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An intricate field of NNW-SSE trending sand ridges was discovered for the first time in 2010 and mapped in detail over the past 6 years during multiple campaigns in the vicinity of the islands of Helgoland and Dune in the German Seabight. The tramline-shaped, elongate, linear and meandering sand ridges are up to 1 km in length, over 10 m in height and spaced 10-50 m apart. The ridges exhibit complex anastomosing patterns, converging and disappearing to the south of the study area and diverging, becoming more subdued and disappearing further to the north. Seismic profiling, surface samples and multibeam data show that the ridges are stationary, while the sediment wave fields to the north and west are migrating westwards. Based on our mapping results, the most likely hypothesis for the development and maintenance of these enigmatic structures is a combination of tidal currents, tides and sediment influx. Whether these structures are associated with the Mesozoic subcrop or are glaciofluvial in origin and have been modified by the last glacial-interglacial intervals or are sedimentary structure linked to Holocene sea level rise in the southern North Sea is a matter of debate. Further studies, including numerical modelling of these sand ridges structures, would provide clues to past sedimentary and hydrodynamic conditions and future changes in the highly dynamic and complex tidal environment of the southern North Sea. [ABSTRACT FROM AUTHOR]

This study examines the dispersal of dense water formed in an idealized coastal polynya on a sloping shelf in the absence of ambient circulation and stratification. Both numerical and laboratory experiments reveal two separate bottom pathways for the dense water: an offshore plume moving downslope into deeper ambient water and a coastal current flowing in the direction of Kelvin wave propagation. Scaling analysis shows that the velocity of the offshore plume is proportional not only to the reduced gravity, bottom slope, and inverse of the Coriolis parameter, but also to the ratio of the dense water depth to total water depth. The dense water coastal current is generated by the along-shelf baroclinic pressure gradient. Its dynamics can be separated into two stages: (i) near the source region, where viscous terms are negligible, its speed is proportional to the reduced gravity wave speed and (ii) in the far field, where bottom drag becomes important and balances the pressure gradient, the velocity is proportional to Hc[ g′/( LC d)]1/2 in which H c is the water depth at the coast, g′ the reduced gravity, C d the quadratic bottom drag coefficient, and L the along-shelf span of the baroclinic pressure gradient. The velocity scalings are verified using numerical and laboratory sensitivity experiments. The numerical simulations suggest that only 3%-23% of the dense water enters the coastal pathway, and the percentage depends highly on the ratio of the velocities of the offshore and coastal plumes. This makes the velocity ratio potentially useful for observational studies to assess the amount of dense water formed in coastal polynyas. [ABSTRACT FROM AUTHOR]

Sinking dense plumes are important in many oceanographic settings, notably the polar formation of deep and bottom waters. The dense water sources feeding such plumes are commonly affected by tidal modulation, leading to plume variability on short time scales. In a simple unsteady theory of one-dimensional plumes (based on conservation equations for volume, momentum, and buoyancy), this plume variability is manifested as waves that travel down the resulting current. Using numerical techniques applied to the hyperbolic conservation equations, this study investigates the novel concept that these waves may break as they travel down the plumes, triggering intense local mixing between the dense fluid and surrounding ocean. The results demonstrate that the waves break at geophysically relevant distances from the plume source. The location of wave breaking is very sensitive to plume drag from the seabed, the properties of the dense source, and the amplitude and period of the source modulation. To the extent that the simple model represents the real world, these results suggest that wave breaking originating from the tidal modulation of dense plumes could lead to a strong and previously unexplored source of local deep-ocean mixing. [ABSTRACT FROM AUTHOR]

Abstract: We use a numerical model to examine the dynamics controlling flushing of the basins of the Orust fjord system on the Swedish west coast over a period of 71days in the autumn of 2010. This fjord system is known for its seasonal and permanent hypoxic basins; Koljöfjord, Byfjord and Havstensfjord. It is shown that wind direction determines in- and outflows of the basins. Westerly winds result in surface currents towards the interior of the system, and counter currents at the bottom. In contrast, easterly winds cause the export of surface water, which is compensated by upwelling of Skagerrak water. Although these easterly winds cause renewal to take place several times a month in southern Havstensfjord, it occurs only once or twice in the northern part, while Koljöfjord and Byfjord are ventilated about every 3years. The reduction in density throughout the water column (i.e. weakening of the vertical stratification) is related to the amount of energy being supplied to the deepwater by internal waves, created by barotropic tides across the sills, and wind blowing across the surface. The model shows that, in the southern part of the system, tidal movement dominates the energy supply to the deepwater. Further into the system, the importance of wind energy increases. In Koljöfjord and Byfjord, wind accounts for about 40% of the supplied energy for deepwater mixing. Concurrent weekly measurements taken in each fjord basin reveal that the monthly monitoring programme currently in place cannot adequately resolve the dynamics of the fjords. Modelled and observed currents in Svanesund and Sunninge Strait show prominent baroclinic motion on time-scales between one day and one week, suggesting that monitoring programs that aim to support numerical modelling need to resolve these time scales. [Copyright &y& Elsevier]

In real ocean environments, currents generally exist simultaneously with ocean waves. However, the most previous investigations for the seabed response have only considered wave loading, ignoring currents. In this study, unlike previous studies, currents are included in the model of seabed response with waves, based on Biot's poroelastic dynamic theory (u-p approximation). Numerical results reveal that the consideration of currents has significant effect on the seabed response. The opposing current is beneficial to prevent liquefaction, while the following current would worsen the stability of the seabed. The parametric studies indicate that the wave period, water depth, saturation, soil permeability, and thickness of the seabed significantly affect the seabed response under combined loading of waves and currents. The maximum relative difference of the pore pressure between the cases with currents (velocity is -2 m/s) and without currents (pcurrent-pnocurrent)/p0 can reach up to 25% in both coarse and fine sand. The analysis of transient liquefaction in the fine sand seabed indicates that the maximum liquefaction depth increases with the following currents, but it decreases with the opposing current. [ABSTRACT FROM AUTHOR]

A form of linear, barotropic potential vorticity equation is derived for an ocean with a free surface, in which only one scalar variable appears (ocean bottom pressure, or subsurface pressure). Unlike quasigeostrophic or rigid-lid derivations, the only approximation made (apart from linearization) is that changes in the circulation must be slow compared with the inertial frequency. Effects of stratification are included, but only parametrically in the sense that density is treated as a given quantity or forcing term rather than a variable. [ABSTRACT FROM AUTHOR]

Gravity Recovery and Climate Experiment (GRACE) gravity data spanning January 2003–November 2005 are used as proxies for ocean bottom pressure (BP) averaged over 1 month, spherical Gaussian caps 500 km in radius, and along paths bracketing the Antarctic Circumpolar Current’s various fronts. The GRACE BP signals are compared with those derived from the Estimating the Circulation and Climate of the Ocean (ECCO) ocean modeling–assimilation system, and to a non-Boussinesq version of the Regional Ocean Model System (ROMS). The discrepancy found between GRACE and the models is 1.7 cmH2O (1 cmH2O ∼ 1 hPa), slightly lower than the 1.9 cmH2O estimated by the authors independently from propagation of GRACE errors. The northern signals are weak and uncorrelated among basins. The southern signals are strong, with a common seasonality. The seasonal cycle GRACE data observed in the Pacific and Indian Ocean sectors of the ACC are consistent, with annual and semiannual amplitudes of 3.6 and 0.6 cmH2O (1.1 and 0.6 cmH2O with ECCO), the average over the full southern path peaks (stronger ACC) in the southern winter, on days of year 197 and 97 for the annual and semiannual components, respectively; the Atlantic Ocean annual peak is 20 days earlier. An approximate conversion factor of 3.1 Sv (Sv ≡ 106 m3 s-1) of barotropic transport variability per cmH2O of BP change is estimated. Wind stress data time series from the Quick Scatterometer (QuikSCAT), averaged monthly, zonally, and over the latitude band 40°–65°S, are also constructed and subsampled at the same months as with the GRACE data. The annual and semiannual harmonics of the wind stress peak on days 198 and 82, respectively. A decreasing trend over the 3 yr is observed in the three data types. [ABSTRACT FROM AUTHOR]

Bottom topography, or more generally the geometry of the ocean basins, is an important ingredient in numerical ocean modeling. With the help of an adjoint model, it is shown that scalar diagnostics or objective functions in a coarse-resolution model, such as the transport through Drake Passage, the strength of the Atlantic Ocean meridional overturning circulation, the Deacon cell, and the meridional heat transport across 32°S, are sensitive to bottom topography as much as they are to surface boundary conditions. For example, adjoint topography sensitivities of the transport through Drake Passage are large in choke-point areas such as the Crozet–Kerguélen Plateau and south of New Zealand; the Atlantic meridional overturning circulation is sensitive to topography in the western boundary region of the North Atlantic Ocean and along the Scotland–Iceland Ridge. Many sensitivities are connected to steep topography and can be interpreted in terms of bottom form stress, that is, the product of bottom pressure and topography gradient. The adjoint sensitivities are found to agree with direct perturbation methods with deviations smaller than 30% for significant perturbations on time scales of 100 yr, so that the assumption of quasi linearity that is implicit in the adjoint method holds. The horizontal resolution of the numerical model affects the sensitivities to bottom topography, but large-scale patterns and the overall impact of changes in topography appear to be robust. The relative impact of changes in topography and surface boundary conditions on the model circulation is estimated by multiplying the adjoint sensitivities with assumed uncertainties. If the uncertainties are correlated in space, changing the surface boundary conditions has a larger impact on the scalar diagnostics than topography does, but the effects can locally be on the same order of magnitude if uncorrelated uncertainties are assumed. In either case, bottom topography variations within their prior uncertainties affect the solution of an ocean circulation model. To this extent, including topography in the control vector can be expected to compensate for identifiable model errors and, thus, to improve the solutions of estimation problems. [ABSTRACT FROM AUTHOR]

This paper presents analyses of the fundamental properties of the fully nonlinear Boussinesq-type equations for waves and currents over a permeable seabed introduced by Chen [2005]. These properties are the wave celerity and the spatial porous damping rate for wave propagation on permeable beds. The governing equations for horizontally two-dimensional wave and current motions are first summarized, citing its methods of derivation, applicability and accuracy, and with emphasis on the inclusion of higher-order dispersive damping terms in the permeable-layer momentum equation. Then the fundamental properties are analyzed by extracting the complex linear dispersion relation embedded in the model equations using a Stokes-type approach and accounting for the coupling of the real and imaginary wavenumbers. Plots of the fundamental properties reveal that the inclusion of higher-order dispersive damping terms in the porous-layer momentum equation improves its agreement with the exact solution from shallow to deep water. Inclusion of these terms is also indispensable in obtaining accurate fundamental properties for high values of the permeable-layer thickness ratio. Determination of the model parameter values based on optimization is discussed. Finally, rational expansion of the dispersion equations is used to elucidate the observed behavior of the model's fundamental properties. [ABSTRACT FROM AUTHOR]

The response of a time-dependent atmospheric function close to a slope region is studied. A forced two-layered model over a sloping topography is examined with focus on the particular solution and interaction between the constructed barotropic and baroclinic mode. The water column responds with long periodic waves along the sloping bottom, and the solution includes a time-dependent Ekman transport in the upper layer. Despite its relative simplicity, the model produces a resonant response in the barotropic mode to a northward-propagating plane-wave wind stress field. The particular solution shows that a state close to resonance in the barotropic mode is given by a 1/λy line for the barotropic coefficient when plotted as a function of the forcing wave period and wavelength λy. From the Norwegian Meteorological Institute (NMI) Hindcast database, the calculated time series of the wind stress curl reveals a high energy level in the forcing period interval between 48 and 185 h. The Vøring Plateau slope has a natural oscillation period around 100 h for topographically trapped waves, and the model shows that an along-slope wavelength of 76 km will produce a resonant response in the water column for this wave period. Hence, when the forcing function is constructed as a sum of (infinitely) many Fourier components, topographic waves will be triggered along the Vøring Plateau slope for a northward-moving atmospheric pressure system. [ABSTRACT FROM AUTHOR]