Abstract. The time dependent nearshore circulation field during 3 days of the SUPERDUCK field experiment is simulated. We consider the generation of nearshore currents due to obliquely incident breaking waves, damping effects due to bottom friction, and diffusion effects due to lateral momentum mixing caused by turbulence and depth-varying current velocities. Because of uncertainties in the friction and lateral mixing coefficients, numerical simulations are carried out for a realistic range of values for these coefficients. The resulting shear instabilities of the longshore current exhibit unsteady longshore progressive vortices with timescales of O(100 s) and length scales of O(100 m) and longer. The time dependent flow involves the strengthening, weakening, and interaction of vortices. Vortex pairs are frequently shed offshore. During this process, locally strong offshore directed currents are generated. We find that a stronger mean current and faster and more energetic vortex structures result as the friction coefficient is decreased. However, the longshore length scales of the resulting flow structures are not altered significantly. An increase in the mixing coefficient causes relatively small variations in the propagation speeds. However, the resulting flow structures are less energetic with larger longshore length scales. Shear instabilities are found to induce significant horizontal momentum mixing in the surf zone and affect the cross-shore distribution of the mean longshore current. Mixing due to the presence of the instabilities is found to be dominant over mixing caused by more traditional mechanisms such as turbulence. For values of the free parameters that reproduce the propagation speed of the observed motions, the frequency range within which shear instabilities are observed as well as the mean longshore current profile are predicted well.

.[1] Three stations, at $80 m water depth on the Oregon shelf between 44.7°N and 43.9 N, were studied under hypoxic conditions in late spring and summer of 2009 to determine benthic oxygen consumption rates. Oxygen fluxes were derived from eddy correlation (EC) measurements made from an autonomous lander deployed for 11-15 h at a time. Average oxygen consumption rates ranged from 3.2 to 9.8 mmol m À2 d À1 and were highest at the southernmost station. Methods for separating eddy components and rotating coordinates were examined for effects on EC fluxes. It was found that oscillations at frequencies associated with surface and internal waves made significant contributions, but horizontal component biasing could be minimized by wave-based rotation methods. Additional measurements included benthic boundary layer properties, and sediment permeability and profiles of sediment organic C, chlorophyll-a, excess 210 Pb and % fines. Comparative flux estimates were determined from benthic chamber measurements and microelectrode profiles at two of the stations. The chamber O 2 consumption rates exceeded the EC fluxes by factors of 1.2-1.8, which may reflect enclosure effects, the different spatial and temporal scales of the measurements, and/or inhomogeneous benthic respiration rates. The magnitudes of the fluxes by either method, however, are low for shelf depths. Thus, for benthic O 2 consumption to contribute to Oregon shelf hypoxia, bottom waters must be slowly renewed and minimally ventilated by along-or across-shelf advection and turbulent mixing. Circulation studies indicate these conditions are favored by increased near-bottom stratification during persistent summer upwelling-relaxation cycles.Citation: Reimers, C. E., H. T. Özkan-Haller, P. Berg, A. Devol, K. McCann-Grosvenor, and R. D. Sanders (2012), Benthic oxygen consumption rates during hypoxic conditions on the Oregon continental shelf: Evaluation of the eddy correlation method,

[1] The rip current field resulting from the transformation of surface gravity waves over offshore submarine canyons is studied. Employing a wave transformation model and a wave-induced circulation model over observed bathymetry we find that wave height variations associated with undulations in the canyon contours cause rip current circulation cells with alongshore spacing of O(100m) even though the nearshore bathymetry displays no variations at these length scales. Further, the predicted rips correspond to observed rip currents during the Nearshore Canyon Experiment (NCEX). Motivated by these results we study the relationship between O(100 m) scale variations in offshore bathymetric contours and the resulting rip current field in the nearshore. To isolate the roles of possible bathymetric features, we construct a series of idealized case studies that include site characteristics found at NCEX that are conducive of rip current development, such as a curved shoreline, an offshore submarine canyon and undulations in the canyon contours. Our results show that the first two components are unable to produce the observed short-scale circulation systems, while wave refraction over undulations in the canyon walls at length scales of O(100 m) provides a sufficient disturbance to generate alongshore wave height variations that drive multiple rip currents for a variety of incident wave conditions. Rips are not generated when the wave period is short, or when the angle of incidence is large. Analysis of the alongshore momentum balances further demonstrates that the rip current locations are also strongly influenced by inertial effects. Hence, nonlinear processes are important within the rip current circulation cell and we find that nonlinear advective acceleration terms balance a large portion of the driving alongshore gradient in the mean water surface elevation in the vicinity of the rip currents with bottom friction accounting for the remainder. Away from the rips, the balance is between the wave forcing and the pressure gradient outside the surf zone and wave forcing and bottom friction inside the surf zone, as expected.

[1] The dynamics of vorticity motions forced by wave groups incident on an alongshore-uniform barred beach are analyzed. For both normally and obliquely incident wave groups, the potential vorticity and enstrophy equations reveal that the temporal variability of wave group-forced vortices is directly linked to the variability in the incoming wave groups rather than bottom friction, as previously hypothesized. Analysis of the lifespan of individual vortices further shows that the wave group forcing is responsible for not only the temporal variations of the vortices but also their eventual demise. Vortices in the simulations persist for 5 to 45 min, which is consistent with recent field observations. For oblique wave groups, the resulting vortices are advected by the mean current, yielding a signature in the frequency-wave number spectrum that is similar to that usually attributed to shear instabilities of the alongshore current. These results may explain previous observations of alongshore-propagating vorticity motions in the presence of a stable alongshore current. For simulations involving an unstable alongshore current, we find that the inclusion of wave group forcing results in velocity spectra that are much broader compared to the simulations that neglect wave grouping, which could explain discrepancies between previously observed and modeled spectral widths of propagating vorticity motions. Finally, the potential enstrophy balance shows that vorticity production due to wave groups may be as important as that due to the instability process and that not all low-frequency vortical motions observed during oblique wave incidence should be attributed to shear instabilities of the alongshore current.

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