Buoyancy-driven Coastal Currents Research Articles

Buoyancy-Driven Coastal Currents off Oregon during Fall and Winter

Abstract During fall/winter off the Oregon coast, oceanographic surveys are relatively scarce because of rough weather conditions. This challenge has been overcome by the use of autonomous underwater gliders deployed along the Newport hydrographic line (NH-Line) nearly continuously since 2006. The discharge from the coastal rivers between northern California and the NH-Line reach several thousands of cubic meters per second, and the peaks are comparable to the discharge from the Columbia River. This freshwater input creates cross-shelf density gradients that together with the wind forcing and the large-scale Davidson Current results in strong northward velocities over the shelf. A persistent coastal current during fall/winter, which the authors call the Oregon Coastal Current (OCC), has been revealed by the glider dataset. Based on a two-layer model, the dominant forcing mechanism of the OCC is buoyancy, followed by the Davidson Current and then the wind stress, accounting for 61% (±22.6%), 26% (±18.6%), and 13% (±11.7%) of the alongshore transports, respectively. The OCC average velocities vary from 0.1 to over 0.5 m s−1, and transports are on average 0.08 (±0.07) Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), with the maximum observed value of 0.49 Sv, comparable to the summertime upwelling jet off the Oregon coast. The OCC is a surface-trapped coastal current, and its geometry is highly affected by the wind stress, consistent with Ekman dynamics. The wind stress has an overall small direct contribution to the alongshore transport; however, it plays a primary role in modifying the OCC structure. The OCC is a persistent, key component of the fall/winter shelf dynamics and influences the ocean biogeochemistry off the Oregon coast.

Laboratory modelling of the effects of temporal changes of estuarine-fresh-water discharge rates on the propagation speed of oceanographic coastal currents

In this paper, results of laboratory experiments simulating buoyancy-driven coastal currents produced by estuarine discharges into the ocean, are discussed. The responses of the propagation speeds of the currents to increases and decreases of the volumetric discharge rate at the source are investigated. For increasing discharge rate, we find that the mean speed of the current head displays a sharp rise some time after the source discharge condition has changed. In contrast, a decrease of the current speed following a decreasing discharge rate proceeds gradually. The current speed after acceleration or deceleration is found to be equal to the speed that would be expected had the discharge been at the higher or lower rate from the start of the experiment. The relative speed at which the information of the changed discharge condition at the source approaches the advancing current head from upstream, for both increasing and decreasing discharge rates, is found to be approximately one to three times the mean speed of the current. Further, we find that this transmission speed is 0.82±0.20 times the propagation speed of a linear, long interfacial Kelvin wave.

Instabilities of buoyancy-driven coastal currents and their nonlinear evolution in the two-layer rotating shallow water model. Part 2. Active lower layer

This paper is the second part of the work on linear and nonlinear stability of buoyancy-driven coastal currents. Part 1, concerning a passive lower layer, was presented in the companion paper Gula & Zeitlin (J. Fluid Mech., vol. 659, 2010, p. 69). In this part, we use a fully baroclinic two-layer model, with active lower layer. We revisit the linear stability problem for coastal currents and study the nonlinear evolution of the instabilities with the help of high-resolution direct numerical simulations. We show how nonlinear saturation of the ageostrophic instabilities leads to reorganization of the mean flow and emergence of coherent vortices. We follow the same lines as in Part 1 and, first, perform a complete linear stability analysis of the baroclinic coastal currents for various depths and density ratios. We then study the nonlinear evolution of the unstable modes with the help of the recent efficient two-layer generalization of the one-layer well-balanced finite-volume scheme for rotating shallow water equations, which allows the treatment of outcropping and loss of hyperbolicity associated with shear, Kelvin–Helmholtz type, instabilities. The previous single-layer results are recovered in the limit of large depth ratios. For depth ratios of order one, new baroclinic long-wave instabilities come into play due to the resonances among Rossby and frontal- or coastal-trapped waves. These instabilities saturate by forming coherent baroclinic vortices, and lead to a complete reorganization of the initial current. As in Part 1, Kelvin fronts play an important role in this process. For even smaller depth ratios, short-wave shear instabilities with large growth rates rapidly develop. We show that at the nonlinear stage they produce short-wave meanders with enhanced dissipation. However, they do not change, globally, the structure of the mean flow which undergoes secondary large-scale instabilities leading to coherent vortex formation and cutoff.

Instabilities of buoyancy-driven coastal currents and their nonlinear evolution in the two-layer rotating shallow-water model. Part 1. Passive lower layer

Buoyancy-driven coastal currents, which are bounded by a coast and a surface density front, are ubiquitous and play essential role in the mesoscale variability of the ocean. Their highly unstable nature is well known from observations, laboratory and numerical experiments. In this paper, we revisit the linear stability problem for such currents in the simplest reduced-gravity model and study nonlinear evolution of the instability by direct numerical simulations. By using the collocation method, we benchmark the classical linear stability results on zero-potential-vorticity (PV) fronts, and generalize them to non-zero-PV fronts. In both cases, we find that the instabilities are due to the resonance of frontal and coastal waves trapped in the current, and identify the most unstable long-wave modes. We then study the nonlinear evolution of the unstable modes with the help of a new high-resolution well-balanced finite-volume numerical scheme for shallow-water equations. The simulations are initialized with the unstable modes obtained from the linear stability analysis. We found that the principal instability saturates in two stages. At the first stage, the Kelvin component of the unstable mode breaks, forming a Kelvin front and leading to the reorganization of the mean flow through dissipative and wave–mean flow interaction effects. At the second stage, a new, secondary unstable mode of the Rossby type develops on the background of the reorganized mean flow, and then breaks, forming coherent vortex structures. We investigate the sensitivity of this scenario to the along-current boundary and initial conditions. A study of the same problem in the framework of the fully baroclinic two-layer model will be presented in the companion paper.

Idealized Two-Dimensional Modeling of a Coastal Buoyancy Front, or River Plume, under Downwelling-Favorable Wind Forcing with Application to the Alaska Coastal Current

AbstractThe cross-shelf structure of a buoyancy-driven coastal current, such as produced by a river plume, is modeled in a two-dimensional cross-shelf slice as a “wide” geostrophically balanced buoyancy front. Downwelling-favorable wind stress applied to this front leads to advection in the surface and bottom boundary layers that causes the front to become steeper so that it eventually reaches a steep quasi-steady state. This final state is either convecting, stable and steady, or stable and oscillatory depending on D/δ* and by /f 2, where D is bottom depth, δ* is an Ekman depth, by is the cross-shelf buoyancy gradient, and f is the Coriolis parameter. Descriptions of the cross-shelf circulation patterns are given and a scaling is presented for the isopycnal slope. The results potentially apply to the Alaska Coastal Current, which experiences strong, persistent downwelling-favorable wind stress during winter, but also likely have application to river plumes subjected to downwelling-favorable wind stress.

Generation of a Buoyancy-Driven Coastal Current by an Antarctic Polynya

Abstract Descent and spreading of high salinity water generated by salt rejection during sea ice formation in an Antarctic coastal polynya is studied using a hydrostatic, primitive equation three-dimensional ocean model called the Proudman Oceanographic Laboratory Coastal Ocean Modeling System (POLCOMS). The shape of the polynya is assumed to be a rectangle 100 km long and 30 km wide, and the salinity flux into the polynya at its surface is constant. The model has been run at high horizontal spatial resolution (500 m), and numerical simulations reveal a buoyancy-driven coastal current. The coastal current is a robust feature and appears in a range of simulations designed to investigate the influence of a sloping bottom, variable bottom drag, variable vertical turbulent diffusivities, higher salinity flux, and an offshore position of the polynya. It is shown that bottom drag is the main factor determining the current width. This coastal current has not been produced with other numerical models of polynyas, which may be because these models were run at coarser resolutions. The coastal current becomes unstable upstream of its front when the polynya is adjacent to the coast. When the polynya is situated offshore, an unstable current is produced from its outset owing to the capture of cyclonic eddies. The effect of a coastal protrusion and a canyon on the current motion is investigated. In particular, due to the convex shape of the coastal protrusion, the current sheds a dipolar eddy.

Interaction of transient shelf currents with a buoyancy-driven coastal current

Observations on the inner New Jersey shelf (1996) showed that transient wind-driven currents of 3–4-day periods strongly interacted with a buoyancy-driven coastal current (or buoyant plume) originating from the Hudson estuary. In particular, the depth-averaged current fluctuations were amplified in the buoyant water. This phenomenon is studied here using a primitive equation numerical model (SPEM5). Transient (periodic) shelf currents are introduced in the form of incident barotropic shelf waves (BSW). The model domain is an idealized channel with open upstream/ downstream boundaries and the depth exponentially increasing offshore, which allows the BSW propagation through the model domain. In most cases, the wave period is five days. The buoyancydriven coastal current is forced by constant buoyant discharge through a coastal gap. Propagating BSWs reduce the growth of a buoyant anticyclonic bulge at the source region while a coastal current downstream from the source contains more buoyant water with a sharper density gradient in the frontal zone compared to the case without BSWs. The amplitude of vertically averaged transient currents increases in the buoyant layer (for instance, by 20–30% for the inflow density anomaly of 3 to 4 kg m ). This amplification depends on the density anomaly of buoyant inflow. On the other hand, variations in the inflow velocity and/or the net transport do not affect the BSW amplification. The amplification of velocity amplitude is associated with the incident BSW scattering into higher wave modes. The total energy flux should remain approximately the same as BSWs propagate through the domain. The higher modes have lower group speeds and thus their amplitudes should be higher in order to maintain the same energy flux. Such interactions of transient currents with buoyant plumes are important for the mixing processes and the across-shelf exchange on the inner shelf.

The influence of shelfbreak forcing on the alongshelf penetration of the Danube buoyant water, Black sea

The buoyancy-driven coastal current propagating along the western coast in the Black Sea is forced by the discharge of several major European rivers including the Danube, Dnepr and South Bug. In this study, we present observational evidence that the buoyant water alongshelf penetration is strongly affected by shelfbreak mesoscale features associated with the Rim Current dynamics. The Rim Current is a major element of the Black Sea general circulation, typically following isobaths over the upper-to-middle slope. Two hydrographic surveys conducted in 1992 and 1994 have been chosen among available archive data for the detailed analysis. In both years, though Danube buoyant discharge was similar prior to the beginning of shipboard observations (varying around 7000 m 3 s −1), the buoyant water exhibited very different downstream (that is, in the direction of Kelvin wave) penetration. In 1992, it spread all the way around the southwestern corner of the Black Sea basin and then further eastward past the Bosporus Strait. In contrast, its downstream penetration was blocked in 1994 and buoyant water did not even reach Cape Kaliakra on the Bulgarian coast. This difference was related to the shelfbreak processes. In 1992, the cyclonic meander of the Rim Current merged with the coastal buoyant water thus promoting its advection from Cape Kaliakra downstream. In 1994, a strong anticyclone in the southwestern corner of the Black Sea completely blocked the propagation of a buoyancy-driven current past Cape Kaliakra. In addition, another anticyclone in the northwestern part of the sea advected buoyant water offshore to the central area of the northwestern shelf. The positions of anticyclonic eddies during a period of observations was confirmed by remote sensing data. As these and other examples indicate, coastal buoyancy driven currents can be effectively blocked and dispersed offshore by the shelfbreak anticyclones if the shelf width allows their interaction with buoyant water. Previous observational studies suggested an upwelling-favorable wind as the principal forcing agent that arrested downstream penetration of buoyancy-driven coastal currents.

The influence of shelfbreak forcing on the alongshelf penetration of the Danube buoyant water, Black sea

The buoyancy-driven coastal current propagating along the western coast in the Black Sea is forced by the discharge of several major European rivers including the Danube, Dnepr and South Bug. In this study, we present observational evidence that the buoyant water alongshelf penetration is strongly affected by shelfbreak mesoscale features associated with the Rim Current dynamics. The Rim Current is a major element of the Black Sea general circulation, typically following isobaths over the upper-to-middle slope. Two hydrographic surveys conducted in 1992 and 1994 have been chosen among available archive data for the detailed analysis. In both years, though Danube buoyant discharge was similar prior to the beginning of shipboard observations (varying around 7000 m3 s−1), the buoyant water exhibited very different downstream (that is, in the direction of Kelvin wave) penetration. In 1992, it spread all the way around the southwestern corner of the Black Sea basin and then further eastward past the Bosporus Strait. In contrast, its downstream penetration was blocked in 1994 and buoyant water did not even reach Cape Kaliakra on the Bulgarian coast. This difference was related to the shelfbreak processes. In 1992, the cyclonic meander of the Rim Current merged with the coastal buoyant water thus promoting its advection from Cape Kaliakra downstream. In 1994, a strong anticyclone in the southwestern corner of the Black Sea completely blocked the propagation of a buoyancy-driven current past Cape Kaliakra. In addition, another anticyclone in the northwestern part of the sea advected buoyant water offshore to the central area of the northwestern shelf. The positions of anticyclonic eddies during a period of observations was confirmed by remote sensing data. As these and other examples indicate, coastal buoyancy driven currents can be effectively blocked and dispersed offshore by the shelfbreak anticyclones if the shelf width allows their interaction with buoyant water. Previous observational studies suggested an upwelling-favorable wind as the principal forcing agent that arrested downstream penetration of buoyancy-driven coastal currents.

Eddy Formation and Interaction in a Baroclinic Frontal Geostrophic Model*

Abstract The authors investigate the behavior of buoyancy-driven coastal currents in a series of numerical experiments based on a two-layer frontal geostrophic model. The model focuses on baroclinic instability, allows for finite amplitude variations in the upper-layer thickness, and includes a topographic background vorticity gradient. Simulations of isolated fronts demonstrate meandering of the frontal outcropping, filamentation, and the development of both warm core and cold core eddies. Eddies can merge with each other, separate, or be reabsorbed into the current. Despite the assumption of only two layers, it is found that growth rates and length scales of the emergent features are in agreement with results of studies based on more sophisticated primitive equation models. It is determined that the cross-front topographic slope has a significant effect on the instability. In particular, a bottom that slopes in the same sense as the fluid interface hinders the growth of perturbations. Simulations with t...

The Summer Hydrography and Surface Circulation of the East Siberian Shelf Sea*

During the ice-free summer season in 1995 the authors deployed and subsequently tracked 39 surface drifters to test the hypothesis that the discharge from the Kolyma River forces a buoyancy-driven coastal current from the East Siberian Sea toward Bering Strait. The observed mean flow is statistically significant at the 95% level of confidence, but its direction contradicts their initial hypothesis. Instead of a coastally trapped eastward flow, the authors find a laterally sheared westward flow with maximum velocities offshore that correlate only weakly with the local winds. At a daily, wind-dominated timescale the drifter data reveal spatially coherent flows of up to 0.5 m s21. The Lagrangian autocorrelation scale is about 3 days and the Lagrangian eddy length scale reaches 40 km. This spatial scale exceeds the nearshore internal deformation radius by a factor of 3; however, it more closely corresponds to the internal deformation radius associated with the offshore ice edge. Bulk estimates of the horizontal mixing coefficient resemble typical values of isotropic open ocean dispersion at midlatitudes. Hydrographic observations and oxygen isotope ratios of seawater indicate a low proportion of riverine freshwater relative to sea ice melt in most areas of the East Siberian Sea except close to the Kolyma Delta. The observations require a reevaluation of the conceptual view of the summer surface circulation of the East Siberian Sea. Eastward buoyancy-driven coastal currents do not always form on this shelf despite large river discharge. Instead, ice melt waters of a retreating ice edge act as a line source of buoyancy that in 1995 forced a westward surface flow in the East Siberian Sea.

Penetration of Buoyant Coastal Discharge onto the Continental Shelf: A Numerical Model Experiment

Abstract Plumes of buoyant water produced by inflow from rivers and estuaries are common on the continental shelf. Typically they turn anticyclonically to flow alongshelf as buoyancy-driven coastal currents. During this passage, mixing with ambient shelf water gradually erodes the plume buoyancy so that its alongshelf penetration is finite. This paper addresses the extent of this penetration and how it is determined by fundamental dimensionless flow parameters. A three-dimensional numerical model is applied to an idealized flow regime. Ambient shelf conditions include tidal motion, but neither wind stress nor ambient alongshelf current. The alongshelf extent of penetration is evaluated after the plume reaches a stationary condition downshelf. A total of 66 model experiments are conducted, including variations in buoyant source and ambient shelf properties. Five dimensionless parameters determine the alongshelf and across-shelf penetration, the latter the coastal current width. The most critical of these i...

Transport and Recruitment of Blue Crab Larvae:a Model with Advection and Mortality

The present paper develops a mathematical model for the transport and recruitment of blue crab (Callinectes sapidus) larvae, and applies it to the inner continental shelf of the Middle Atlantic Bight near Delaware Bay, U.S.A. Blue crab larvae develop through seven or eight planktonic zoeal stages to a megalopa stage suitable for recruitment to adult populations of east coast estuaries. The larvae are concentrated near the surface, and the currents are primarily forced by alongshelf winds and river discharge through major estuaries. Model currents are prescribed based on a realistic synthesis of their observed relationship to wind and river discharge. Besides the resulting advection, particle diffusion and biological mortality are added to determine the fate of larvae released from their parent estuary. Groups of particles were released across the source region of the outflowing buoyancy-driven current in the model estuary mouth. Most larvae were swept alongshelf to the south with the buoyancy-driven coastal current, and thus were lost as recruits to the population of their parent estuary. However, some larvae released close to the seaward edge of the emerging coastal current were able to cross the coastal current front and move seaward into inner shelf water during upwelling-favorable (northward) wind events. Some of these, in turn, were suitably placed near the parent estuary mouth so that they could be advected landward as megalopae into the estuary during a subsequent downwelling-favorable (southward) wind event and thus join the adult population. The model results for megalopae returns were computed from consecutive daily release of 1000 particles, and were compared with 4 years of blue crab megalopa settlement data for Delaware Bay. The model results for 1989 and 1990 matched the observed data remarkably well, with both years showing dominance by a single return event of a few days duration. For 1991 and 1992, the observed results showed multiple return events, and the model results did not follow these well.

Subtidal currents from a shipboard acoustic Doppler current profiler in tidally dominated waters

As part of a comprehensive study of inner shelf dynamics in the vicinity of a major estuary, we test the performance of a shipboard acoustic Doppler current profiler (ADCP). We compare velocities from the ADCP with those from current meters located 6 and 10 m below the surface in water shallower than 30 m. We find the ADCP suitable to estimate subtidal currents of O(10 cm s −1) embedded in a tidal flow exceeding 100 cm s −1. We obtain the former currents by subtracting predicted tidal currents which we computed from empirical models. Three different models all result in similar subtidal flows; none of them appears superior. However, spatial and temporal averaging of the velocity data are necessary to reduce noise in the ADCP record to acceptable levels. Our subtidal flow estimates are reliable, robust and physically meaningful. We find that bottom friction influences the tidal currents over the entire water column. We also find a narrow, baroclinic, subtidal jet exiting the estuary on the right looking seaward. This jet is the source of the local buoyancy-driven coastal current.

The stability of buoyancy-driven coastal currents

Griffiths, R.W. and Linden, P.F., 1981. The stability of buoyancy-driven coastal currents. Dyn. Atmos. Oceans, 5: 281--306. Buoyancy-driven boundary currents were generated in the laboratory by releasing buoyant fluid from a source adjacent to a vertical boundary in a rotating container. The boundary removed the Coriolis force parallel to it, allowing the buoyant fluid to spread in a current along the boundary. Use of a cylindrical boundary and a line source that released fluid uniformly around the circumference enabled an axisymmetric (zonal) current to be produced. With the continuous release of fluid from the source, the current grew in width and depth until it became unstable to non-axisymmetric disturbances. The wavelength and phase velocities of the disturbances were consistent with a model of baroclinic instability of two-layer flow when frictional dissipation due to Ekman layers is included. However, when the current only occupied a small fraction of the total depth, barotropic processes were also thought to be important, with the growing waves gaining energy from the horizontal shear. In other experiments, gravity currents were produced by a point source adjacent to either a zonal (circular) or a meridional (radial) vertical boundary. The currents were also observed to become unstable to the same upstream breaking waves as those on the continuous zonal current. Finally, some comparisons are made with oceanic coastal currents.

The stability of buoyancy-driven coastal currents : Griffiths, R.W. and P.F. Linden, 1981. Dynam. Atmos. Oceans, 5(4):281–306

Griffiths, R.W. and Linden, P.F., 1981. The stabil i ty of buoyancy-dr iven coastal currents. Dyn. Atmos . Oceans, 5: 281--306. Buoyancy-dr iven boundary currents were generated in the labora tory by releasing buoyan t fluid f rom a source adjacent to a vertical boundary in a rota t ing container . The boundary removed the Coriolis force parallel to it, a l lowing the buoyan t fluid to spread in a current along the boundary . Use of a cylindrical boundary and a line source that released fluid un i formly around the c i rcumference enabled an ax isymmetr ic (zonal) current to be produced. With the con t inuous release of fluid f rom the source, the current grew in width and depth unti l it became unstable to non-ax isymmetr ic disturbances. The wavelength and phase velocit ies o f the disturbances were consistent wi th a mode l of barocl inic instabil i ty of two-layer f low when fr ict ional dissipation due to Ekman layers is included. However , when the current only occupied a small f ract ion of the total depth, baro t ropic processes were also thought to be impor tan t , with the growing waves gaining energy f rom the hor izonta l shear. In o ther exper iments , gravity currents were produced by a poin t source adjacent to ei ther a zonal (circular) or a meridional (radial) vertical boundary . The currents were also observed to become unstable to the same upst ream breaking waves as those on the cont inuous zonal current . Finally, some compar isons are made with oceanic coastal currents. 1. I N T R O D U C T I O N Buoyancy-driven coastal currents occur in many parts of the world oceans. A particularly striking example is the East Greenland Current in which cold, fresh polar water flows southward from the Arctic Ocean along the east coast of Greenland. It occupies a wedge-shaped region some 200 km in width, with the maximum depth (200 m) at the coast. The primary driving force appears to be the density difference between the polar water and the water of the Norwegian Sea. Where land masses are not present, the southward spreading of the less dense polar water is inhibited by the effects of the Earth's rotation. Southward motion induces an east--west flow to conserve angular momentum, and this flow, in turn, produces a north--south Coriolis force which opposes the buoyancy force. Consequently, further southward motion is inhibited. However, at a meridional barrier such as the 0 3 7 7 0 2 6 5 / 8 1 / 0 0 0 0 0 0 0 0 / $ 0 2 . 5 0 © 1981 Elsevier Scient if ic Publishing Company