Frontal Flows Research Articles

The Annual Cycle of Upper-Ocean Potential Vorticity and Its Relationship to Submesoscale Instabilities

AbstractThe evolution of upper-ocean potential vorticity (PV) over a full year in a typical midocean area of the northeast Atlantic is examined using submesoscale- and mesoscale-resolving hydrographic and velocity measurements from a mooring array. A PV budget framework is applied to quantitatively document the competing physical processes responsible for deepening and shoaling the mixed layer. The observations reveal a distinct seasonal cycle in upper-ocean PV, characterized by frequent occurrences of negative PV within deep (up to about 350 m) mixed layers from winter to mid-spring, and positive PV beneath shallow (mostly less than 50 m) mixed layers during the remainder of the year. The cumulative positive and negative subinertial changes in the mixed layer depth, which are largely unaccounted for by advective contributions, exceed the deepest mixed layer by one order of magnitude, suggesting that mixed layer depth is shaped by the competing effects of destratifying and restratifying processes. Deep mixed layers are attributed to persistent atmospheric cooling from winter to mid-spring, which triggers gravitational instability leading to mixed layer deepening. However, on shorter time scales of days, conditions favorable to symmetric instability often occur as winds intermittently align with transient frontal flows. The ensuing submesoscale frontal instabilities are found to fundamentally alter upper-ocean turbulent convection, and limit the deepening of the mixed layer in the winter-to-mid-spring period. These results emphasize the key role of submesoscale frontal instabilities in determining the seasonal evolution of the mixed layer in the open ocean.

Using surface flow image velocimetry to analyse flow approaching grated inlets

This paper describes the use of surface flow image velocimetry (SFIV) to analyse flow velocity fields near grated inlets. This will help to understand the flow-capture process of grated inlets and improve their design. The technique allows approximation of the surface flow velocity field where other techniques can fail. Especially in cases when the flow near the inlet is supercritical, and with small depths, the introduction of other devices to measure velocity fields can dramatically modify the flow pattern around the gauge and lead to unreliable measurements. SFIV provides a non-intrusive way of estimating flow velocities in the area covered by the images. Full-scale experiments were carried out using a high-resolution camera and commercial codes to characterise the flow patterns approaching grated inlets commonly used in Barcelona, Spain. The results from image processing algorithms were used to differentiate frontal and lateral flows reaching the inlets.

On the Dynamics of Canyon–Flow Interactions

This paper explores the dynamical origin and physical characteristics of flow disturbances induced by ocean currents in interaction with shelf-incised submarine canyons. To this end, a process-oriented hydrodynamic model is applied in a series of case studies. The focus of studies is the canyon-upwelling process in which seawater is moved from the upper continental slope onto the shelf within a shelf-break canyon. Results reveal that the generation of canyon upwelling, to zero-order approximation, is a barotropic and friction-independent quasi-geostrophic process. Hence, the principle of conservation of potential vorticity for such flows is sufficient to explain the fundamental physical properties of the canyon-upwelling process. For instance, this principle explains the direction-dependence of the canyon-upwelling process. This principle also explains the formation of stationary topographic Rossby waves downstream from the canyon that can lead to far-field effects. Density effects, being of secondary influence to the canyon-upwelling process, result in the intensification of canyon-upwelling flows via the formation of narrow near-bottom density fronts and associated baroclinic geostrophic frontal flows. Findings of this work reveal that the apparently complex canyon-upwelling process is much more basic than previously thought.

Patchiness and depth-keeping of copepods in response to simulated frontal flows

Patchiness and depth-keeping of copepods in response to simulated frontal flows

Variability of mesozooplankton spatial distribution in the North Aegean Sea, as influenced by the Black Sea waters outflow

The North Aegean Sea constitutes an important region of the Mediterranean Sea since in its eastern part the mesotrophic, low salinity and relatively cold water from the Black Sea (outflowing from the Dardanelles strait) meets the oligotrophic, warm and very saline water of Levantine origin, thus forming a thermohaline front. Mesozooplankton samples were collected at discrete layers according to the hydrology of the upper 100 m, during May 1997 and September 1998. In May highest biomass and abundance values (up to 66.82 mg m − 3 and 14,157 ind m − 3 ) were detected in the 10–20 m layer (within the halocline) of the stations positioned close to the Dardanelles strait. The front moved slightly southwards in September, characterized by high biomass and abundance values within the halocline layer. The areas moderately or non influenced by Black Sea water revealed lower standing stock values than the frontal area in both cruises and maxima were detected in the uppermost low salinity layer. Samples collected at the stations and/or layers more influenced by Black Sea water were distinguished from those collected at layers and/or stations more affected by Levantine waters in both periods. In May the former samples were characterized by the copepods Acartia clausi, Centropages typicus, Paracalanus parvus. The abundance of the above species decreased gradually with increasing salinity, in the horizontal and/or in the vertical dimension, with a parallel increase of the copepods Oithona plumifera, Oithona copepodites, Oncaea media, Ctenocalanus vanus, Farranula rostrata. During September the frontal area as well as that covered by the modified Black Sea water, were highly dominated by the cladoceran Penilia avirostris and doliolids. For both seasons, MDS plots, issued from the combination of mesozooplankton and water-type data, revealed the gradual differentiation of zooplankton composition from the frontal area towards the area covered by Levantine water, following the spreading and mixing of the Black sea water. The observed temporal and spatial variability in the distribution pattern of mesozooplankton standing stock and species composition seems to depend considerably on the variability of circulation and frontal flows.

Cyclogenesis in the deep ocean beneath Western Boundary Currents: A process‐oriented numerical study

A two‐layer shallow‐water equation model is applied to a flat‐bottom ocean on the f plane to explore instability mechanisms in Western Boundary Current (WBC) that lead to the formation of strong cyclones in the deep ocean underneath. Findings reveal a tight coupling of surface meandering and deep cyclogenesis, in agreement with observational evidence. Barotropic cyclones develop in timescales of 5–10 days and attain swirl speeds of >50 cm/s (depends on initial strength of WBC) on a diameter of ∼100 km. Cyclogenesis is driven by advection of relative vorticity in the surface ocean and failure of the thermocline to respond rapidly enough to the associated sea level variations. Findings suggest that cyclogenesis and the associated strong abyssal flows (benthic storms) are ubiquitous features of WBCs and other frontal flows.

The theory of quasi-geostrophic von Kármán vortex streets in two-layer fluids on a beta-plane

In this study self-organized periodic coherent vortex structures arising in geophysical turbulent flows at low Rossby number are investigated by developing a conceptual model based on an analytical theory of von Kármán vortex streets affected by stratification and differential rotation. In the framework of a quasi-geostrophic (QG) two-layer beta-plane model vortex streets with three different types of vertical structures (barotropic, upper layer and hetonic) are analysed using the point vortex approximation. The streets are found to be exact solutions of the potential vorticity equation and to be characterized by four non-dimensional parameters. Von Kármán streets are semi-localized solutions which form a bridge between vortex pairs (limit of symmetric dilute streets) and two parallel vortex sheets (limit of dense streets). On the beta-plane QG von Kármán streets can only move to the east, i.e. with a speed outside the range of speeds of Rossby waves, so that a dynamical asymmetry in the zonal direction is introduced. A complete classification on a diagram of states shows that critical bounds exist in the parameter space, prescribing for example a maximum distance between vortex rows beyond which no QG vortex streets can be found. Typically a fast and a slow vortex street with different flow structures are found in the region of existence. As a function of distance between vortex rows baroclinic QG vortex streets show a characteristic non-monotonic speed behaviour at scales of the order of the baroclinic Rossby radius. A wide region of possible existence of QG von Kármán streets is found in atmospheric, oceanic and planetary conditions as well as in rotating tank experiments. The theory can be applied to describe the coherent part of turbulent baroclinic intermittent zonal jet-like and frontal flows and provides a scaling for such flows.

Glacial–interglacial (MIS 1–10) migrations of the Subtropical Front across ODP Site 1119, Canterbury Bight, Southwest Pacific Ocean

Ocean Drilling Program (ODP) Site 1119 is located at water depth 395 m near the subtropical front (STF; here represented by the Southland Front), just downslope from the shelf edge of eastern South Island, New Zealand. The upper 86.19 metres composite depth (mcd) of Site 1119 sediment was deposited at an average sedimentation rate of 34 cm/kyr during Marine Isotope Stages (MIS) 1–8 (0–252 ka), and is underlain across a ∼25 kyr intra-MIS 8 unconformity by MIS 8.5–11 (277–367 ka) and older sediment deposited at ∼14 cm/kyr. A time scale is assigned to Site 1119 using radiocarbon dates for the period back to ∼39 ka, and, prior to then, by matching its climatic record with that of the Vostok ice core, which it closely resembles. Four palaeoceanographic proxy measures for surface water masses vary together with the sandy-muddy, glacial–interglacial (G/I) cyclicity at the site. Interglacial intervals are characterised by heavy δ 13C, high colour reflectance (a proxy for carbonate content), low γ-ray (a proxy for clay content) and light δ 18O; conversely, glacial intervals exhibit light δ 13C, low reflectance, high γ-ray and heavy δ 18O signatures. Early interglacial intervals are represented by silty clays with 10–105-cm-thick beds of sharp-based ( Chondrites-burrowed), shelly, graded, fine sand. The sands are rich in foraminifera, and were deposited distant from the shoreline under the influence of longitudinal flow in relatively deep water. Glacial intervals comprise mostly micaceous silty clay, though with some thin (2–10 cm thick) sands present also at peak cold periods, and contain the cold-water scallop Zygochlamys delicatula. Interglacial sandy intervals are characterised by relatively low sedimentation rates of 5–32 cm/kyr; cold climate intervals MIS 10, 6 and 2 have successively higher sedimentation rates of 45, 69 and 140 cm/kyr. Counter-intuitively, and forced by the bathymetric control of a laterally-moving shoreline during G/I and I/G transitions, the 1119 core records a southeasterly (seaward) movement of the STF during early glacial periods, accompanied by the incursion of subtropical water (STW) above the site, and northwesterly (landward) movement during late glacial and interglacial times, resulting in a dominant influence then of subantarctic surface water (SAW). The history of passage of these different water masses at the site is clearly delineated by their characteristic δ 13C values. The intervals of thin, graded sands–muds which occur within MIS 2–3, 6, 7.4 and 10 indicate the onset at times of peak cold of intermittent bottom currents caused by strengthened and expanded frontal flows along the STF, which at such times lay near Site 1119 in close proximity to seaward-encroaching subantarctic waters within the Bounty gyre. In common with other nearby Southern Hemisphere records, the cold period which represents the last glacial maximum lasted between ∼23–18 ka at Site 1119, during which time the STF and Subantarctic Front (SAF) probably merged into a single intense frontal zone around the head of the adjacent Bounty Trough.

The complete classification of large-amplitude geostrophic flows in a two-layer fluid

Abstract We examine two-layer geostrophic flows over a flat bottom on the β-plane. If the displacement of the interface is of the order of the depth of the upper layer, the dynamics of the flow depends on the following non-dimensional parameters: (i) the Rossby number ∊, (ii) the ratio δ of the depth of the upper layer to the total depth of the fluid, (iii) the “β-effect number” α = Ro/Re cot θ, where Ro is the deformation radius, Re, is the earth's radius and θ is the latitude. In this paper We derive four sets of asymptotic equations which cover the parameter space (∊ ≪ 1,α,δ). In order to find out, which asymptotic regimes are relevant to the real ocean, we estimate ∊, δ and α a number of frontal flows in the Northern Pacific and Southern oceans We also discuss the stability properties of large-amplitude geostrophic flows and classify them in the (∊, α, δ)

Domain-Independent Attribution. Part II: Its Value in the Verification of Dynamical Theories of Frontal Waves and Frontogenesis

Abstract Theories of frontogenesis and frontal waves describe development in terms of the interaction of a basic state or environmental flow with a frontal flow. The basic-state flow may comprise a large-scale confluent–diffluent deformation field and/or an alongfront temperature gradient. The frontal flow is seen as evolving as a result of its interaction with the environmental flow. Such theories make specific predictions about the effect of the basic-state flow on the frontal flow. To test these predictions, counterparts of the basic-state flows and frontal flows used in theoretical models must be extracted from atmospheric data. Here the concept of attribution is used to identify such counterparts. In the present context, attribution refers to the process whereby a part of the wind field is attributed to a part of the vorticity or divergence field. It is mathematically equivalent to the process by which a part of a field of electric potential is associated with an element of total charge density in el...

Stability of Steady Fronts with Uniform Potential Vorticity

Abstract The linear stability of steady frontal zones is considered using the primitive equations. The frontal zones chosen have uniform potential vorticity and form a sequence of “snapshots” of the deformation-induced front as it evolves toward frontal collapse. The stability for a given alongfront wavelength is determined by integrating a numerical model forward from an initial condition of the basic-state front plus white noise. Two classes of instabilities emerge. In one class, the modes are modified versions of familiar, synoptic-scale baroclinic waves. The other class consists of Kelvin–Helmholtz (KH) instabilities. These modes have wavelengths of order 1 km and exist only when the cross-frontal scale is of order 10 km or smaller. With e-folding times of a few minutes, the growth of KH waves may limit the cross-frontal scale of active, time-dependent frontal flows, although boundary layer processes are probably important, at least for surface fronts, before KH modes appear. No other instabilities ar...

An Explanation for Intense Frontal Updrafts and Narrow Cold-Frontal Rainbands

Abstract Measurements with Doppler radar, and instrumented aircraft and towers, have revealed that surface cold fronts often have cross-frontal circulations organized on a scale of a kilometer or less. These circulations include intense updrafts (1 to 20 m s−1) that result in a narrow band of heavy rainfall. We used a nonhydrostatic model to investigate the mechanism for these updrafts and to isolate those characteristics of the prefrontal environment that result in intense updrafts and narrow bands of heavy rainfall. Our simulations were initialized with a cold reservoir in a manner analogous to that used to produce a gravity current. The similarity between the observations and our simulated frontal flows supports the hypothesis that the flow at the leading edge of surface cold fronts can sometimes be represented by gravity-current dynamics. We also found that the differences between frontal circulations and classical dry gravity currents can be explained by the effects of precipitation and vertical shea...

A note on the use of zero potential vorticity models

Abstract The acceptability of zero potential vorticity models as approximations for natural systems of small, but finite, potential vorticity is studied for bounded frontal flows of arbitrary profile. It is demonstrated that all (infinitely) long-wave solutions of the zero potential vorticity front are asymptotic limits for some (not necessarily long-wave) solutions of the small potential vorticity front. In contrast, for downstream-varying solutions there is no simple way of demonstrating this property. These findings suggest that the use of zero potential vorticity models should be carefully examined in other, non-frontal, problems as well. Finally we show that the longwave solutions of the zero potential vorticity flow are at most neutral (quasi-stable).

Regional cerebral metabolic activity in chronic schizophrenia

Results from measurements of regional cerebral blood flow (rCBF) by 133Xe inhalation are presented for neuropathologically proven cases of frontal lobe degeneration of non-Alzheimer type, Pick's disease, Alzheimer's and Creutzfeldt-Jakob's diseases with frontal predominance and one case of bilateral thalamic infarction. The pathological and neuropsychiatric findings in these cases were described in the preceding articles. rCBF-data were available in nine patients with the diagnosis of frontal lobe degeneration of non-Alzheimer type and in all four patients with Pick's disease. All 13 patients showed a pathologic flow pattern with focally reduced values in frontal, especially pre-frontal, areas of both hemispheres. The most pronounced focal pathology was seen in the Pick group. Post-central areas were typically better preserved. Two cases of Alzheimer's disease with frontal accentuation had, in addition to low frontal values, also a parietal focal reduction. Frontal flows were also reduced in two cases of Creutzfeldt-Jakob's disease while the hypofrontality fluctuated from one study to another in the case of thalamic infarcts. The extent and localization of cerebral dysfunction in patients with organic dementing disorders, including those affecting the frontal lobes, can thus be described by means of rCBF measurements. Previous studies have shown about 90% accuracy of diagnosis by rCBF compared to diagnosis at autopsy. The possibility for exact localization of the dysfunction has recently been improved by the development of a high-resolution system (254 detectors). Results from the first recordings with this system are presented.

Frontal lobe degeneration of non-Alzheimer type. III. Regional cerebral blood flow

Results from measurements of regional cerebral blood flow (rCBF) by 133Xe inhalation are presented for neuropathologically proven cases of frontal lobe degeneration of non-Alzheimer type, Pick's disease, Alzheimer's and Creutzfeldt-Jakob's diseases with frontal predominance and one case of bilateral thalamic infarction. The pathological and neuropsychiatric findings in these cases were described in the preceding articles. rCBF-data were available in nine patients with the diagnosis of frontal lobe degeneration of non-Alzheimer type and in all four patients with Pick's disease. All 13 patients showed a pathologic flow pattern with focally reduced values in frontal, especially pre-frontal, areas of both hemispheres. The most pronounced focal pathology was seen in the Pick group. Post-central areas were typically better preserved. Two cases of Alzheimer's disease with frontal accentuation had, in addition to low frontal values, also a parietal focal reduction. Frontal flows were also reduced in two cases of Creutzfeldt-Jakob's disease while the hypofrontality fluctuated from one study to another in the case of thalamic infarcts. The extent and localization of cerebral dysfunction in patients with organic dementing disorders, including those affecting the frontal lobes, can thus be described by means of rCBF measurements. Previous studies have shown about 90% accuracy of diagnosis by rCBF compared to diagnosis at autopsy. The possibility for exact localization of the dysfunction has recently been improved by the development of a high-resolution system (254 detectors). Results from the first recordings with this system are presented.

Oceanic fronts and their effects on underwater acoustics

Oceanic fronts are dynamical phenomena with many properties in common with atmospheric fronts. Fronts are the locus of intense horizontal gradients in one or more properties. Oceanic fronts are generated by atmospheric forcing (momentum, heat, and mass transfer), river runoff, and differential advection and mixing. Mixing is intense in fronts; thus, persistent fronts must be maintained through convergent flows. Typically, fronts have strong thermal and haline gradients. Depending upon the relative signs of these gradients, thermohaline fronts are “density-reinforced fronts” (or, simply “density fronts”) or “density-compensated fronts.” These two classes of fronts differ in their dynamics and, thus, flow fields. Sound speed fronts accompany thermal fronts, which leads to strong acoustic refraction across oceanic fronts. Because of the convergent flow, organic and inorganic particles are concentrated at fronts, leading to aggregation of phytoplankton, zooplankton, fish, and marine mammals and birds. Hence, frontal zones are rich in biological scatterers and noise. The microstructure of fluid velocity, temperature, salinity, and thus, sound velocity is intense in frontal zones, which also enhances volume scattering. Due to gravity wave refraction and breaking by frontal flows, marked sea state variations occur across fronts, producing inhomogeneities in surface scattering and ambient noise.