Horizontal Mixing Processes Research Articles

Discovery of Magnetically Guided Metal Accretion onto a Polluted White Dwarf

Dynamically active planetary systems orbit a significant fraction of white dwarf stars. These stars often exhibit surface metals accreted from debris disks, which are detected through infrared excess or transiting structures. However, the full journey of a planetesimal from star-grazing orbit to final dissolution in the host star is poorly understood. Here, we report the discovery that the cool metal-polluted star WD 0816–310 has cannibalized heavy elements from a planetary body similar in size to Vesta, and where accretion and horizontal mixing processes have clearly been controlled by the stellar magnetic field. Our observations unveil periodic and synchronized variations in metal line strength and magnetic field intensity, implying a correlation between the local surface density of metals and the magnetic field structure. Specifically, the data point to a likely persistent concentration of metals near a magnetic pole. These findings demonstrate that magnetic fields may play a fundamental role in the final stages of exoplanetary bodies that are recycled into their white dwarf hosts.

The Response of Tropical Rainfall to Idealized Small‐Scale Thermal and Mechanical Forcing

AbstractPredicting the spatiotemporal distribution of rainfall remains a key challenge in Tropical Meteorology, partly due to an incomplete understanding of the effects of different environmental factors on atmospheric convection. In this work, we use numerical simulations of tropical ocean domains to study how rainfall responds to imposed localized thermal and mechanical forcings to the atmosphere. We use the Normalized Gross Moist Stability—NGMS—to quantify the net precipitation response associated with a given net atmospheric heating. We find that NGMS values differ considerably for different forcings, but show that the relationship between precipitation and column relative humidity collapses along a universal curve across all of them. We also show that the contributions from mean vertical advection of moist and dry static energy only approximate the NGMS well at scales larger than a couple hundred kilometers, indicating that general horizontal mixing processes are not negligible at smaller scales.

Ageostrophic Contribution by the Wind and Waves Induced Flow to the Lateral Stirring in the Mediterranean Sea

AbstractWe study the impact of the Ekman currents and Stokes drift on the surface horizontal transport and mixing properties of the Mediterranean Sea. Finite Size Lyapunov Exponents (FSLE) at the ocean surface are computed over the whole basin using 25 years of satellite altimetry derived geostrophic currents, 10‐m wind velocity and wave fields. We find that the transport pathways unveiled by the geostrophic Lagrangian Coherent Structures (LCS) are significantly modified by the ageostrophic currents (i.e., Ekman and Stokes induced velocities), often leading to a decrease of the retention capacity of the eddies. An exhaustive assessment of the regional and temporal variability of the FSLE shows an increase of the horizontal mixing processes, due to the ageostrophic component, up to 37% in regions such as the Gulf of Lion or the Aegean Sea, during the seasons where wind and waves are intense and persistent. Positive trends in the total FSLE (up to 1.2% of the value of FSLE per year in some regions) suggest that Mediterranean Sea has experienced a significant increase in the intensity of the mixing processes over the last decades. Ageostrophic features are considered to play a role in determining the properties of the relative dispersion. Through the analysis of the Lagrangian Anisotropy Index (LAI) using virtual and real pair of drifters, we observe that the particle dispersion is mainly dominated by the zonal flow, and that the ageostrophic currents induce meridional dispersion, particularly in regions where wind and wave are intensified.

Prospects for acoustic remote sensing and acoustic system performance in the Beaufort Gyre region

In the Beaufort Gyre region north of Alaska, the vertical interleaving of the near-surface temperature maximum, the warm Pacific Summer Water (PSW), the cool Pacific Winter Water, and the Atlantic layer make for unusual acoustic conditions. The dynamics of these upper ocean layers cause typical complex and turbulent-like oceanic flow that causes geographically variable heat content and acoustics. A sound duct in the PWW, below the PSW and above the Atlantic Layer, filling ∼75 to 225 m depth, is prominent but not universal. Erosion of the PSW warm layer by either vertical or horizontal mixing processes often breaks the duct, with an additional seasonal cycle of ducting effectiveness, and allows sound to escape to interact with the scattering and attenuating surface. The duct enables communication and navigation signaling, but with that interruption caveat. It also allows passive sensing, best for sound that enters the duct and moves through it. A challenge is to utilize sound that exists outside the duct. This includes sounds emitted outside the duct by mammals, fracturing ice, or anything else. This sound will either cycle through the duct at steep angle, or stay above the duct and repeatedly reflect from the top boundary. The combination of unducted sound being subject to strong attenuation by ice or wave interaction (the typical high-latitude situation) and high variability of the duct effectiveness, imposes limitations on the usefulness of the otherwise convenient duct.

Natural 222Rn as a tracer of mixing and volatilization in a shallow aquifer during a CO2 injection experiment

AbstractThis study aims to evaluate the application of 222Rn in groundwater as a tracer for monitoring CO2 plume migration in a shallow groundwater system, which is important to detect potential CO2 leakage in the carbon capture and storage (CCS) project. For this research, an artificial CO2‐infused water injection experiment was performed in a shallow aquifer by monitoring hydrogeochemical parameters, including 222Rn. Radon in groundwater can be a useful tracer because of its sensitivity to sudden changes in subsurface environment. To monitor the CO2 plume migration, the data were analysed based on (a) the influence of mixing processes on the distribution of 222Rn induced by the artificial injection experiment and (b) the influence of a carrier gas role by CO2 on the variation of 222Rn. The spatio‐temporal distributions of radon concentrations were successfully explained in association with horizontal and vertical mixing processes by the CO2‐infused water injection. Additionally, the mixing ratios of each monitoring well were calculated, quantitatively confirming the influence of these mixing processes on the distribution of radon concentrations. Moreover, one monitoring well showed a high positive relationship between 222Rn and Total dissolved inorganic carbon (TIC) by the carrier gas effect of CO2 through volatilization from the CO2 plume. It indicated the applicability of 222Rn as a sensitive tracer to directly monitor CO2 leakage. When with a little effect of carrier gas, natural 222Rn in groundwater can be used to compute mixing ratio of CO2‐infused water indicative of CO2 migration pathways. CO2 carrier gas effect can possibly increase 222Rn concentration in groundwater and, if fully verified with more field tests, will pose a great potential to be used as a natural tracer for CO2.

Inter-annual variability of shelf water volume on the northeastern Canadian and United States continental shelf, 1973–2013

Understanding the variability of waters confined to the northeastern United States and Canadian continental shelf is essential in determining factors that influence hydrographic patterns and structure, changes in fisheries stocks and planktonic assemblages, and the impact of Arctic warming. Shelf waters (SHW) located between Newfoundland and Cape Hatteras are characterized by lower salinity (<34 PSU) and lower temperatures than the generally warmer and more saline slope waters located farther offshore. Significant inter-annual variability of SHW volume has been noted from previous studies within the more limited Mid Atlantic Bight sub-region. Multiple factors may impact the volume of SHW, including local and remote river runoff and precipitation, release and equator-ward transport of fresh water from the Arctic and sub-polar gyre, ice melt from Greenland, cross-shelf removal, and both vertical and horizontal mixing processes. Here, we estimate inter-annual variability of SHW volume over the large shelf domain located between Newfoundland and Cape Hatteras from 1973 through 2013. Objectively-analyzed (OA) annual mean SHW thickness and error (as a percent of data variance) is computed for each of eleven sub-regions spanning our study domain for the 41-year period. Results show significant decreasing trends in SHW volume within 5 of the 11 sub-regions (The Grand Banks through the Eastern Scotian Shelf sub-regions) in agreement with previous work which has shown a decreasing trend of northern Labrador Current transport. OA annual mean SHW volume anomalies were cross-correlated with annual mean shelf slope front (SSF) position anomalies for corresponding sub-regions, displaying a highly significant relationship for the Georges Bank, the Mid Atlantic Bight, and the DelMarVa Hatteras shelf, with higher volumes corresponding to more offshore SSF positions. Annual mean near-surface salinity anomalies, when cross-correlated with OA annual mean SHW volume anomalies, displayed significant positive correlations from the Gulf of St. Lawrence northeast to the Newfoundland shelf and displayed primarily negative correlations from the Gulf of Maine to the DelMarVa Hatteras sub-region. The North Atlantic Oscillation annual indices displayed significant cross-correlations with annual mean OA SHW volume anomalies only for the Tail of the Grand Banks and Georges Bank., There were no significant cross correlations computed between the OA annual mean SHW volume anomalies and the NAO winter index for corresponding years.

Entrainment and Energy Transfer Variability Along the Descending Path of the Denmark Strait Overflow Plume

AbstractThe descent of the Denmark Strait overflow plume is an important process in the Atlantic Meridional Overturning Circulation. Downstream of the sill, the plume entrains ambient water, increasing its volume transport. The entrainment and related transfer of energy can be driven by vertical or horizontal turbulent mixing, and varies spatially, as the plume descends, and temporally, as the volume transport at the sill changes. Using over 30 years of profile data, this spatial and temporal variability in the first 200 km downstream of the sill was investigated. Dissipation and entrainment rates were derived from Thorpe scales, and each profile was identified as either a low‐ or high‐transport flow, defined as below or above‐average volume transport at the sill. In the first 175 km flow type explains most of the variability in entrainment and dissipation rates, with high‐transport flow producing order of magnitude higher rates. Sections crossing the plume and yo‐yo casts (continuous profiling) indicate that dissipation and entrainment are likely driven by the formation of shear instabilities in the interfacial layer, when the vertical velocity shear overcomes the stratification. This vertical turbulent mixing explains most of the variability within the first 175 km, suggesting horizontal turbulent mixing processes may not play as important a role in this region. The importance of temporal flow variability means that further improvements to our understanding of plume dynamics in the Denmark Strait will require a novel observational approach to fully account for spatial and temporal contributions.

Asymmetric and axisymmetric dynamics of tropical cyclones

Abstract. We present the results of idealized numerical experiments to examine the difference between tropical cyclone evolution in three-dimensional (3-D) and axisymmetric (AX) model configurations. We focus on the prototype problem for intensification, which considers the evolution of an initially unsaturated AX vortex in gradient-wind balance on an f plane. Consistent with findings of previous work, the mature intensity in the 3-D model is reduced relative to that in the AX model. In contrast with previous interpretations invoking barotropic instability and related horizontal mixing processes as a mechanism detrimental to the spin-up process, the results indicate that 3-D eddy processes associated with vortical plume structures can assist the intensification process by contributing to a radial contraction of the maximum tangential velocity and to a vertical extension of tangential winds through the depth of the troposphere. These plumes contribute significantly also to the azimuthally averaged heating rate and the corresponding azimuthal-mean overturning circulation. The comparisons show that the resolved 3-D eddy momentum fluxes above the boundary layer exhibit counter-gradient characteristics during a key spin-up period, and more generally are not solely diffusive. The effects of these eddies are thus not properly represented by the subgrid-scale parameterizations in the AX configuration. The resolved eddy fluxes act to support the contraction and intensification of the maximum tangential winds. The comparisons indicate fundamental differences between convective organization in the 3-D and AX configurations for meteorologically relevant forecast timescales. While the radial and vertical gradients of the system-scale angular rotation provide a hostile environment for deep convection in the 3-D model, with a corresponding tendency to strain the convective elements in the tangential direction, deep convection in the AX model does not suffer this tendency. Also, since during the 3-D intensification process the convection has not yet organized into annular rings, the azimuthally averaged heating rate and radial gradient thereof is considerably less than that in the AX model. This lack of organization results broadly in a slower intensification rate in the 3-D model and leads ultimately to a weaker mature vortex after 12 days of model integration. While azimuthal mean heating rates in the 3-D model are weaker than those in the AX model, local heating rates in the 3-D model exceed those in the AX model and at times the vortex in the 3-D model intensifies more rapidly than AX. Analyses of the 3-D model output do not support a recent hypothesis concerning the key role of small-scale vertical mixing processes in the upper-tropospheric outflow in controlling the intensification process. In the 3-D model, surface drag plays a particularly important role in the intensification process for the prototype intensification problem on meteorologically relevant timescales by helping foster the organization of convection in azimuth. There is a radical difference in the behaviour of the 3-D and AX simulations when the surface drag is reduced or increased from realistic values. Borrowing from ideas developed in a recent paper, we give a partial explanation for this difference in behaviour. Our results provide new qualitative and quantitative insight into the differences between the asymmetric and symmetric dynamics of tropical cyclones and would appear to have important consequences for the formulation of a fluid dynamical theory of tropical cyclone intensification and mature intensity. In particular, the results point to some fundamental limitations of strict axisymmetric theory and modelling for representing the azimuthally averaged behaviour of tropical cyclones in three dimensions.

두 가지 연직혼합방안에 따른 해양대순환모형 혼합층깊이 및 상층수온 모사 민감도 비교

Vertical and horizontal mixing processes in the ocean mixed layer determine sea surface temperature and temperature variability. Accordingly, simulating these processes properly is crucial in order to obtain more accurate climate simulations and more reliable future projections using an ocean general circulation model (OGCM). In this study, by using Modular Ocean Model version 4 (MOM4) developed by Geophysical Fluid Dynamics Laboratory, the upper ocean temperature and mixed layer depth were simulated with two different vertical mixing schemes that are most widely used and then compared. The resultant differences were analyzed to understand the underlying mechanism, especially in the Tropical Pacific Ocean where the differences appeared to be the greatest. One of the schemes was the so-called KPP scheme that uses K-Profile parameterization with nonlocal vertical mixing and the other was the N scheme that was rather recently developed based on a second-order turbulence closure. In the equatorial Pacific, the N scheme simulates the mixed layer at a deeper level than the KPP scheme. One of the reasons is that the total vertical diffusivity coefficient simulated with the N scheme is ten times larger, at maximum, in the surface layer compared to the KPP scheme. Another reason is that the zonal current simulated with the N scheme peaks at a deeper ocean level than the KPP scheme, which indicates that the vertical shear was simulated on a larger scale by the N scheme and it enhanced the mixed layer depth. It is notable that while the N scheme simulates a deeper mixed layer in the equatorial Pacific compared to the KPP scheme, the sea surface temperature (SST) simulated with the N scheme was cooler in the central Pacific and warmer in the eastern Pacific. We postulated that the reason for this is that in the central Pacific atmospheric forcing plays an important role in determining SST and so does a strong upwelling in the eastern Pacific. In conclusion, what determines SST is crucial in interpreting the relationship between SST and mixed layer depth.

Mixing processes in the deep water of the Gulf of Elat (Aqaba): Evidence from measurements and modeling of the triple isotopic composition of dissolved oxygen

A time series of the 17O excess (17Δ) was measured in the Gulf of Elat (Aqaba) between May 2007 and August 2009. 17Δ is unaffected by respiration; thus it is a unique, conservative tracer that preserves the signature acquired in the photic zone, the source region for deep‐water formation. In this study we used 17Δ to assess the ratio of photosynthetic O2 to atmospheric O2 in deep water. We observed an increase of 17Δ by 20 per meg in the water residing below 300 m over a period of 3 months, followed by a decrease of 60 per meg over the next 7 months. These changes indicated penetration of photosynthetic O2, followed by penetration of atmospheric O2 into the deep water. To test whether vertical mixing could explain the observed variations in 17Δ, we compared our results with simulated values obtained from a one‐dimensional hydrodynamic model, which was extended to include dissolved O2 isotopes. Although successfully reproducing the observed temperatures, salinities, and dissolved O2 concentrations in the gulf, the model could not reproduce the observed variations in 17Δ in the deep water. This discrepancy shows that horizontal mixing processes have an important role in the interaction between deep and surface water in the gulf. We suggest that for the most part, these processes occur along the coastal boundaries of the gulf.

Towards a mesoscale eddy closure

A turbulence closure for the effect of mesoscale eddies in non-eddy-resolving ocean models is proposed. The closure consists of a prognostic equation for the eddy kinetic energy (EKE) that is integrated as an additional model equation, and a diagnostic relation for an eddy length scale ( L), which is given by the minimum of Rhines scale and Rossby radius. Combining EKE and L using a standard mixing length assumption gives a diffusivity ( K), corresponding to the thickness diffusivity in the [Gent, P.R., McWilliams, J.C. 1990. Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr. 20, 150–155] parameterisation. Assuming downgradient mixing of potential vorticity with identical diffusivity shows how K is related to horizontal and vertical mixing processes in the horizontal momentum equation, and also enables us to parameterise the source of EKE related to eddy momentum fluxes. The mesoscale eddy closure is evaluated using synthetic data from two different eddy-resolving models covering the North Atlantic Ocean and the Southern Ocean, respectively. The diagnosis shows that the mixing length assumption together with the definition of eddy length scales is valid within certain limitations. Furthermore, implementation of the closure in non-eddy-resolving models of the North Atlantic and the Southern Ocean shows consistently that the closure has skill at reproducing the results of the eddy-resolving model versions in terms of EKE and K.

Dispersion processes and transport pattern in the ROFI system of the eastern English Channel derived from a particle-tracking model

A random-walk Lagrangian particle-tracking model is applied to study horizontal mixing processes in the ROFI system of the eastern English Channel. A particle-tracking model is incorporated within the three-dimensional hydrodynamic model, POM, which is run under realistic tidal forcing and freshwater input. Our computations based on both the Lagrangian and Eulerian approaches show the existence of a classical coastal current along the French coast whose dynamics is strongly dominated by the freshwater runoff and modulated by the neap-spring tidal cycle. The prevailing near-shore northward residual current is also identified by Argos drifter tracking in the vicinity of the French coast and by field observations. However, the Lagrangian and Eulerian residual velocity fields do not agree throughout the model domain. The convergence of the residual currents, on the offshore margin of the ROFI, in combination with the northward residual transport, strongly affects the spatial distribution of neutrally buoyant particles and can provide patchiness in the horizontal distribution of material. With respect to the dispersive part of watermass displacement, our simulations reveal two current regimes in the eastern English Channel: the offshore regime with small values of effective dispersion and nearly isotropic spreading, and the near-shore regime of high dispersion and stretching. Shear dispersion is assumed to be a basic mechanism operating in the ROFI system, giving rise to a relatively large horizontal dispersion near the French coast. This happens because, in the near-shore waters, turbulence interacts with vertical shear producing horizontal dispersion. In turn, that mechanism interacts with the horizontal shear of residual currents, giving rise to a large effective dispersion in the direction of the residual current. A combined effect of tidal motions and freshwater runoff produces horizontal variations in the residual velocity field. The efficiency of spreading increases toward the Dover Strait where velocity variations are the highest. The cross-shore dispersion appears to be small and does not produce stretching. A watermass travelling through both regimes is therefore strongly deformed when it reaches the ROFI system.

Chaotic mixing in a basin due to a sinusoidal wind field

AbstractThis paper presents a novel numerical study of wind‐induced mixing in a circular shallow lake. The wind‐induced hydrodynamic field is first predicted by a Godunov‐type shallow water equation solver based on a quadtree grid. Then the horizontal mixing processes are analysed in terms of chaotic advection. The particle dynamics under a sinusoidal wind field are found to change from regular to chaotic states with increasing period of wind oscillation. The numerical approach is applicable to a wide range of environmental flows. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Numerical model of nearshore flow on quadtree grids

Shorelines are seldom straight or smooth, but instead, often contain rhythmical seaward projections of sediment in series with fairly uniform spacing ranging from 10 cm to 1,500 m. These cusps are formed resulting from interaction between nearshore hydrodynamics, sediment dynamics and beach morphology in the surf zone. An accurate prediction of wave-current interaction is very important for coastal zone management, because wave-induced currents and tidal currents have main influence on horizontal mixing processes of suspended sand and buoyant pollutants. In this paper, details of an adaptive quadtree grid based numerical model of the coupled wave climate and depth-averaged current field are described. This model accounts for hydrodynamics in the surf-zone such as wave breaking, shoaling, refraction, diffraction, wave-current interaction, mixing process and bottom friction effects. The model has been validated for the nearshore currents at a three-cusp beach installed in the U.K. Coastal Research Facility.

Application of the Tidal Random Walk Model to Calculate Water Dispersion Coefficients in the Ems Estuary

Longitudinal and lateral dispersion coefficients are computed for the Ems Estuary using a modified version of the tidal random walk model developed by Zimmerman. This model relates the mixing properties of the flow to the velocity and length scales of the tidal current and the residual eddies. It is found that values of the longitudinal dispersion coefficient are 200–1200m2s−1, depending on the position in the estuary, and lateral dispersion coefficients are a factor ofc. 50 smaller. These results are three to four times larger than earlier reported values, which were determined from field observations. This difference is attributed to the fact that in the tidal random walk model, the dependence of the dispersion mechanism on the detailed kinematics of the flow is neglected, since a statistical distribution of residual eddies is assumed. However, the model predicts the spatial distribution of the dispersion coefficients in the estuary quite well. Particular attention is paid to the presence of three peaks in the dependence of these coefficients on the distance to the river mouth. Using the concepts underlying the tidal random walk model, these peaks are related to the presence of tidally induced residual circulations around sand banks. It is also concluded that the effect of shear dispersion on the horizontal mixing processes in the Ems Estuary can be neglected.

Observations of jets in density stratified crossflows

The results are presented of a water tunnel study undertaken to determine the characteristics of a circular jet in a density stratified crossflow. Flow visualization indicates that the salient characteristics include a stable potential core of length L in the vicinity of the jet exit. The length of the stable potential core depends on the velocity ratio α = U J U ∞ and the non-dimensional frequency ND U ∞ . As α is increases the value of L increases; L also increases for increasing values of ND U ∞ < 0.03 . However, L decreases with increasing ND U ∞ for values of ND U ∞ > 0.03 . Knowledge of this variability is necessary for quantifying mixing rates. Further downwind, the jet trajectory evolves to a maximum height of rise z m , and subsequently relaxes to a lower equilibrium level, z e. Values of z m and z e are in agreement with the predictions of scaling arguments. Mean density measurements indicate rapid increase of dilution for small distances from the jet exit; however, for large distances predominantly horizontal mixing processes result in much slower increase of dilution with distance.

Modeling Outfall Plume Behavior Using Far Field Circulation Model

The near field behavior of the new Boston sewage outfall plume, as simulated by a far field circulation model of Massachusetts Bay, has been compared with similar predictions from a well-known and often-used near field plume model (ULINE). The three-dimensional circulation model applied to Massachusetts Bay is based upon far field physical processes that may or may not apply to the near field plume physics. An accurate characterization of initial mixing is important for reliable prediction of the environmental impact of the new outfall's discharge. Two parameters predicted by both models, plume rise (or trap) height and dilution, have been compared to determine how well the circulation model represents near field behavior. The comparison indicates that the Massachusetts Bay far field model predicts trap heights and initial dilutions that are surprisingly similar to those generated by the near field model. The parameterization of the horizontal and vertical mixing processes in the far field circulation model, together with appropriate numerical grid resolution and a good prediction of three-dimensional currents and vertical mixing near the new outfall, apparently simulates the small scale and very near field entrainment processes sufficiently well to provide confidence that far field results are not biased due to poorly simulated initial dilution and trap height.

A SF6 tracer study of horizontal mixing in Lake Constance

Horizontal mixing processes in the hypolimnion of the western part of Lake Constance are studied by measuring the dispersion of a sulfur hexafluoride (SF6) tracer plume. Only 1 liter gaseous SF6 (STP) was released at a central hypolimnic depth of 60 m in August 1990. Over a period of 3 months the horizontal dispersion of the tracer plume was measured by 19 surveys using a new, vertically integrating sampling device. The observed horizontal dispersion is marked by strong storm-induced stirring events. Nevertheless mean turbulent diffusion coefficients for the whole period can be computed. They rise about linear from 0.7 ⋅ 105 cm2/s to 3.0 ⋅ 105 cm2/s with the distance from the western end of the lake. For the hypolimnion of Uberlingersee, a sill-separated basin in the western part of Lake Constance, a simple budget model gives an exchange time of 67 ± 6 days with the main basin (Obersee).

Carbon uptake experiments with a zonally-averaged global ocean circulation model

A model which simulates the zonally averaged thermohaline circulation in the major ocean basins and includes the balance of stable and decaying tracers, is presented. The model is used to estimate oceanic uptake of tracers due to transiently varying atmospheric concentrations, with steady thermohaline circulation. 2 × CO 2 -experiments with an inorganic carbon cycle component yield an evolution that is consistent with results from 3-dimensional models on time scales of decades to millennia. The model's average uptake from 1980–1989 is 2.1 Gt C yr -1 when the industrial evolution of pCO 2 in the atmosphere is prescribed. Only about 10% of excess carbon is taken up by waters sinking in the North Atlantic but 30% by waters sinking in the Southern Ocean. The influence of vertical and horizontal mixing processes on the uptake in various regions the ocean is investigated. Uptake experiments of bomb-produced radiocarbon demonstrate possible limitations of the model. Agreement with the observations can be obtained if a parameterization is introduced that accounts for the near-surface meridional mixing of tracers due to the wind-driven circulation. Inventories and penetration depth of bomb radiocarbon are compared with estimates from 3-dimensional model simulations and observations. Global uptake is close to these estimates, however, inventories in the Southern Ocean are considerably larger. DOI: 10.1034/j.1600-0889.1994.t01-1-00003.x

Mechanisms for nitrogen supply to the Australian North West Shelf

An upper bound for the rate of supply of new nitrate required to maintain the observed primary production on the North West Shelf is estimated to be 0.1 g N m-2 day -1. Nitrate concentrations over the shelf and slope regions are high ( &gt; 100 mg N m-3, in water deeper than - 100 m and usually low (~10 mg N m-3), on the shelf. River flow is weak and carries little nutrient into the shelf waters and so it remains for ocean physical processes to advect and mix the nutrient-rich deep waters onto the shallower shelf regions to meet the nutrient demand. Several mechanisms are reviewed to determine their potential in carrying out the required transport processes. Estimates of the advection of nitrate onto the shelf show that both semi-diurnal tidal flow and low-frequency (periods &gt; 35 h) upwelling events can each contribute approximately half the required demand, providing there is rapid use of nutrients. The upwelling events occur in summer and are associated with reversals of the south-west-flowing Leeuwin Current. Tropical cyclones are also shown to be capable of meeting a small, but significant, portion of the demand through enrichment of the surface layers in the offshelf waters by upwelling and vertical mixing. The enriched water can then be advected onto the shelf. Both tidal and internal tidal motion have the potential to transport nitrate onto the shelf from deeper water through vertical and horizontal mixing processes. However, these processes are difficult to quantify accurately. It is concluded that nitrogen is supplied to this shelf ecosystem by physical processes that are regular throughout the year, as opposed to large sporadic events that occur only once or twice a year.