Observations of nonlinear momentum fluxes over the inner shelf

Nonlinear momentum fluxes over the inner continental shelf are examined using moored observations from multiple years at two different locations in the Middle Atlantic Bight. Inner shelf dynamics are often described in terms of a linear alongshore momentum balance, dominated by frictional stresses generated at the surface and bottom. In this study, observations over the North Carolina inner shelf show that the divergence of the cross-shelf flux of alongshore momentum is often substantial relative to the wind stress during periods of strong stratification. During upwelling at this location, offshore fluxes of alongshore momentum in the surface layer partially balance the wind stress and reduce the role of the bottom stress. During downwelling, onshore fluxes of alongshore momentum reinforce the wind stress and increase the role of bottom stress. Over the New England inner shelf, nonlinear terms have less of an impact in the momentum balance and exhibit different relationships with the wind forcing. Differences between locations and time periods are explained by variations in bottom slope, latitude, vertical shear and cross-shelf exchange. Over the New England inner shelf, where moored density data are available, variations in vertical shear are only partially explained by thermal wind balance with the cross-shelf density gradient. Residuals of the vertical shear not explained by thermal wind balance are correlated with the alongshore wind stress during periods of relatively strong stratification. An important implication of this study is that cross-shelf winds can potentially influence the alongshore momentum balance over the inner shelf, in contrast with deeper locations over the middle to outer shelf.