Frontal Dynamics Research Articles

Frontal dynamics of powder snow avalanches

We analyze frontal dynamics of dilute powder snow avalanches sustained by rapid blow‐out behind the front. Such material injection arises as a weakly cohesive snow cover is fluidized by the very pore pressure gradient that the particle cloud induces within the snowpack. We model cloud fluid mechanics as a potential flow consisting of a traveling source of denser fluid thrust into a uniform airflow. Stability analysis of a mass balance involving snow cover and powder cloud yields relations among scouring depth, frontal height, speed, mixed‐mean density, and impact pressure when the frontal region achieves a stable growth rate. We compare predictions with field measurements, show that powder clouds cannot reach steady frontal speed on a uniform snowpack with constant cloud width and derive a criterion for cloud ignition. Because static pressure is continuous across the mean air‐cloud interface and deviatoric stresses are negligible, frontal acceleration is insensitive to local slope, but instead arises from a deficit of flow‐induced suction in the wake. We calculate how far a powder cloud travels until its frontal mixed‐mean density becomes stable, and show how topographic spread can hasten its collapse.

Relative Contributions of Synoptic and Low-Frequency Eddies to Time-Mean Atmospheric Moisture Transport, Including the Role of Atmospheric Rivers

The relative contributions to mean global atmospheric moisture transport by both the time-mean circulation and by synoptic and low-frequency (periods greater than 10 days) anomalies are evaluated from the vertically integrated atmospheric moisture budget based on 40 yr of “chi corrected” NCEP–NCAR reanalysis data. In the extratropics, while the time-mean circulation primarily moves moisture zonally within ocean basins, low-frequency and synoptic anomalies drive much of the mean moisture transport both from ocean to land and toward the poles. In particular, during the cool-season low-frequency variability is the largest contributor to mean moisture transport into southwestern North America, Europe, and Australia. While some low-frequency transport originates in low latitudes, much is of extratropical origin due to large-scale atmospheric anomalies that extract moisture from the northeast Pacific and Atlantic Oceans. Low-frequency variability is also integral to the Arctic (latitudes > 70°N) mean moisture budget, especially during summer, when it drives mean poleward transport from relatively wet high-latitude continental regions. Synoptic variability drives about half of the mean poleward moisture transport in the midlatitudes of both hemispheres, consistent with simple “lateral mixing” arguments. Extratropical atmospheric transport is also particularly focused within “atmospheric rivers” (ARs), relatively narrow poleward-moving moisture plumes associated with frontal dynamics. AR moisture transport, defined by compositing fluxes over those locations and times where column-integrated water vapor and poleward low-level wind anomalies are both positive, represents most of the total extratropical meridional moisture transport. These results suggest that understanding potential anthropogenic changes in the earth ’s hydrological cycle may require understanding corresponding changes in atmospheric variability, especially on low-frequency time scales.

Bringing physics to life at the submesoscale

A common dynamical paradigm is that turbulence in the upper ocean is dominated by three classes of motion: mesoscale geostrophic eddies, internal waves and microscale three‐dimensional turbulence. Close to the ocean surface, however, a fourth class of turbulent motion is important: submesoscale frontal dynamics. These have a horizontal scale of O(1–10) km, a vertical scale of O(100) m, and a time scale of O(1) day. Here we review the physical‐chemical‐biological dynamics of submesoscale features, and discuss strategies for sampling them. Submesoscale fronts arise dynamically through nonlinear instabilities of the mesoscale currents. They are ephemeral, lasting only a few days after they are formed. Strong submesoscale vertical velocities can drive episodic nutrient pulses to the euphotic zone, and subduct organic carbon into the ocean's interior. The reduction of vertical mixing at submesoscale fronts can locally increase the mean time that photosynthetic organisms spend in the well‐lit euphotic layer and promote primary production. Horizontal stirring can create intense patchiness in planktonic species. Submesoscale dynamics therefore can change not only primary and export production, but also the structure and the functioning of the planktonic ecosystem. Because of their short time and space scales, sampling of submesoscale features requires new technologies and approaches. This paper presents a critical overview of current knowledge to focus attention and hopefully interest on the pressing scientific questions concerning these dynamics.

Habitat partitioning by fish larvae among coastal, offshore and frontal zones in the southern North Sea

This study compared marine and estuarine larval fish assemblages among near- shore, offshore and frontal habitats of the southern North Sea. In late June and early July 2005, parallel cruises collected larvae along offshore transects crossing tidal mixing and river plume frontal zones in the German Bight, and at shallow nearshore stations within the Wadden Sea and associated estuaries of major tributaries. A mixture of larvae from 17 species occurred including the (then) newly re-established 'southern' clupeids (European sardine Sardina pilchardus and anchovy Engraulis encrasicolus) as well as traditional North Sea species such as horse mackerel Trachurus trachurus, solenette Buglossidium luteum and various species of gobies. Multivariate statistics on taxonomic composition indicated that shallow and deep areas were 2 distinct habitats. Wadden Sea stations had a conspicuously different species composition and low species richness compared to all deeper areas. In deeper waters, species composition was similar regardless of frontal strength, and larger larvae occurred most frequently in the cooler areas of the German Bight. Altered species composition observed at some nearshore stations indicated that frontal dynamics and tidal advection may lead to expatriation of larval fish to presumably less favorable areas. Frontal habitats in the German Bight may not necessarily promote higher survival of larval fish than other areas but appear to form important boundaries between species-rich, deeper waters of the German Bight and relatively species-poor, shallower waters of the Wadden Sea.

The Subpolar Front of the Japan/East Sea. Part III: Competing Roles of Frontal Dynamics and Atmospheric Forcing in Driving Ageostrophic Vertical Circulation and Subduction

ABSTRACT The effects of wind stress and surface cooling on ageostrophic vertical circulation and subduction at the subpolar front of the Japan/East Sea are investigated using a nonhydrostatic numerical model. In experiments forced by wind and/or cooling, ageostrophic vertical circulation is enhanced relative to the unforced case. Both surface cooling and wind stress intensify the circulation by enhancing frontogenesis associated with frontal meandering. Winds further strengthen vertical motions by generating internal gravity waves. Downfront winds (i.e., oriented along the frontal jet) transport surface water from the denser to lighter side of the front, causing it to migrate toward the region of higher stratification and enhancing the vertical mixing at the front. This induces outcropping of isopycnals from the middle of the pycnocline along which surface water is subducted. Hence downfront winds enhance subduction down to the middle of the pycnocline, but not beneath. On the other hand, cooling uplifts isopycnals from greater depths to the surface so that it allows for the subduction of fluid to greater depths. In contrast to the vertical circulation, frontal subduction is more intensified by surface cooling than wind stress, because part of wind-forced circulation (e.g., internal gravity wave) does not contribute to subduction. Ageostrophic vertical circulation and frontal subduction are most intense when both wind stress and surface cooling are at play.

Simulating the dynamics and intensification of cyclonic Loop Current Frontal Eddies in the Gulf of Mexico

The dynamics associated with the Loop Current (LC) variability in the Gulf of Mexico (GoM) are studied using a 5‐year, free‐running numerical simulation with the Hybrid Coordinate Ocean Model (HYCOM). The dynamics of major GoM circulation features are represented: the extension of the LC and the associated anticyclonic, warm core Loop Current Eddies (LCEs) and cyclonic Loop Current Frontal Eddies (LCFEs). The study focuses on the dynamics of the LCFEs and their role during the LCEs shedding, which dramatically affects the GoM circulation. We analyze several characteristics of the LC frontal dynamics. Modeled LCFEs have a coherent vertical structure, which extends to the deep layers of the GoM. They may split in two separate upper and lower layer eddies. Deep and surface remnants from different frontal eddies are able to align to form new, coherent structures. LCFEs intensify along the extended LC northern edge when flowing over the deep northern GoM shelf slope that forms the Mississippi Fan, through a “promontory effect” in which the incoming cyclone aggregates positive potential vorticity anomalies in lower layers, leading to the intensification of the whole vortex structure. LCFEs may also expand further along the LC path by horizontal vortex merging, when they are blocked between the LC and the northeast corner of the continental shelf in the GoM. The intensification and merging due to topographic effects explain the enlarged frontal eddies observed on the eastern side of the Loop Current. These larger eddies further migrate along the LC front and may play a role in the shedding sequence.

Volume growth of a powder snow avalanche

AbstractWe contrast the frontal dynamics of dilute powder snow avalanches with the behavior of their tail. While the former can be regarded as a fast-moving eruption current dominated by synergistic material injection into a short head, the latter behaves as a nearly arrested dilute cloud of particles expanding by progressive incorporation of ambient air, or by entrainment of snow-cover material by late avalanches trailing well behind the front.

Shifts in an epibenthic trophic web across a marine frontal area in the Southwestern Atlantic (Argentina)

Marine benthic trophic relationships and food web structures may be influenced by benthic–pelagic coupling processes, which could also be intensified by the physical dynamics of marine fronts. In this work, we employed stable isotope (δ 13C and δ 15N) analysis to investigate the influence of the Southwest (SW) Atlantic shelf-break front (SBF; 38–39°S, 55–56°W; Argentina) on an epibenthic trophic web. Epibenthic organisms were sampled, at depths of ~ 100 m, with a non-selective dredge from a sandy bottom community located in frontal (F) and marginal (M) areas. The SBF position and the chlorophyll-a (chl-a) concentrations were inferred using satellite data of the sea surface temperature (SST) and satellite chl-a concentration, respectively. The most noticeable shifts in stable isotopes between the sampled areas were those of the Patagonian scallop, Zygochlamys patagonica (δ 13C), and those of the sea urchin, Sterechinus agassizi (δ 15N). Diet analyses inferred from stable isotopes and mixing models demonstrated that the dominant component of this community, Z. patagonica, had variable contributions to higher trophic levels between areas. More importantly, the epibenthic assemblage in F areas showed δ 13C-enriched and δ 15N-depleted isotopic signatures with respect to the M areas. Collectively, this evidence suggests that frontal dynamics promotes the accumulation of δ 13C-enriched phytoplankton in the seabed in F areas, while in M areas the more degraded organic matter becomes more important in the trophic web, decreasing the δ 15N isotopic signature of the assemblage. Therefore, the trophic web was sustained by fresher food in F areas than in M areas, demonstrating the role of frontal dynamics in the shaping of these communities.

The Effect of Contact Angle on Saturation Overshoot

For certain porous media and initial conditions, constant flux infiltrations show a saturation profile that exhibits overshoot. This overshoot, which is the cause of gravity‐driven fingering, cannot be described by standard models of unsaturated flow, and it is likely controlled by the exact nature of the pore filling at the initial front. These frontal dynamics have recently been studied as a function of infiltrating flux and infiltrating fluid and were present in our earlier paper. The overshoot behavior is found to disappear below a certain flux which has been named the overshoot flux. We found that, except for water, the overshoot flux qover is a linear function of surface tension σ divided by the viscosity μ, qover ∼ σ/μ. This is expected if the physics at the front that controls overshoot are the same physics that controls the capillary desaturation curve, but the overshoot flux for water did not fall on this curve. To resolve this discrepancy, we conducted additional experiments to test how wettability affects the saturation overshoot.

Frontal dynamics in a California Current System shallow front: 1. Frontal processes and tracer structure

The three‐dimensional dynamics in a shallow front are examined using density and current data from two surveys 100 km offshore of Monterey Bay, California. Survey 1 is forced by down‐front winds, and both surveys have considerable cross‐front density gradients and flow curvature. The maximum Rossby numbers on the dense side reached maxima of +0.60 in survey 1 and +0.45 in survey 2. Downwelling occurs in regions of confluence (frontogenesis) associated with potential vorticity (PV) change and thermal wind imbalance. Streamers of particulate matter and PV are advected southeastward by the frontal jet and downward. Nonlinear Ekman currents advect dense water over light water in the presence of down‐front winds, which leads to upwelling along the front and downwelling on the light side of the front. At sites of active ageostrophic secondary circulation (ASC), induced by frontogenesis or Ekman effects, the observed cross‐front ageostrophic velocity is consistent with the diagnosed vertical velocity. Furthermore, in survey 2, ageostrophic divergence may play an important role at the curved front, presumably counteracting quasi‐geostrophic frontogenesis due to isopycnal confluence. Downward frictional vertical PV flux below the surface extracts PV from the pycnocline and reinforces the frontogenetic vertical PV flux. PV destruction at the surface is inferred from a low PV anomaly below the mixed layer in survey 2. Since the magnitude of the frontogenetic ASC is only twice the magnitude of Ekman suction, external forcing may have a considerable impact on the vertical heat and PV fluxes.

The Transition between Sharp and Diffusive Wetting Fronts as a Function of Imbibing Fluid Properties

The efficiency of one fluid displacing another in a permeable medium depends on the pore‐scale dynamics at the main wetting front. Experiments have shown that the frontal dynamics can result in two different flow regimes: a sharp and a diffuse front. In the sharp front regime, the displacing fluid occupies nearly all the pores and throats behind the main wetting front and the saturation changes abruptly. In contrast, in the diffuse front regime, pores are filled gradually at the main wetting front, and the saturation change is gradual in space. The different fronts can greatly alter the relative permeability curves, the trapping mechanisms, and the displacement efficiency. Directly measuring the sharpness of the front is difficult. Instead, we correlated the front sharpness to saturation overshoot, which occurs for moderate‐ to high‐flux vertical displacements of low‐density fluid by a higher density fluid in one‐dimensional, homogenous, permeable media. We hypothesized that the sharpness of wetting front can be explained by competition between two different pore‐filling mechanisms (called snap off and piston like), with the competition controlled by the velocity of the front and thus the injected flux. We conducted a series of infiltration experiments to determine the saturation profile as a function of flux for seven different fluids. We found that for each fluid there is a flux (called overshoot flux) below which saturation overshoot ceases and the front is diffuse. We found that the overshoot flux depends inversely on the invading fluid's viscosity and shows little or no dependence on the invading fluid's surface tension, vapor pressure, or miscibility with water.

Simulation of wave fronts on dry beds using Lagrangian blocks

Simulations of wave fronts on a dry bed are conducted using blocks as the computational elements. The non-negativity of the blocks allows simulations of the wave fronts to be carried out without the oscillation problem that limits the applicability of many existing computational methods. At the leading edge of the advancing front, the velocity is maximum and friction dominates. Simulations of this dominant friction effect at the wave front are carried out for the release of water from a dam-break outflow, a levee overflow and a sump outflow. Despite geometric differences, the frontal dynamics of all three flows are similarly affected by friction.

Dynamics of physical and biological systems of the Prince Edward Islands in a changing climate

Sub-Antarctic islands are classified as isolated, hostile, impoverished regions, in which the terrestrial and marine ecosystems are relatively simple and extremely sensitive to perturbations. They provide an ideal ecological laboratory for studying how organisms, ecological processes and ecosystems respond to a changing ocean climate in the Southern Ocean. 1hese islands are characterised by large populations of top predators and subsequently any changes in the oceanographic frontal dynamics associated with the Antarctic Circumpolar Current, either in the vicinity of these islands or further afield, may have strong implications on their foraging behaviour. The relatively easy accessibility of the Prince Edward Islands from South Africa and their location between the main frontal systems bordering the Antarctic Circumpolar Current enable high-resolution synoptic field studies to be undertaken. Such studies have provided information on the impact changes in the large-·scale ocean dynamics have on the local marine ecosystems.

Downwelling and upwelling regimes: Connections between surface fronts and abyssal circulation

Recent observations in the Sea of Japan show evidence of convection to a depth of roughly 1000 m in the winter of 2000, situated along the polar front. Numerical simulations have shown that this deep mixing is associated with both ageostrophic frontal circulations and pre-existing larger-scale downwelling regimes. The downwelling regimes appear to be a result of interactions between frontal meandering and deep circulation in this basin over bottom topography anomalies. The coupling between the frontal dynamics and the deep circulation are explored by analogy to atmospheric frontal circulations through the semigeostrophic Sawyer–Eliassen equation, solved numerically for the case of the Sea of Japan. As in the atmospheric case, a vertical coupling between the upper and lower circulations can produce a localized region of downwelling that can be conducive to deeper mixing than that forced solely from surface fluxes.

Rio de la Plata estuarine system: Relationship between river flow and frontal variability

This paper deals with the application of SeaWIFS images to characterize spatial and temporal variability of fronts in the Rio de la Plata estuarine system over the period 2000–2003. We aim to depict the relationship between river outflow and variability of fronts’ loci on monthly to ENSO-related timescales and the influence of the winds along Rio de la Plata (axial winds) on the abrupt changes in frontal dynamics over synoptic timescales. During the studied period both La Niña (July 1999–June 2000) and El Niño (April 2002–May 2003) events induced significant displacements of fronts. Three distinct fronts were analyzed between river, estuarine, coastal and marine waters of the Rio de la Plata: Main Turbidity Front, Main Marine Front, and Secondary Marine Front. Their number, location and separation seem to be mainly related to river outflow and second, to fresh (>8 m/s) axial winds. During low discharge periods (i.e. summer time and/or La Niña events) these winds induce abrupt changes in the location of fronts (100–200 km) and greater separation between them over synoptic timescale, whereas during high river discharge or ENSO years some of the variability of fronts location is explained by the river’s outflow fluctuations, especially by the high variability of the River Uruguay discharge.

Evolution of solitary marginal disturbances in baroclinic frontal geostrophic dynamics with dissipation and time-varying background flow

A two-layer frontal geostrophic flow corresponds to a dynamical regime that describes the low-frequency evolution of baroclinic ocean currents with large amplitude deflections of the interface between the layers on length-scales longer than the internal deformation radius within the context of a thin upper layer overlying a dynamically active lower layer. The finite-amplitude evolution of solitary disturbances in baroclinic frontal geostrophic dynamics in the presence of time-varying background flow and dissipation is shown to be governed by a two-equation extension of the unstable nonlinear Schrödinger (UNS) equation with variable coefficients and forcing. The soliton solution of the unperturbed UNS equation corresponds to a saturated isolated coherent anomaly in the baroclinic instability of surface-intensified oceanographic fronts and currents. The adiabatic evolution of the propagating soliton and the uniformly valid first-order perturbation fields are determined using a direct perturbation approach together with phase-averaged conservation relations when both dissipation and time variability are present. It is shown that the soliton amplitude parameter decays exponentially due to the presence of the dissipation but is unaffected by the time variability in the background flow. On the other hand, the soliton translation velocity is unaffected by the dissipation and evolves only in response to the time variability in the background flow. The adiabatic solution for the induced mean flow exhibits a dissipation-generated ‘shelf region’ in the far field behind the soliton, which is removed by solving the initial-value problem.

Atlantic Southern Ocean productivity: Fertilization from above or below?

Primary productivity and the associated uptake of atmospheric carbon dioxide in the Southern Ocean (SO) is thought to be generally limited by bioavailable iron (Fe). Two sources of Fe for the surface waters of the SO have been proposed: (1) oceanic input of nutrient‐rich (i.e., Fe) waters from upwelling and lateral flows from continental margins; and (2) atmospheric input from the deposition of mineral dust emanating from the arid regions of South America and Australia. In this work, analysis of weekly remotely sensed sea surface temperature (SST), ocean chlorophyll a content [Chl a] and model‐derived atmospheric dust‐Fe fluxes are used to identify the predominant source of Fe during phytoplankton blooms in the surface waters of the south Atlantic Ocean between 40°S and 60°S. The results of our study suggest that oceanic source through upwelling of nutrient‐rich waters due to mesoscale frontal dynamics is the major source of bioavailable Fe controlling biological activity in this region. This result is consistent with the idea that acidification of aeolian dust prior to its deposition to the ocean may be required to solubilize the large fraction of mineral‐iron and make it bioavailable.

Comparative analysis of the dynamics of human postural control during fixation and pursuit of a visual target

The properties of the system maintaining the upright posture were compared in different states of the oculomotor system: during target fixation and horizontal fast and slow pursuit (0.1 and 0.01 Hz), recording the trajectories of the center of pressure in the frontal and the sagittal planes. Methods of nonlinear analysis were applied to assess the similarity in pairwise comparisons. The overall similarity of the frontal plane dynamics proved to be higher than that of the sagittal plane dynamics. However, differences were revealed in fast pursuit versus slow pursuit or fixation in the frontal but not in the sagittal plane. Such differences may reflect the different inertia of the oculomotor and the balance control systems. In general, the results are consistent with the current notions on the two orthogonal subsystems of postural control.

On Step‐Function Reaction Kinetics Model in the Absence of Material Diffusion

We propose a precise definition of the step‐function kinetics suitable for approximating diffuse propagating reaction fronts in one‐dimensional gasless‐combustion‐type models when a Lewis number is large. We investigate this kinetics in the context of free‐radical frontal polymerization (FP) in which a monomer‐initiator mixture is converted into a polymer via a propagating self‐sustaining reaction front. The notion of step‐function kinetics has been extensively used in studies of the frontal dynamics both in FP and in combustion problems when the material diffusion is negligible. However, the models have always been effectively reduced to their point‐source approximations without defining exactly what the step‐function kinetics is for diffuse reaction fronts. We demonstrate numerically that dynamics of diffuse fronts in systems modeled with step‐function kinetics and in systems modeled with Arrhenius kinetics are qualitatively the same at time scales at which the bulk reaction ahead of the front can be ignored. We perform stability analysis for the traveling reaction wave and show that the stability threshold is in close agreement with numerical simulations as well as with other existing kinetics approximations. The benefits of using step‐function kinetics are two‐fold. The reaction dynamics predicted by the step‐function kinetics approximates the dynamics predicted by the Arrhenius kinetics over a wider range of system parameters than the point‐source approximation. Second, the systems governed by the step‐function kinetics can be analyzed both analytically and numerically within the framework of a single model.

Phytoplankton size classes competitions at sub-mesoscale in a frontal oceanic region

The effects of mesoscale and sub-mesoscale dynamics on the competition between two different phytoplankton size classes are investigated with a 3D primitive equations model. The model reproduces realistic simulations of mesoscale turbulence generated by a westward current in the southern hemisphere at statistical equilibrium in a summer situation. Effects of two different grazing pressures on phytoplankton competitions are compared and the role of eddy variability is quantified comparing high and low resolution simulations. High resolution simulations reveal a filamentary distribution of biomass and nutrients induced by the combination of vertical advection and horizontal stirring. This fine scale variability is observed not only on the horizontal but also on the vertical into the subsurface chlorophyll maximum. One of the key results is that such a dynamics induces a spatial segregation of the phytoplankton in the southern part of the frontal region that is mainly filamentary. This spatial segregation consists in biomass maxima for large phytoplankton in rich nutrients filaments and maxima for small phytoplankton outside these filaments. This anti-correlation is particularly strong when grazing pressure is low and is confirmed by statistical analysis. In the central frontal region, dominated by mesoscale dynamics, the two phytoplankton classes are strongly correlated together and biomass maxima are located close to downwelling regions that are poor in nutrients. It is shown that the effect of grazing is significantly amplified by the fine scale dynamics and that the combination of these two mechanisms is responsible of a switch of the ecosystem dominance in the surface layers. In addition, the effect of frontal dynamics on the detritus export is very sensitive to grazing pressure: increasing grazing induces a significant decrease of the export in the presence of frontal dynamics whereas it induces an increase of the export without small-scale variability.