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The steep continental slope off the Lofoten–Vesterålen islands of northern Norway appears to be the source of the most intense mesoscale eddy field in all of the Nordic Seas. Here we use linearized two-layer shallow-water equations to study the stability of the Norwegian Atlantic Current in this region. The study extends previous works that used linearized quasigeostrophic vertical mode equations to examine the effects of bottom topography on baroclinic instability here. We find evidence of baroclinic instability in the stacked shallow-water model but also of barotropic instability that is associated with the upper-layer lateral shear. The calculations give an indication that growth rates of barotropic instability may be comparable to or larger than those of baroclinic instability over the steepest parts of the continental slope. [ABSTRACT FROM AUTHOR]

Low-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea are generated at Macquarie Ridge, south of New Zealand, and propagate northwest as a focused beam before impinging on the Tasmanian continental slope. In situ observations from the Tasman Sea capture synoptic measurements of the incident semidiurnal mode-1 internal-tide, which has an observed wavelength of 183 km and surface displacement of approximately 1 cm. Plane-wave fits to in situ and altimetric estimates of surface displacement agree to within a measurement uncertainty of 0.3 cm, which is the same order of magnitude as the nonstationary (not phase locked) mode-1 tide observed over a 40-day mooring deployment. Stationary energy flux, estimated from a plane-wave fit to the in situ observations, is directed toward Tasmania with a magnitude of 3.4 ± 1.4 kW m−1, consistent with a satellite estimate of 3.9 ± 2.2 kW m−1. Approximately 90% of the time-mean energy flux is due to the stationary tide. However, nonstationary velocity and pressure, which are typically 1/4 the amplitude of the stationary components, sometimes lead to instantaneous energy fluxes that are double or half of the stationary energy flux, overwhelming any spring–neap variability. Despite strong winds and intermittent near-inertial currents, the parameterized turbulent-kinetic-energy dissipation rate is small (i.e., 10−10 W kg−1) below the near surface and observations of mode-1 internal tide energy-flux convergence are indistinguishable from zero (i.e., the confidence intervals include zero), indicating little decay of the mode-1 internal tide within the Tasman Sea. [ABSTRACT FROM AUTHOR]

The reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of the mode-1 internal tide as mode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget. [ABSTRACT FROM AUTHOR]

The volume transport of the Agulhas Current was measured over a 3-yr period by an array of seven current meter moorings and four current- and pressure-recording inverted echo sounders (CPIES) deployed at 34°S. CPIES extended the array farther offshore in order to capture, for the first time, the full Agulhas Current during meander events. Transports derived from CPIES are well correlated with overlapping current meter transports (0.89). The Eulerian mean current is 219 km wide and 3000 m deep, with peak surface speeds of 1.8 m s−1 and a weak northward undercurrent on the continental slope below 1200 m. A new algorithm to capture the western boundary jet transport at each time step T is defined as the poleward transport out to the first maximum of the vertically integrated velocity beyond the half-width of the mean jet. The mean transport of the Agulhas Current jet, so defined, is −84 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) with a standard error of 2 Sv. Sampling and instrumental errors are explicitly estimated and amount to an additional 9 Sv. A more traditional estimate, based on net transport integrated to a fixed distance offshore Tbox, gives a mean transport of −77 ± 5 Sv. This transport is 10 Sv greater than an equivalent transport at 32°S, corresponding to a latitudinal increase equal to that predicted by Sverdrup dynamics. The time series of T and Tbox show important differences during solitary meander events and at longer time scales. In terms of an annual cycle, the Agulhas Current appears strongest during austral summer, a similar phase to the Gulf Stream and Kuroshio. [ABSTRACT FROM AUTHOR]

The article examines the accelerating dense-water flow on continental slopes. It states that water is denser on a shallow shelf than in the adjacent deep ocean. Its acceleration is affected by several factors such as gravity, density, and the component of the earth's rotation. Moreover, downslope flows can follow winter cooling of shallow shelf seas at mid to high latitudes.

Stratified flow over topography is studied, with oceanic applications in mind. A model is developed for a fluid with arbitrary vertical stratification and a free surface, flowing over three-dimensional topography of arbitrary size and steepness, with background rotation, in the linear hydrostatic regime. The model uses an expansion of the flow fields in terms of a set of basis functions, which efficiently capture the vertical dependence of the flow. The horizontal structure may then be found by solving a set of coupled partial differential equations in two horizontal directions and time, subject to simple boundary conditions. In some cases, these equations may be solved analytically, but, in general, simple numerical procedures are required. Using this formulation, the internal tide generated by a time-periodic barotropic tidal flow over a continental shelf and slope is calculated in various idealized configurations. The topography and fluid motion are taken to be independent of one coordinate direction and the fluid to be either two-layer or uniformly stratified. For the two-layer case, expressions for the shoreward and oceanward energy fluxes associated with the internal tide are derived. For the uniformly stratified case, it is studied numerically how the accuracy of the solutions depends upon the number of basis functions used, and it is shown that good solutions and energy flux estimates can often be obtained with only a few basis functions. In both cases, the results show that the position of the coastline, through its effect on the form of the barotropic tide, significantly influences the strength of the internal tide generation. [ABSTRACT FROM AUTHOR]

A reduced-gravity model is used to examine the dynamics of dense water descending a continental slope. The model solves for the geostrophically adjusted state before it is subjected to significant frictional decay. For such bottom-mounted flow, it is argued that frictional torque would dominate the net vorticity balance to equalize the edge flows, resulting in double velocity cores. Constrained by the geostrophic balance, the dense water thus may settle only over a concave bottom and is sheetlike, covering typically the whole slope rise. As such, the adjustment is characterized by a spreading rather than sinking of the layer—with little descent of the upper edge but a swift downslope current propelling the lower edge. Through the mechanical energy balance, it is found in addition that a greater density anomaly would increase the total entrainment flux to more strongly dilute the original anomaly, yielding a product water that is less varied in the water-mass properties. Model predictions compare favorably with some observed dense outflows, in support of the entrainment and friction control of the geostrophic adjustment. [ABSTRACT FROM AUTHOR]

In an attempt to understand the degree to which the stability of a shelfbreak front, characterized by continuous horizontal and vertical shear, is affected by topography, a linear stability analysis was conducted for a range of frontal jets and bottom-slope configurations. Three-dimensional perturbations superposed on a continuously stratified shelfbreak front were investigated using linearized, hydrostatic primitive equations. For all model runs in the study, the frontal instability mode, which is the fastest-growing mode for a baroclinic flow, was not influenced by the bottom: Retrograde, prograde, and flat-bottom jets all share the same stability characteristics. In contrast, weakly baroclinic jets are strongly influenced by bottom topography. The presence of a bottom slope stabilizes prograde jets and destabilizes retrograde jets, a difference attributed to the orientation of the isopycnals relative to the bottom slope. Temporal and/or downstream changes in the bottom slope and/or background stratification are shown to produce sizeable changes in the instability of a weakly baroclinic jet. [ABSTRACT FROM AUTHOR]

High-resolution hydrographic and velocity measurements across the East Greenland shelf break south of Denmark Strait have revealed an intense, narrow current banked against the upper continental slope. This is believed to be the result of dense water cascading over the shelf edge and entraining ambient water. The current has been named the East Greenland Spill Jet. It resides beneath the East Greenland/Irminger Current and transports roughly 2 Sverdrups of water equatorward. Strong vertical mixing occurs during the spilling, although the entrainment farther downstream is minimal. A vorticity analysis reveals that the increase in cyclonic relative vorticity within the jet is partly balanced by tilting vorticity, resulting in a sharp front in potential vorticity reminiscent of the Gulf Stream. The other components of the Irminger Sea boundary current system are described, including a presentation of absolute transports. [ABSTRACT FROM AUTHOR]

Recent measurements in a region of continental slope characterized by ridges and valleys running up and down the slope reveal interesting high mode structure in the tidal band velocity signals, with enhanced mixing above the corrugations. In order to understand these observations, numerical simulations of the internal tide generation in this region of topography were performed. Here the focus is on the response of the flow to along;chslope barotropic tidal forcing. For small-amplitude barotropic forcing, internal waves are generated over the continental slope that propagate toward the ocean surface and toward shallower water. When higher-amplitude forcing is combined with large-amplitude corrugations, the flow is locally supercritical downstream of ridges, and transient internal hydraulic jumps result. As the flow relaxes each half tidal period, these jumps are released as internal wave packets, which propagate up into the thermocline. The internal hydraulic jumps are a source of mixing in the valleys, while the small-scale shears associated with the internal waves could lead to mixing higher up the water column. Over the forcing range considered, the response is dominated by slightly higher harmonics of the tidal forcing frequency than predicted by existing analytic theories. [ABSTRACT FROM AUTHOR]