Laurence Armi

Maximal two-layer exchange over a sill and through the combination of a sill and contraction with barotropic flow

The analysis of two-layer exchange flow through contractions with a barotropic component treated by Armi & Farmer (1986) is extended to include exchange flows over sills and through a combination of a sill and contraction. It is shown that exchange over a sill is fundamentally different from exchange through a contraction. Control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer. This asymmetry in the control results in asymmetrical flows. The interface depth above the crest is not one half the total depth, as assumed in other studies by analogy with flow through contractions, but is somewhat deeper; the maximal exchange rate is less than for flow through a contraction of equal depth. When both a sill and a contraction are present, the contraction influences control at the sill crest only if it lies between the sill and the source of denser water. The response to barotropic flow is also asymmetrical: the transition to single-layer flow occurs at much lower speeds for a barotropic component in one direction than the other. Results of the analysis are applied to exchange flow through the Strait of Gibraltar, which includes both a sill and a contraction. It is shown that maximal exchange conditions apply throughout part of the tidal cycle, and observations illustrate several of the analytical predictions for barotropic flows, including the formation of fronts, single-layer flow, submaximal exchange and reverse flow.

The hydraulics of two flowing layers with different densities

This is a theoretical and experimental study of the basic hydraulics of two flowing layers. Unlike single-layer flows, two-layer flows respond quite differently to bottom depth as opposed to width variations. Bottom-depth changes affect the lower layer directly and the upper layer only indirectly. Changes in width can affect both layers. In fact for flows through a contraction control two distinct flow configurations are possible; which one actually occurs depends on the requirements of matching a downstream flow. Two-layer flows can pass through internally critical conditions at other than the narrowest section. When the two layers are flowing in the same direction, the result is a strong coupling between the two layers in the neighbourhood of the control. For contractions a particularly simple flow then exists upstream in which there is no longer any significant interfacial dynamics; downstream in the divergent section the flow remains internally supercritical, causing one of the layers to be rapidly accelerated with a resulting instability at the interface. A brief discussion of internal hydraulic jumps based upon the energy equations as opposed to the more traditional momentum equations is included. Previous uniqueness problems are thereby avoided.

Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

Abstract Detailed observations of the structure within internal solitary waves propagating shoreward over Oregons continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By as...

Two Years in the Life of a Mediterranean Salt Lens

Abstract A lens of Mediterranean water (Meddy) was tracked in the eastern North Atlantic for two years with SOFAR floats. The Meddy was first found between the Canary Islands and the Azores in October 1984. It center moved in an irregular pattern, at speeds of a few cm s−1, and translated 1100 km to the south in two years. This Meddy was surveyed four times by CTD and velocity profilers, and once with the microstructure profiler EPSONDE. When observed during the first two surveys the Meddy had a core that was stably and smoothly stratified in both salinity and temperature, nearly uniform in the horizontal, and was saltier than the surrounding ocean by 0.65 psu. The Meddy was eroded from its edges, top and bottom, and lost salt and hat with an e-folding time of about one year. The salinity at the center remained at its original value during the first year and decreased during the second year. Evidence was seen for mixing by lateral intrusions, double diffusion, and turbulence; the intrusions are thought to...

The flow of Atlantic water through the Strait of Gibraltar

Abstract We describe and analyze observations of the water exchange through the Strait of Gibraltar. The primary observations were taken in April 1986 and included data from moored recording instruments at four locations in the strait, together with an intensive survey by ship. These measurements included extensive CTD profiling, acoustic Doppler current profiling, numerous profiles from expendable instruments, and imaging using a high-frequency echo sounder. The analysis focuses on the internal hydraulics of the Strait and in particular the presence of hydraulic controls and their influence on the exchange. Our observations during April 1986 show that the maximal exchange condition, in which a subcritical flow is bounded by supercritical flow at both ends of the Strait, did occur, although with various subtleties not explicitly incorporated in our previous theoretical developments. The Atlantic water, moving east along the surface, encounters a control at Tarifa Narrows in the eastern part of the Strait. The Mediterranean water passes through controls both at Camarinal Sill and at Spartel Sill, further west. At our westernmost mooring at Spartel, the outflowing Mediterranean water is continuously supercritical, with little tidal variability. The control at Camarinal Sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide. Tangier Basin, bounded by Camarinal and Spartel Sills, acts as an internal reservoir for the outflowing Mediterranean water, the interface rising and falling through each tidal cycle. During spring tides, the Tangier Basin interface rises high enough to flood the control at Camarinl Sill and produces a reverse flow of the lower layer. When control is lost at Camarinal Sill a travelling bore is released. This bore may modify the location of the control acting on the surface layer in Tarifa Narrows. The Atlantic water enters the Alboran Sea as a jet which may have a well-defined northern boundary where it separates from the European coast; this boundary may be identified by the presence of a slick. Although the internal jump west of Camarinal Sill is almost certainly an area of intense mixing, frictional effects do not appear to be dominant in the subcritical portion of the flow.

Large Lenses of Highly Saline Mediterranean Water

Isolated compact anticyclonic eddies or salt lenses were found in the Canary Basin. Hydrographic surveys of three such lenses show large anomalies of salinity and temperature (∼0.8, 2.5°C). They are centered at ∼1100 m, have a vertical extent of up to 900 m and radii of ∼50 km. Current meter records indicate anticyclonic velocities up to 29 cm s−1. Fine structure with vertical scales of ∼20 m and less, possibly due to intrusive decay, appears at the outer edges of the lenses whereas the centers are free of such structure. The probability of finding a salt lens at any station in the Canary Basin is fairly high (∼0.08).

Tracking Three Meddies with SOFAR Floats

Abstract Three Meddies were tracked for up to two years in the Canary Basin using neutrally buoyant SOFAR floats. These Meddies have cores of warm, salty Mediterranean Water and are approximately 100 km in diameter, 800 m thick, and are centered at a depth of 1100 m. Meddy 1 was tracked for two years (1984–86) with five floats as it drifted 1090 km southward with a mean velocity of 1.8 cm s−1. Four shipboard surveys made during these two years revealed the nearly total decay of Meddy 1 by gradual mixing processes. Meddy 2 drifted 530 km southwestward over 8.5 months with a mean velocity of 2.3 cm s−1 until it collided with Hyeres Seamount near 31°N, 29°W. The floats trapped in this Meddy then stopped looping abruptly, implying a major disruption of this Meddy. Meddy 3 drifted 500 km southwestward for a year and a half with a mean translation velocity of 1.1 cm s−1. A comparison of the velocity of Meddies to the velocity of nearby floats at 1100 m depth outside of the Meddies shows clearly that all three M...

Spirals on the sea

Spiral eddies were first seen in the sunglitter on the Apollo Mission 30 years ago; they have since been recorded on synthetic aperture radar (SAR) images and in the infrared. We present a small sample of images. The spirals are broadly distributed over the worlds oceans, 10–25 km in size and overwhelmingly cyclonic. Under light winds favourable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. The linear features are wound into spirals in vortices associated with horizontal shear instability, modified by rotation, in regions where the shear is comparable with the Coriolis frequency. Two models for concentrating shear are presented: a softened version of the classical sharp Margules front, and the time–dependent Lagrangian model of Hoskins & Bretherton. Horizontal shear instabilities and both frontal models favour cyclonic shear and cyclonic spirals, but for different reasons.

Lagrangian Observations of Meddy Formation during A Mediterranean Undercurrent Seeding Experiment

Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediterranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent. Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15‐20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3‐7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast. The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D’Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.

Stratified flow over topography: the role of small-scale entrainment and mixing in flow establishment

Stratified flow over topography is examined in the context of its establishment from rest. A key element of numerical and steady–state analytical solutions for large amplitude topographic flow is the splitting of streamlines, which then enclose a trapped wedge of mixed fluid above the rapidly moving deeper layer. Measurements have been acquired that illustrate the development of this wedge and the role played by small–scale instabilities and mixing formed initially by the acceleration of subcritical stratified flow over the obstacle crest. The volume of trapped fluid progressively increases with time, permitting the primary flow to descend beneath it over the lee face of the obstacle. Throughout the evolution of this flow, small–scale instability and consequent entrainment would seem to be a prime candidate for producing the weakly stratified wedge, thus allowing establishment of the downslope flow to take place. Velocity structure of instabilities within the entrainment zone is observed and the associated entrainment rate determined. The entrainment is sufficient to produce a slow downstream motion within the upper layer and a density step between the layers that decreases with downstream distance. The resulting internal hydraulic response is explained in terms of a theory that accommodates the spatially variable density difference across the sheared interface. The measurements described here were acquired in a coastal inlet subject to gradually changing tidal currents. It is proposed that the observed mechanism for flow establishment also has application to atmospheric flow over mountains.