wind balance wit h 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 g radient) that reproduces accurately the observed cross-shelf variation of t he depth-averaged, along-shelf current between the surf zone and midshelf.