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Harvey E. Seim

Abstract

The contribution of salinity changes to sound speed fluctuations is often neglected in estimating the scattering cross section at high frequencies (>10 kHz). To examine when salinity might be important, an expression is formulated for the scattering cross section σ that includes salinity and an estimate of the cospectrum of temperature and salinity. Profiles from the southern New England shelf, the Bosphorus, and Puget Sound are used to estimate levels of σ as a function of depth and acoustic frequency. Salinity can increase σ by more than an order of magnitude, particularly at frequencies greater than 100 kHz, when salinity controls the density field. The cospectrum is expected to be large under the same conditions and can potentially negate strong scattering at lower frequencies. An f +1 dependence of σ is expected over two decades in frequency when salinity controls density. Multifrequency acoustic systems may be able to distinguish biology and microstructure based on this spectral dependence.

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Harvey Seim and Catherine Edwards

Abstract

A 3-month-long field program conducted in winter 2012 inshore of the seaward deflection of the Gulf Stream at the Charleston Bump observed several 7–21-day periods of strong (>0.5 m s−1) equatorward along-shelf flow over the upper continental slope. In sea surface temperature images, these phenomena resemble and appear linked to warm filaments, features known to be associated with meanders of the Gulf Stream as it traverses the southeast coast of North America. However, the character of these upper-slope features differs from previous descriptions of filaments, hence we describe them as “upper-slope jets.” We document the characteristics of the jets, which are approximately 30 km in width, centered on the 200-m isobath, with a maximum temperature variation at depth, and reasonably long-lived. Southwestward flow within the jet extends to 200 m and is in approximate thermal wind balance below a surface mixed layer. Maximum transport is estimated to be about 2.0 Sv (1 Sv ≡ 106 m3 s−1), driving a net equatorward along-shelf velocity over the deployment period. For this time period, at least, the jets form the equatorward flow of the shoreward flank of the Charleston Gyre. We suggest the features resemble the Pinocchio’s Nose Intrusion recently described by Zhang and Gawarkiewicz. Large-amplitude meander crests with sufficiently strong curvature vorticity are a plausible source of initiation of the upper-slope jets.

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Sara Haines, Harvey Seim, and Mike Muglia

Abstract

Quality control procedures based on nonvelocity parameters for use with a short-range radar system are applied with slight modification to long-range radar data collected offshore of North Carolina. The radar footprint covers shelf and slope environments and includes a segment of the Gulf Stream (GS). Standard processed and quality controlled (QCD) radar data are compared with 4 months of acoustic Doppler current profiler (ADCP) time series collected at three different sites within the radar footprint. Two of the ADCP records are from the shelf and the third is on the upper slope and is frequently within the GS. Linear regression and Bland–Altman diagrams are used to quantify the comparison. QCD data at all sites have reduced scatter and improved correlation with ADCP observations relative to standard processed data. Uncertainty is reduced by approximately 20%, and linear regression slopes and correlation coefficients increase by about 0.1. At the upper slope site, the QCD data also produced a significant increase in the mean speed. Additionally, a significant increase, averaging roughly 20%, in mean speed in the GS is apparent when comparing standard processed data and QCD data, concentrated at large range and at the azimuthal extremes of radial site coverage. Shifts in the distributions of the standard processed and QCD velocity estimates are consistent with the removal of zero-mean noise from the observations, which has minimal impact where the radial site range is <70 km and a large impact at greater range in the GS where mean currents exceed 1 m s−1.

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Catherine R. Edwards and Harvey E. Seim

Abstract

Defining the vertical depth average of measured currents to be barotropic is a widely used method of separating barotropic and baroclinic tidal currents in the ocean. Away from the surface and bottom boundary layers, depth-averaging measured velocity is an excellent estimate of barotropic tidal flow, and internal tidal dynamics can be well represented by the difference between the measured currents and their depth average in the vertical. However, in shallow and/or energetic tidal environments such as the shelf of the South Atlantic Bight (SAB), bottom boundary layers can occupy a significant fraction of the water column, and depth averaging through the bottom boundary layer can overestimate the barotropic current by several tens of centimeters per second near bottom. The depth-averaged current fails to capture the bottom boundary layer structure associated with the barotropic tidal signal, and the resultant estimate of baroclinic tidal currents can mimic a bottom-trapped internal tide.

Complex empirical orthogonal function (CEOF) analysis is proposed as a method to retain frictional effects in the estimate of the barotropic tidal currents and allow an improved determination of the baroclinic currents. The method is applied to a midshelf region of the SAB dominated by tides and friction to quantify the effectiveness of CEOF analysis to represent internal structure underlying a strong barotropic signal in shallow water. Using examples of synthesized and measured data, EOF estimates of the barotropic and baroclinic modes of motion are compared to those made using depth averaging. The estimates of barotropic tidal motion using depth-averaging and CEOF methods produce conflicting predictions of the frequencies at which there is meaningful baroclinic variability. The CEOF method preserves the frictional boundary layer as part of the barotropic tidal current structure in the gravest mode, providing a more accurate representation of internal structure in higher modes. The application of CEOF techniques to isolate internal structure co-occurring with highly energetic tidal dynamics in shallow water is a significant test of the method. Successful separation of barotropic and baroclinic modes of motion suggests that, by fully capturing the effects of friction associated with the barotropic tide, CEOF analysis is a viable technique to facilitate examination of the internal tide in similar environments.

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Harvey E. Seim and Catherine R. Edwards

Abstract

Simultaneous ADCP profile measurements are compared over a 2-month period in late 2003. One set of measurements comes from a National Data Buoy Center (NDBC) buoy-mounted ADCP, the other from a bottom-mounted, upward-looking ADCP moored roughly 500 m from the buoy. The study was undertaken to evaluate the proficiency of an experimental configuration by NDBC; unfortunately, the ADCP was not optimally configured. The higher temporally and vertically resolved bottom-mounted ADCP data are interpolated in time and depth to match the buoy-mounted ADCP measurements. It is found that the two ADCP measurements are significantly different. The buoy-mounted measurements are affected by high-frequency (<10 h period) noise that is vertically coherent throughout the profiles. This noise results in autospectra that are essentially white, unlike the classic red spectra formed from the bottom-mounted ADCP observations. The spectra imply a practical noise floor of 0.045 m s−1 for the buoy-mounted system. Contamination by surface waves is the likely cause of this problem. At tidal frequencies the buoy-mounted system underestimates major axis tidal current magnitude by 10%–40%; interference from the buoy chain and/or fish or plankton are considered the most likely cause of the bias. The subtidal velocity field (periods greater than 40 h) is only partially captured; the correlation coefficient for the east–west current is 0.49 and for the north–south current is 0.64.

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Harvey E. Seim, Michael C. Gregg, and R. T. Miyamoto

Abstract

Acoustic backscatter has produced spectacular images of internal ocean processes for nearly two decades, but interpretation of the images remains ambiguous because several mechanisms can generate measurable backscatter. The authors present what is thought to be the first simultaneous measurements of calibrated acoustic returns and turbulent microstructure, collected in a set of 20-m-tall billows. The observations are from Admiralty Inlet, a salt-stratified tidal channel near Puget Sound. Scattering due to turbulent microstructure alone is strong enough to explain the measured backscatter at specific sites within the billows. Existing formulations underestimate the strength of acoustic backscatter from turbulent microstructure. Due to a misinterpretation of the high-wavenumber temperature spectrum, some previous formulations underestimate the differential Scattering cross section (σ) when scattering from the viscous-convective subrange. Also, the influence of salinity on refractive-index fluctuations can be as large as or greater than that of temperature when the density stratification is dominated by salinity. Using temperature alone to estimate σ in coastal and estuarine waters may lead to significant underestimates. A simple formulation is derived that takes these two factors into account. Because of high ambient scattering from zooplankton in Admiralty Inlet, the acoustic data are conditionally sampled along modeled profiler trajectories to avoid using bulk statistics. Scalar dissipation is greatest in the bounding surfaces of the billows, consistent with these surfaces producing the most intense scattering. Acoustic backscatter can be used to remotely sense the spatial structure of scalar dissipation in turbulent events where σ due to turbulent microstructure exceeds the background level set by scattering from biology. In lakes and the deep ocean where scattering from zooplankton is expected to be negligible, scattering from microstructure may be the dominant mechanism. The largest uncertainties in the comparison result from the very large difference in sampling volume of the acoustic system and microstructure profiler.

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Harvey E. Seim, David P. Winkel, Glen Gawarkiewicz, and Michael C. Gregg

Abstract

Observations from CTD tow-yos and microstructure profiles indicate the presence of a benthic front near 450-m depth on the western side of the Straits of Florida at 27°N. The front is at midslope and approximately beneath the axis of the Florida Current. There is a 0.2 kg m−3 jump in density across the front, and the bottom boundary layer changes thickness from 10–20 m upslope to 50–70 m downslope of the front. Axial currents exhibit very strong vertical shear and cyclonic vertical vorticity upslope of the front in contrast to the significantly weaker vertical shears and anticyclonic vertical vorticity observed downslope. All these features, with the exception of the anticyclonic vorticity downslope of the front, are consistent with a model of barotropically forced flow through a stratified channel with a sloping sidewall. An arrested boundary layer regime is predicted to develop on the sloping sidewall upslope of a height h pfL/N, where f is the Coriolis parameter, L is the width of the flat-bottomed portion of the channel, and N is the buoyancy frequency. The arrested boundary layer supports little cross-slope transport. Below h p a cross-channel circulation cell develops in the model, driven by the bottom Ekman layer. Transition between the boundary layer regimes leads to convergence and frontogenesis near a height h p on the sloping sidewall. Strong mixing at the front acts to destroy the salinity minimum signature of Antarctic Intermediate Water moving into the North Atlantic. The development of anticyclonic vorticity in the Florida Current as it flows through the Florida Straits forces the strongest flow onto the western slope, where an arrested boundary layer develops. This configuration supports strong vertical and lateral shears, all largely in geostrophic balance.

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Lu Han, Harvey Seim, John Bane, Robert E. Todd, and Mike Muglia

Abstract

Carbon-rich Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) shelf waters typically converge on the continental shelf near Cape Hatteras. Both are often exported to the adjacent open ocean in this region. During a survey of the region in mid-January 2018, there was no sign of shelf water export at the surface. Instead, a subsurface layer of shelf water with high chlorophyll and dissolved oxygen was observed at the edge of the Gulf Stream east of Cape Hatteras. Strong cooling over the MAB and SAB shelves in early January led to shelf waters being denser than offshore surface waters. Driven by the density gradient, the denser shelf waters cascaded beneath the Gulf Stream and were subsequently entrained into the Gulf Stream, as they were advected northeastward. Underwater glider observations 80 km downstream of the export location captured 0.44 Sv of shelf waters transported along the edge of the Gulf Stream in January 2018. In total, as much as 7×106 kg of carbon was exported from the continental shelf to a greater depth in the open ocean during this 5-day-long cascading event. Earlier observations of near-bottom temperature and salinity at a depth of 230 m captured several multiday episodes of shelf water at a location that was otherwise dominated by Gulf Stream water, indicating that the January 2018 cascading event was not unique. Cascading is an important, yet little-studied pathway of carbon export and sequestration at Cape Hatteras.

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