Shipboard ADCP Research Articles

Energy Partition between Submesoscale Internal Waves and Quasigeostrophic Vortical Motion in the Pycnocline

Abstract Shipboard ADCP velocity and towed CTD chain density measurements from the eastern North Pacific pycnocline are used to segregate energy between linear internal waves (IW) and linear vortical motion [quasigeostrophy (QG)] in 2D wavenumber space spanning submesoscale horizontal wavelengths λx ∼ 1–50 km and finescale vertical wavelengths λz ∼ 7–100 m. Helmholtz decomposition and a new Burger number (Bu) decomposition yield similar results despite different methodologies. While these wavelengths are conventionally attributed to internal waves, both QG and IW contribute significantly at all measured scales. Partition between IW and QG total energies depends on Bu. For Bu < 0.01, available potential energy EP exceeds horizontal kinetic energy EK and is contributed mostly by QG. In contrast, energy is nearly equipartitioned between QG and IW for Bu ≫ 1. For Bu < 2, EK is contributed mainly by IW, and EP by QG, while, for Bu > 2, contributions are reversed. Finescale near-inertial IW dominate vertical shear variance, implying negligible QG contribution to vertical shear instability. In contrast, both QG and IW at the smallest λx ∼ 1 km contribute large horizontal shear variance, so that both may lead to horizontal shear instability, while QG, with its longer time scales, likely dominates isopycnal stirring. Both QG and IW contribute to vortex stretching at small vertical scales. For QG, the relative vorticity contribution to linear potential vorticity anomaly increases with decreasing horizontal and increasing vertical scales.

Two-Dimensional Wavenumber Spectra on the Horizontal Submesoscale and Vertical Finescale

Abstract Horizontal and vertical wavenumbers (kx, kz) immediately below the Ozmidov wavenumber (N3/ε)1/2 are spectrally distinct from both isotropic turbulence (kx, kz > 1 cpm) and internal waves as described by the Garrett–Munk (GM) model spectrum (kz < 0.1 cpm). A towed CTD chain, augmented with concurrent Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling float microstructure measurements and shipboard ADCP surveys, are used to characterize 2D wavenumber (kx, kz) spectra of isopycnal slope, vertical strain, and isopycnal salinity gradient on horizontal wavelengths from 50 m to 250 km and vertical wavelengths of 2–48 m. For kz < 0.1 cpm, 2D spectra of isopycnal slope and vertical strain resemble GM. Integrated over the other wavenumber, the isopycnal slope 1D kx spectrum exhibits a roughly +1/3 slope for kx > 3 × 10−3 cpm, and the vertical strain 1D kz spectrum a −1 slope for kz > 0.1 cpm, consistent with previous 1D measurements, numerical simulations, and anisotropic stratified turbulence theory. Isopycnal salinity gradient 1D kx spectra have a +1 slope for kx > 2 × 10−3 cpm, consistent with nonlocal stirring. Turbulent diapycnal diffusivities inferred in the (i) internal wave subrange using a vertical strain-based finescale parameterization are consistent with those inferred from finescale horizonal wavenumber spectra of (ii) isopycnal slope and (iii) isopycnal salinity gradients using Batchelor model spectra. This suggests that horizontal submesoscale and vertical finescale subranges participate in bridging the forward cascade between weakly nonlinear internal waves and isotropic turbulence, as hypothesized by anisotropic turbulence theory.

Enhanced Near-Inertial Waves and Turbulent Diapycnal Mixing Observed in a Cold- and Warm-Core Eddy in the Kuroshio Extension Region

Abstract Shipboard observations of upper-ocean current, temperature–salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in a cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6f. The mode regression method was proposed to extract NIWs from the shipboard ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the “inertial chimney” effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs’ downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean. Significance Statement We provide observational evidence for the downward propagation of near-inertial waves enhanced by mesoscale eddies. This is significant because the down-taking of wind energy by the near-inertial waves is an important energy source for turbulent mixing in the interior ocean, which is essential to the shaping of ocean circulation and climate. The anticyclonic eddies are widely regarded as a conduit for the downward near-inertial energy propagation, while the cyclonic eddies activity influencing the near-inertial waves propagation lacks clear cognition. In this study, enhanced near-inertial waves and turbulent dissipation were observed inside both cyclonic and anticyclonic eddies in the Kuroshio Extension region, which has significant implications for improving the parameterization of turbulent mixing in ocean circulation and climate models.

Off-Equatorial Deep-Cycle Turbulence Forced by Tropical Instability Waves in the Equatorial Pacific

AbstractThe equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer—“deep-cycle turbulence”—that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri ≈ 0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0°, 140°W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear υz off of the equator. Parameterized turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon. Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which, when combined with shear from shipboard ADCP data, suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator.

How a Small Reef in the Kuroshio Cultivates the Ocean

AbstractVertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin‐Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo‐sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10−1 m2 s−1), resulting in strong nitrate fluxes of O(1–103 mmol m−2 day−1). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10−3 m s−1), as well as an upwelling nitrate flux of O(102 mmol m−2 day−1) in the entire subsurface layer.

Spatiotemporal structures of tidal currents and upwelling around the Hachiri-ga-se marine hill in the southwestern Japan Sea

Spatiotemporal structures of tidal and mean currents in August 2008 and May 2010 were estimated by shipboard ADCP data around the Hachiri-ga-se marine hill in the southwestern Japan Sea. The time variations of the tidal currents were quite similar both in August and May. The vertical structure of tidal and mean currents in August was more baroclinic than that in May. Upwelling region moved when the tidal current varied with time. The upwellings occurred in the convergence region when the horizontal current flowed across the marine hill. The upwellings in August and May mainly occurred at the central and northwestern parts of the Hachiri-ga-se marine hill, respectively. In the case of May 2010, the thermocline was lifted about 10 m near the top of hill along the cross-hill temperature section. It is considered that many zooplankton measured by quantitative echosounder were distributed in the upwelling region.

Origin and Attenuation of Mesoscale Structure in Circumpolar Deep Water Intrusions to an Antarctic Shelf

AbstractCross-isobath transport of Upper Circumpolar Deep Water (UCDW) provides a major source of heat to the Antarctic continental shelves. Adaptive sampling with a Slocum glider revealed that the UCDW regularly intrudes onto the western Antarctic Peninsula shelf within mesoscale eddies, and a linear stability analysis of the shelf-break current upstream confirmed eddy length scales and vertical structure are consistent with the baroclinic instability of the current. The properties of the most unstable mode are insensitive to current orientation but sensitive to bottom slope in accordance with modified Eady theory. Once on the shelf, the eddies’ core properties mix with ambient shelf water to form modified CDW (mCDW). Concurrent shipboard CTD and ADCP data are used to diagnose the responsible mixing processes and highlight the importance of thermohaline intrusions. The genesis mechanism of the interleaving layers cannot be confirmed, however a simple analytic model suggests the upper limit contribution of advection by internal waves cannot account for the observed temperature variance unless the cross-eddy temperature gradient is large. Data-adaptive sampling of an eddy with the glider revealed it lost heat across two isopycnals and a fixed radius at a rate of 7 × 109 J s−1 over 3.9 days. This rate is corroborated by a diffusion model initialized with the eddy’s initial hydrographic properties and informed by the heat fluxes parameterized from the shipboard data. The results suggest eddies predominately lose heat laterally and downward, which preserves subsurface heat for melting of marine-terminating glaciers.

Mean circulation of the Mid-Atlantic Bight from a climatological data assimilative model

The along-shelf momentum balance of the Mid-Atlantic Bight (MAB) coastal ocean includes a significant contribution from the along-shelf gradient in sea level. This sea level tilt, of order 10−7, and other features of the mean sea level are not captured well in global mean dynamic topography (MDT) derived from hydrographic observations or satellite altimetry and gravity data, and is poorly represented in global and basin scale dynamical models. This is problematic for applications that would use coastal satellite altimeter data to estimate total water level above datum. We have produced a MDT for the MAB using the Regional Ocean Modeling System (ROMS) with 4-dimensional variational (4D-Var) data assimilation configured to obtain climatological annual and monthly mean results. The observations assimilated were a regional hydrographic climatology of temperature and salinity, and long-term mean velocity from HF-radar, shipboard ADCP, and current-meters. Assimilation adjusts the 3-dimensional ocean state, boundary conditions, and air-sea fluxes to minimize the model-data misfit. The assimilation of mean velocity data is vital to obtaining a realistic circulation result. The MDT exhibits a strong across-shelf sea level slope in geostrophic balance with the southwestward mean flow. The subtle along-shelf tilt is recovered and is relatively uniform throughout the MAB inside the 50 m isobath, but on the southern outer shelf it reverses sign and drives significant across-isobath flow, partially draining the southward mean transport. In the north, across-shelf flow is offshore in the surface and bottom Ekman layers, but largely balanced locally by inflow in the interior depth range.

Subthermocline and Intermediate Zonal Currents in the Tropical Pacific Ocean: Paths and Vertical Structure

AbstractThe mean subthermocline and intermediate zonal circulation in the tropical Pacific is investigated using a compilation of shipboard ADCP measurements and absolute geostrophic velocities constructed from a high-resolution 0–2000-m Argo climatology referenced to a 1000-m velocity field derived from Argo float drifts. This reference field is dominated by basinwide alternating zonal jets with a meridional wavelength of about 3°. In regions where the sampling of SADCP data is sufficient, the consistency between the two independent datasets is striking; using the Argo drift reference is crucial to capture the current structures. Two apparently distinct systems of alternating westward and eastward zonal jets are seen in both datasets equatorward of 10°: a series of low-latitude subthermocline currents (LLSCs) below the thermocline, extending from about 200 to 800 m, including the eastward Tsuchiya jets; and a series of low-latitude intermediate currents (LLICs), extending from about 700 to at least 2000 m. These systems seem to merge poleward of 10°. Both series shoal to lighter densities eastward. The subthermocline currents and their associated potential vorticity structures undergo a major shift near 155°W, suggesting some difference in the dynamic regime between the regions west and east of this longitude. Differing behaviors (the LLSCs tend to angle poleward to the east, whereas the LLICs angle slightly equatorward) suggest that these jets may be dynamically distinct, with different forcing mechanisms.

A tale of two eddies: The biophysical characteristics of two contrasting cyclonic eddies in the East Australian Current System

AbstractMesoscale cyclonic eddies are known to be highly productive. Less well‐known are the dynamics and productivity of smaller cyclonic eddies, known as frontal eddies, that form on the landward side of western boundary currents. In this study, we investigate the physical and biogeochemical properties of two contrasting cyclonic eddies in the East Australian Current (EAC). The first (“Murphy”), a mesoscale cyclonic eddy that formed at ∼28°S with a diameter of ∼160 km and high surface chlorophyll‐a concentrations, which lived ∼47 days. The second (“Freddy”), a smaller frontal eddy (∼35 km diameter) that formed from a shelf water billow ∼7 days prior to sampling at ∼31.5°S and was advected off the shelf along the EAC front (from ∼200 m to 4000 m of water). Both eddies were at least 1000 m deep with a similar steric height anomaly. We introduce and employ “the method of closest approach” using shipboard ADCP velocities to estimate the eddy centers, which reveals significant tilting through the water column. We estimate rotation rates of 4–10 days and 1–9 days and Rossby numbers 0.25–0.1 and 0.6–0.1, from the surface to 600 m for Murphy and Freddy, respectively. High‐resolution altimetry measurements from the SARAL/AltiKA satellite provide estimates of the ageostrophic component of rotation. Our results show that the frontal eddy is significantly more ageostrophic, energetic, and productive than the mesoscale cyclone, despite its small size and short life (∼4 weeks). We suggest that frontal eddies have potential to contribute significantly to the net productivity of the Tasman Sea region.

Observed evidence of the anomalous South China Sea western boundary current during the summers of 2010 and 2011

AbstractSeven years of directly measured current data from a mooring in the Xisha area of the South China Sea (SCS), together with shipboard ADCP and satellite data, have shown the western boundary current (WBC) anomaly and its vertical structure during the summers of 2010 and 2011. The observed WBC presented obvious year‐to‐year variability, especially in the summer. Overall, the summer mean velocity at the mooring site over 7‐year (2007–2013) was northeastward. The moored ADCP showed that the northeastward velocity was particularly strong in the summer of 2010, but the increase was confined in the upper 120 m. In contrast, the northeastward current disappeared throughout the observed depth range (from 50 to 450 m) in the summer of 2011. Even at the deepest observed position, the monthly velocity anomalies reached 14 cm s−1 westward and 12 cm s−1 southward in the zonal and meridional directions, respectively. Both the Vietnam offshore current (VOC) and double gyres in the western SCS disappeared and the southern anticyclonic gyre expanded to strengthened the northward WBC in the summer of 2010. However, in summer of 2011, the VOC intensified, and the northern cyclonic gyre enlarged with its northern edge reaching 18°N, slightly north of mooring site, which weakened the northeastward WBC. The observed SCS circulation anomalies during 2010 and 2011 were mainly induced by the basin‐scale wind field anomalies associated with the 2009/2010 El Niño and 2010/2011 La Niña.

Mesoscale to Submesoscale Wavenumber Spectra in Drake Passage

AbstractThis study discusses the upper-ocean (0–200 m) horizontal wavenumber spectra in the Drake Passage from 13 yr of shipboard ADCP measurements, altimeter data, and a high-resolution numerical simulation. At scales between 10 and 200 km, the ADCP kinetic energy spectra approximately follow ak−3power law. The observed flows are more energetic at the surface, but the shape of the kinetic energy spectra is independent of depth. These characteristics resemble predictions of isotropic interior quasigeostrophic turbulence. The ratio of across-track to along-track kinetic energy spectra, however, significantly departs from the expectation of isotropic interior quasigeostrophic turbulence. The inconsistency is dramatic at scales smaller than 40 km. A Helmholtz decomposition of the ADCP spectra and analyses of synthetic and numerical model data show that horizontally divergent, ageostrophic flows account for the discrepancy between the observed spectra and predictions of isotropic interior quasigeostrophic turbulence. In Drake Passage, ageostrophic motions appear to be dominated by inertia–gravity waves and account for about half of the near-surface kinetic energy at scales between 10 and 40 km. Model results indicate that ageostrophic flows imprint on the sea surface, accounting for about half of the sea surface height variance between 10 and 40 km.

Assimilating temperature and salinity profiles using Ensemble Kalman Filter with an adaptive observation error and T-S constraint

Temperature (T) and salinity (S) profiles from conductivity-temperature-depth data collected during the Northern South China Sea Open Cruise from August 16 to September 13, 2008 are assimilated using Ensemble Kalman Filter (EnKF). An adaptive observational error strategy is used to prevent filter from diverging. In the meantime, aiming at the limited improvement in some sites caused by the T and S biases in the model, a T-S constraint scheme is adopted to improve the assimilation performance, where T and S are separately updated at these locations. Validation is performed by comparing assimilated outputs with independent in situ data (satellite remote sensing sea level anomaly (SLA), the OSCAR velocity product and shipboard ADCP). The results show that the new EnKF assimilation scheme can significantly reduce the root mean square error (RMSE) of oceanic T and S compared with the control run and traditional EnKF. The system can also improve the simulation of circulations and SLA.

Seasonal variability of the Kuroshio Current at the PN Section in the East China Sea based on in-situ observation from 1987 to 2010

As the spatio-temporal variability of the Kuroshio is highly influenced by mesoscale eddies, representing its seasonal variability characteristics requires sufficiently long term observations to reduce the uncertainties. Geostrophic velocity data estimated from hydrographic observation from 1987 to 2010 and the shipboard ADCP velocity data from 1993 to 2008 at the PN Section in the central East China Sea are collected to view the seasonal variability objectively. From both types of observation, it is found that the seasonal climatology mean of the Kuroshio Current exhibits significant difference in three areas, which are located at the Kuroshio Current core and its two flanks in a shallow layer less than 300 m, with the weakest northeast current at the core in autumn, the strongest counter current on the right flank in spring, and the strongest northeast current on the left flank in autumn, respectively. The seasonal variance of the Kuroshio Current also exhibits significant difference on the offshore side of the Kuroshio, with larger variance in spring and summer while smaller variance in autumn and winter. For the current parallel to the PN Section, the ratio of the seasonal variability component to the intraseasonal variability component is relatively smaller than that for the current perpendicular to the PN Section. Further analyses indicate that the seasonal variability at the PN Section is tightly linked to the upstream and downstream current variability.

Shelf water and chlorophyll export from the Hatteras slope and outer shelf

AbstractUsing high‐resolution data acquired from a shipboard ADCP and a towed Scanfish equipped with a CTD and fluorometer, we examine the properties and transport of Middle Atlantic Bight (MAB) shelf water over a region of the Hatteras outer shelf and slope where MAB shelf water is commonly deflected offshore and entrained into the Gulf Stream. The data are from a period in early August 2004 when the seasonal pycnocline of the MAB is well developed and situated over a weakly stratified, near‐bottom shelf water mass commonly referred to as the cold pool. Our data show chlorophyll‐rich cold pool water carried rapidly southward over the slope and outer shelf, at a rate of up to 60 cm s−1, as part of the shelf‐edge frontal jet. This southward transport of chlorophyll‐rich cold pool water is shunted eastward and entrained into the Gulf Stream. However, the latitude band over which this export occurs varies significantly over the 7 day course of our study, a variation which is linked to an order 50 km shift in the latitude at which the Gulf Stream separates from the continental margin. The coupled rapid translation of the Gulf Stream frontal separation and the cold pool export zone is likely to have a significant impact on the movement and accumulation of biogenic material over the Hatteras slope and rise.

Near-coastal circulation in the Northern Humboldt Current System from shipboard ADCP data

The near-coastal circulation of the Northern Humboldt Current System is described analyzing ∼8700 velocity profiles acquired by a shipboard acoustic Doppler current profiler (SADCP) during 21 surveys realized between 2008 and 2012 along the Peruvian coast. This data set permits observation of (i) part of the Peru Coastal Current and the Peru Oceanic Current that flow equatorward in near-surface layers close to the coast and farther than ∼150 km from the coast, respectively; (ii) the Peru-Chile Undercurrent (PCUC) flowing poleward in subsurface layers along the outer continental shelf and inner slope; (iii) the near-surfacing Equatorial Undercurrent renamed as Ecuador-Peru Coastal Current that feeds the PCUC; and (iv) a deep equatorward current, referred to as the Chile-Peru Deep Coastal Current, flowing below the PCUC. A focus in the PCUC core layer shows that this current exhibits typical velocities of 5-10 cm s−1. The PCUC deepens with an increasing thickness poleward, consistent with the alongshore conservation of potential vorticity. The PCUC mass transport increases from ∼1.8 Sv at 5°S to a maximum value of ∼5.2 Sv at 15°S, partly explained by the Sverdrup balance. The PCUC experiences relatively weak seasonal variability and the confluence of eddy-like structures and coastal currents strongly complicates the circulation. The PCUC intensity is also affected by the southward propagation of coastally trapped waves, as revealed by a strong PCUC intensification in March 2010 coincident with the passage of a downwelling coastal wave associated with a weak El Nino event.

Manifestation of two meddies in altimetry and sea-surface temperature

Abstract. Two meddies were identified in the Iberian Basin using shipboard ADCP (Meddy 1) and Argo float (Meddy 2) in contrasting background conditions. Meddy 1 was observed while interacting with the Azores Current (AzC), while Meddy 2 was observed in a much calmer dynamical background, north from the AzC jet. In both cases the meddies formed a clear anticyclonic surface signal, detectable in altimetry as well as in sea-surface temperature (SST). Analysis of the in situ observations of the dynamic signal over Meddy 1 showed that the signal, generated by the moving meddy, dominated the AzC dynamics at least up to the base of the seasonal thermocline even at the late stages of its interaction with the jet. The centre of rotation of the surface signal was shifted south-westward from the axis of the meddy by about 18 km, and its dynamic radius was 2 times bigger than that of the meddy. In the centre of the anticyclonic surface signals of both meddies, SST was colder than that of the surrounding water, in contrast to warm SST anomalies in the cores of surface anticyclones generated by meandering surface currents. The latter difference gives ground for identification of meddies (as well as other sub-surface anticyclones) in comparatively dynamically calm regions using coupled altimetry–SST remote sensing data. An identification of Meddy 1 prior to the shipboard ADCP measurements was the first successful experience. At the same time, SST anomalies over the meddies were rather weak, often unstable and statistically significant only over periods of months.

Numerical modelling of the effect of turbines on currents in a tidal channel – Tory Channel, New Zealand

Numerical modelling is used to assess the effect of a turbine array on tidal currents in the Tory Channel, New Zealand. The Tory Channel is the smaller of two entrances from Cook Strait to the Queen Charlotte Sound with a large island separating the narrow Tory Channel from the main entrance. The 2D depth-averaged finite element model is validated against velocities from shipboard ADCP transects from a survey during spring tide conditions, and water levels recorded at the study site. Turbine drag is introduced to the model as a stress term in the momentum equations, and includes both the turbine thrust and the structural drag. Turbine array drag is a function of the number and size of turbines, which can be parameterised in a non-dimensional number. This non-dimensional turbine drag number D can be used to represent the drag of several different turbine designs. Restrictions are placed on the size of the array to ensure that turbines are placed in realistic locations. In this study, turbines are restricted to areas with water depths greater than 30 m, and where spring tide currents (in the absence of turbines) are greater than 2.0 m s−1, and consequently the turbine array does not span the entire channel width or length. The modelling shows turbines will reduce current speeds both within the turbine array, and also throughout much of the Tory Channel, with local increases in speed immediately adjacent the array. Cut-in and maximum or rated turbine speeds are also incorporated to compare how these factors influence both the power production and effect on currents. The study shows that, due to the restrictions placed on the array location, the likely power production that can be achieved is considerably less than what an analytical prediction suggests might be obtained from the channel. Due to the effects of turbines on current speeds, optimising the area occupied by an array is likely to be an iterative procedure. The power produced per turbine unit could be substantially improved, with little impact on total power produced by the array, by removing turbines from areas where power produced was low. Turbine operational limits, applied in the form of cut-in speeds below which no power is produced, and design speeds above which load shedding occurs, affect both the magnitude and spatial distribution of power production and thus need to be considered in array design.

Tidal and residual currents over abrupt deep-sea topography based on shipboard ADCP data and tidal model solutions for three popular bathymetry grids

The response of tidal and residual currents to small-scale morphological differences over abrupt deep-sea topography (Seine Seamount) was estimated for bathymetry grids of different spatial resolution. Local barotropic tidal model solutions were obtained for three popular and publicly available bathymetry grids (Smith and Sandwell TOPO8.2, ETOPO1, and GEBCO08) to calculate residual currents from vessel-mounted acoustic Doppler current profiler (VM-ADCP) measurements. Currents from each tidal solution were interpolated to match the VM-ADCP ensemble times and locations. Root mean square (RMS) differences of tidal and residual current speeds largely follow topographic deviations and were largest for TOPO8.2-based solutions (up to 2.8 cm s−1) in seamount areas shallower than 1,000 m. Maximum RMS differences of currents obtained from higher resolution bathymetry did not exceed 1.7 cm s−1. Single depth-dependent maximum residual flow speed differences were up to 8 cm s−1 in all cases. Seine Seamount is located within a strong mean flow environment, and RMS residual current speed differences varied between 5 % and 20 % of observed peak velocities of the ambient flow. Residual flow estimates from shipboard ADCP data might be even more sensitive to the choice of bathymetry grids if barotropic tidal models are used to remove tides over deep oceanic topographic features where the mean flow is weak compared to the magnitude of barotropic tidal, or baroclinic currents. Realistic topography and associated flow complexity are also important factors for understanding sedimentary and ecological processes driven and maintained by flow–topography interaction.

Variation in the Hatteras Front density and velocity structure Part 2: Historical setting

On the continental shelf near Cape Hatteras, cool fresh Mid-Atlantic Bight and warm salty South Atlantic Bight shelf waters converge alongshelf 90% of the time, causing strong alongshelf gradients in temperature and salinity known as the ‘Hatteras Front’. The resulting density gradient supports strong shoreward velocities in the cross-shelf oriented ‘nose’, of the Front in wintertime. To investigate further, the Frontal Interactions near Cape Hatteras (FINCH) project used shipboard ADCP and a towed undulating CTD to examine Hatteras Front property, density and velocity fields in August 2004, January 2005, and July 2005. Strong property gradients were encountered across the nose of the Hatteras Front in all cases, but the density gradient, dynamic height gradient, and observed along-front cross-shelf velocities evolved in time. FINCH along-Front velocities were strong and shoreward in fall and winter, and weakly mixed shoreward and seaward in July. Several archived data sets were examined, and demonstrate that the density evolution and wind forcing seen in FINCH are characteristic of other years. Evidence suggests the width of the Hatteras Front does not vary dramatically in time, so that consistently large fall and winter density contrast across the Front implies consistently large shoreward velocities along it in winter. Weak density contrasts across the Hatteras Front in spring suggest the magnitude and sign of springtime density gradients and along-Front velocities could vary interannually. Recruitment success of commercially important stocks on the shelf that depend on cross-shelf transport may thus be affected year to year.