bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytical and numerical modeling of sandbanks dynamics
Authors: Idier Déborah , Astruc Dominique
Published: May 2003

Linear and nonlinear behavior of large‐scale underwater bedform patterns like sandbanks are studied using linear stability analysis and numerical modeling. The model is based on depth‐integrated hydrodynamics equations with a quadratic bottom friction law and a bed load sediment transport model including a bottom slope effect. First, the linear stability analysis of a flat erodible bottom subject to a steady current is performed. The direction of the most amplified bedform is controlled by the current, whereas its wavelength is selected by the gravity driven sediment flow. The instability proved to be due to the velocity component parallel to the mean flow, whereas the transverse velocity and the bottom slope effect are damping processes. The growth rate is then related to the spatial phase‐lag between flow velocity and bathymetry. In order to validate the numerical model in this linear regime, the growth rate and the phase celerity of an infinitesimal bottom perturbation are computed as a function of its wavevector. A good agreement with linear stability results is found. Second, using the property that the growth rates for block‐function and steady current are the same, the nonlinear behavior of the instability is investigated for a simple tidal current. The saturation height of the theoretically most amplified mode is estimated using the numerical model. The analysis of the sediment fluxes gradients shows that the saturation is mostly due to the hydrodynamic processes, although the saturation height slightly depends on the value of the bottom slope effect coefficient. Third, a Landau equation, whose coefficients are computed from the previous results, is used to predict the temporal evolution of the bedform amplitude from the initial infinitesimal perturbation to the saturation. A comparison with the characteristics of continental shelf sandbanks shows that the model gives a reasonable estimation of the temporal dynamics of these large‐scale bedforms. However, the saturation height seems to be slightly overestimated. The limitations of the method are also discussed.
Use of ship mean data for validating model and satellite flux fields during the FETCH experiment
Authors: Eymard L. , Weill A. , Bourras D. , Guérin C. , Le Borgne P. , Lefèvre J. M.
Published: May 2003

During the FETCH experiment in NW Mediterranean Sea (March and April 1998), surface fluxes were measured on board a research vessel and on a moored buoy. To provide the ocean surface forcing at the mesoscale, output fields of three meteorological models (European Centre for Medium‐Range Weather Forecasts (ECMWF) and Meteo‐France ARPEGE and ALADIN) were gathered as well as satellite data. To evaluate the consistency of model fields, we compare ship measurements to the predicted radiative and turbulent surface fluxes and to turbulent fluxes derived from analyses using a bulk formula. We first analyze the quality of the ship bulk fluxes in terms of possible effects of the flow distortion and of intrinsic variability as function of the integration time. The airflow distortion over the ship is found to significantly affect fluxes, but the mean fluxes over the experiment remain unchanged because of compensation between positive and negative errors due to the varying apparent wind direction. Spectral analysis of the ship time series as well as comparison of fluxes obtained by using several averaging times from 1 to 90 min show that the optimal scale for computing ship fluxes is about 20 min, ensuring consistency with the derivation of bulk drag and exchange coefficient. These fluxes are then taken as reference for the comparison with models and satellites. None of the predicted radiative fluxes is consistent with ship measurements. Model turbulent fluxes differ from each other, and the main reason is the discrepancy between modeled and observed atmospheric features. Large discrepancies are found between predicted fluxes and ship fluxes in strong wind conditions due to the different flux parameterization for heat fluxes. Model bulk fluxes thus compare better to ship than predicted fluxes, particularly during the middle period of the experiment. On the contrary, the Meteosat‐derived downward radiative fluxes are comparable to the ship data, and their high time sampling allows us to describe the diurnal cycle of the solar heating. They can be used either at an instantaneous scale (comparison of the satellite flux averaged over 0.1° × 0.1° with 10 min averaged ship fluxes) or at a larger scale (0.3° × 0.3°, 1 hour). It is shown that the surface latent heat flux, derived from a combination of Special Sensor Microwave Imager (SSMI) brightness temperatures and sea surface temperature (SST), as obtained from IR sensors (Advanced Very High Resolution Radiometer (AVHRR), is of a quality similar or better to model bulk fluxes.
Observations of ice thickness and frazil ice in the St. Lawrence Island polynya from satellite imagery, upward looking sonar, and salinity/temperature moorings
Authors: Drucker Robert , Martin Seelye , Moritz Richard
Published: May 2003

For the 1999 winter, this paper examines the behavior of the Bering Sea St. Lawrence Island polynya using a combination of Advanced Very High Resolution Radiometer (AVHRR), RADARSAT synthetic aperture radar (SAR), meteorological data, over‐winter moored upward looking sonars (ULS) and SeaBird salinity/temperature sensors. We define a thermal ice thickness from the AVHRR retrieval of ice surface temperature combined with meteorological observations and a heat flux model. South of the island, we compare the ULS and thermal thicknesses for congelation and frazil ice. When the satellites observe congelation ice over the ULSs, the ULS and thermal ice thicknesses generally agree. When SAR observes Langmuir plumes over the ULSs, which indicate frazil ice formation, the ULSs show scatterers at 5–20 m depths in the water column and the seawater temperatures are either within 0.01°C of freezing or are slightly supercooled. This suggests that during frazil events, crystals either nucleate at depth or are transported to depth by the Langmuir circulation. The combination of the SAR imagery and ULS observations also allow measurement of the pack ice advection velocity, the polynya width and the downwind frazil accumulation thickness, giving widths of 10 to 30 km and thicknesses of 0.1–0.2 m. Substitution of these observed values with the heat flux into the Pease polynya model yields polynya widths that approximately agree with the observed.
Sensitivities of deep‐ocean heat uptake and heat content to surface fluxes and subgrid‐scale parameters in an ocean general circulation model with idealized geometry
Authors: Huang Boyin , Stone Peter H. , Hill Christopher
Published: May 2003

Sensitivities of the net heat flux into the deep ocean (Qnet) and of the deep‐ocean heat content (DOC) below 700 m are studied using an ocean general circulation model and its adjoint. Both are found to have very similar sensitivities. The sensitivity to the surface freshwater flux (E‐P‐R) is positive in the Atlantic but negative in the Pacific and Southern Ocean. A positive sensitivity to the downward net surface heat flux is found only in the North Atlantic north of 40°N and the Southern Ocean. The diapycnal diffusivity of temperature affects Qnet and DOC positively in a large area of the tropics and subtropics in both the Pacific and Atlantic Oceans. The isopycnal diffusivity contributes to Qnet and DOC mainly in the Southern Ocean. Detailed analysis indicates that the surface freshwater flux affects Qnet and DOC by changing vertical velocity, temperature stratification, and overturning circulation. The downward net surface heat flux appears to increase Qnet and DOC by strengthening vertical advection and isopycnal mixing. The contribution of isopycnal diffusivity to Qnet and DOC is largely associated with the vertical heat flux due to isopycnal mixing. Similarly, the diapycnal diffusivity of temperature modulates Qnet and DOC through the downward heat flux due to diapycnal diffusion. The uncertainties of Qnet and DOC are estimated on the basis of the sensitivities and error bars of observed surface forcing and oceanic diffusivities. For DOC they are about 0.7 K (1 K = 3.7 × 1024J) for the isopycnal diffusivity, 0.4 K for the diapycnal diffusivity of temperature, 0.3 K for the surface freshwater flux, and 0.1 K for the net surface heat flux and zonal wind stress. Our results suggest that the heat uptake by ocean general circulation models in climate experiments is sensitive to the isopycnal diffusivity as well to the diapycnal thermal diffusivity.
Turbulence in an estuarine embayment: Observations from Beatrix Bay, New Zealand
Authors: Stevens Craig L.
Published: May 2003

This study characterizes turbulence in a stratified estuarine embayment using measurements of temperature gradient microstructure. The water column was generally strongly stratified with only moderate tidal forcing. Dissipation levels and turbulence length scales in the surface mixing layer, the pycnocline, the fluid interior, and the benthic boundary layer were measured and compared with expectations based on previous work in different systems. Parameters important for modeling, including the measured dissipation ratio (Γd) and the gradient Richardson number (Rig), were calculated. The observations highlight a number of points. (1) The lack of strong tidal flows means that buoyancy effects in estuarine embayments are more important than in the estuary‐proper. (2) The combination of reduced tidal mixing and interacting thermohaline stratification suggests the importance, in certain situations, of diffusive convection. (3) Despite the shallow depths (<40 m), a number of hydrodynamic regimes, vertically adjacent to one another, exist, resulting in a highly variable distribution of turbulence properties.
Impact of a deep ozone hole on Southern Ocean primary production
Authors: Arrigo Kevin R. , Lubin Dan , van Dijken Gert L. , Holm‐Hansen Osmund , Morrow Esther
Published: May 2003

Field studies show that photosynthesis by Antarctic phytoplankton is inhibited by the increased ultraviolet radiation (UVR) resulting from springtime stratospheric ozone (O3) depletion. To extend previous observations, a numerical model utilizing satellite‐derived distributions of O3, clouds, sea ice, surface temperature, and phytoplankton biomass was developed to study the hemispheric‐scale seasonal effects of a deep Antarctic O3 hole on primary production in the Southern Ocean. UVR‐induced losses of surface phytoplankton production were substantial under all O3 conditions, mostly due to UVA. However, when integrated to the 0.1% light depth, the loss of primary production resulting from enhanced fluxes of UVB due to O3 depletion was <0.25%. The loss of primary production is minimized by the strong attenuation of UVR within the water column and by sea ice which is at its peak extent at the time of the most severe O3 depletion.
Low‐frequency variability of the Mediterranean undercurrent downstream of Portimão Canyon
Authors: Chérubin Laurent M. , Serra Nuno , Ambar Isabel
Published: May 2003

Data from current meters deployed during the CANIGO Project from June 1997 to August 1998 are analyzed. We focus on two meridionally aligned current meter moorings located west of the Portimão Canyon. Periodically, an XBT line was performed during the period of the current meter measurements, upstream of Portimão Canyon; it preceded the release of RAFOS floats in each of the two cores of the Mediterranean undercurrent, and some of these floats drifted near the current meter moorings. Two flow regimes are identified at the mooring locations: the first one is an oscillation of the velocity direction (northwest to southwest) with a regular period of about 10 days, which alternates with a second regime similar to mesoscale turbulence in which the currents flow either northward or southward during several days. The trajectories of the RAFOS floats revealed that some of them were trapped in eddies west of Cape St. Vincent when released during the first flow regime. RAFOS released during the second flow regime were trapped in coherent structures (cyclone or anticyclone) passing through or formed in the Portimão Canyon. The regular oscillation of the currents are analyzed in the light of the propagation of Rossby waves. First, the strong stratification above and below the undercurrent is used to diagnose the presence of topographic Rossby waves along the Iberian continental slope. Then, the observed sharp potential vorticity gradients in the undercurrent are considered in order to estimate the characteristics of vortical Rossby waves in a horizontally sheared flow. The comparison of the wavelengths for the two types of waves shows good agreement with the size of the meddies observed in the vicinity of Portimão Canyon and Cape St. Vincent. The waves resonant interaction leads to baroclinic and barotropic instabilities known as mechanisms for meddy formation.
Cross‐shore sediment transport partitioning in the nearshore during a storm event
Authors: Conley Daniel C. , Beach Reginald A.
Published: May 2003

Vertical profiles of fluid velocity and sediment concentration have been collected across the surf‐zone during the SandyDuck '97 field experiment. Analysis from data collected during a 3‐day storm event shows that, in agreement with preceding observations, the mean component of cross‐shore sediment transport throughout the water column and across the surf‐zone is almost uniformly in the offshore direction. Similarly, the wave coherent component is generally onshore directed but does on rare occasions exhibit offshore transport. The infragravity component in these conditions, while exhibiting a definite offshore bias, is generally a negligible component. Thus net cross‐shore transport is predominately a balance between the wave coherent component and the mean component. In observations that stress the importance of the near bed region to cross‐shore sediment transport, it is shown that this balance is a function of elevation above the bed. The increasing importance of the wave coherent component as the bed is approached can lead to situations where the net transport direction shifts from being directed offshore higher in the water column to onshore directed near the bed.
Salt transport and the time‐dependent salt balance of a partially stratified estuary
Authors: Bowen Melissa M. , Geyer W. Rockwell
Published: May 2003

The transport of salt along the channel of an estuary determines the length of the saline intrusion and provides bounds on the transfer of other substances. Observations of salt transport have been hampered by a lack of long time series measurements and complexities of the sampling situation (e.g., local topography and simultaneous variations of tides, freshwater, and winds). Here we estimate salt transport at a relatively uniform section of the Hudson Estuary over a 70‐day period during drought conditions. The salt balance of the estuary is also assessed by comparing salt transport at the section with surveys of total salt content in the estuary. Most salt enters the estuary in monthly pulses of estuarine salt transport (i.e., spatial variations of tidally‐averaged velocity and salinity) during apogean neap tides when vertical stratification is greatest. Oscillatory salt transport (i.e., the covariance of tidally‐varying velocity and salinity) is a more minor player in the salt balance and shows a surprising lack of spring‐neap variation. Integrated salt transport at the section is consistent with estimates of total salt content of the estuary; however, the mismatch in magnitude of the variations indicates that the salinity intrusion is not in equilibrium within the spring‐neap cycle. In contrast, the salt balance adjusts rapidly to an increase in freshwater flow at the end of the deployment. A scaling argument is derived to illustrate the adjustment of the salinity intrusion to the tides and freshwater flow. The results suggest that time dependence is an intrinsic part of the estuarine salt balance.
Modeling study of turbulent mixing over the continental shelf: Comparison of turbulent closure schemes
Authors: Wijesekera H. W. , Allen J. S. , Newberger P. A.
Published: May 2003

The sensitivity of model‐produced time‐dependent wind‐driven circulation on the continental shelf to the turbulent closure scheme employed is studied with a two‐dimensional approximation (variations across‐shelf and in depth) using the Princeton Ocean Model. The level 2.5 Mellor‐Yamada closure (MY), k‐ε closure, and K‐Profile Parameterization schemes are used to evaluate the mesoscale fields and the spatial and temporal variability of mixing. All three submodels produce similar features in the mesoscale circulation. They produce qualitatively similar eddy diffusivities and eddy viscosities, although the turbulent structure and the mixing intensities can differ quantitatively. The k‐ε length scale follows the buoyancy length scale when stratification is important. In contrast, the length scale produced by the q2l equation in the MY scheme deviates substantially from the buoyancy scale unless a stratification‐dependent limitation is imposed. During upwelling‐favorable winds, the majority of turbulent mixing occurs in the top and the bottom boundary layers and in the vicinity of the vertically and horizontally sheared coastal jet. Turbulent mixing in the coastal jet is primarily driven by shear‐production. The near‐surface flow on the inner shelf becomes convectively unstable as wind stress forces the upwelled water to flow offshore in the surface layer. During downwelling‐favorable winds, the strongest mixing occurs in the vicinity of the downwelling front. The largest turbulent kinetic energy and dissipation are found near the bottom of the front. Turbulence in the bottom boundary layer offshore of the front is concentrated between recirculation cells which are generated as a result of symmetric instabilities in the boundary layer flow.
Nonlinear modes of decadal and interannual variability of the subsurface thermal structure in the Pacific Ocean
Authors: Tang Youmin , Hsieh William W.
Published: May 2003

The nonlinear principal component analysis, a neural network technique, is applied to the observed upper ocean heat content anomalies (HCA) in the Pacific basin from 1961 to 2000. By applying the analysis to high‐passed and low‐passed data, nonlinear interannual and decadal modes are extracted separately. The first nonlinear interannual mode is mainly characterized by the El Niño‐Southern Oscillation (ENSO) structure in the tropical Pacific, with considerable asymmetry between warm El Niño and cool La Niña episodes; for example, during strong El Niño, the negative HCA in the western tropical Pacific is much stronger than the corresponding positive HCA during strong La Niña. The first nonlinear decadal mode goes through several notable phases. Two of the phases are related to decadal changes in the La Niña and El Niño characteristics, revealing that the decadal changes for La Niña episodes are much weaker than the changes for El Niño episodes. Other phases of the decadal mode show a possible anomaly link from the middle latitudes to the western tropical Pacific via the subtropical gyre. The decadal changes in the HCA around 1980 and around 1990 were compared and contrasted.
Model simulations of CFC uptake in North Atlantic Deep Water: Effects of parameterizations and grid resolution
Authors: Beismann Jens‐Olaf , Redler René
Published: May 2003

A series of numerical experiments with models of the Atlantic Ocean is analyzed with respect to the uptake of CFC‐11 and its export from the subpolar gyre with the North Atlantic Deep Water. We discuss the influence of parameterizations for air‐sea gas exchange and subgrid‐scale processes on the rate of CFC‐11 that enters the North Atlantic Ocean and its dependence on horizontal grid spacing in models from medium (4/3°) to eddy‐permitting (1/3°) horizontal resolution. Model results are compared with observational estimates of tracer inventories in order to evaluate to what degree the simulations capture realistic CFC distributions. While higher resolution is needed to model details of the CFC distribution, for example, in the Deep Western Boundary Current, the medium resolution models are able to simulate quantitatively satisfying CFC inventories in different water masses. Nevertheless, the inventories derived from the medium‐resolution experiments show a critical dependence on details of the parameterization of the mixing effect of mesoscale eddies and on the representation of bottom boundary layer processes. The numerical representation of eddy activity turns out to be of crucial importance in order to obtain modeled CFC inventories in agreement with observed values, which can be achieved either by carefully choosing the mixing parameterization or by applying higher horizontal resolution. The ratio of CFC‐11 being exported southward from the subpolar North Atlantic to the total CFC‐11 inventory in NADW does not vary significantly over the suite of model experiments.
Wave orbital velocity skewness and linear transition ripple migration: Comparison with weakly nonlinear theory
Authors: Crawford Anna M. , Hay Alex E.
Published: May 2003

Field observations of linear transition ripples during an autumn storm exhibit high correlation between cross‐shore ripple migration rate and the skewness of the near‐bed wave orbital velocity. For a bimodal spectrum of mixed sea and swell, negatively skewed near‐bed orbital velocities were accompanied by offshore ripple migration, while for unimodal swell, ripples migrated onshore under positively skewed velocities. Bispectral analysis and weakly nonlinear wave theory are used to show that the positive and negative velocity skewness arose from higher‐order interactions between frequency components of the incident wave spectrum. There is good quantitative agreement between predicted and observed velocity skewness, indicating a connection between ripple migration and wave‐wave interactions.
Role of aerosols on the Mediterranean solar radiation
Authors: Tragou Elina , Lascaratos Alex
Published: May 2003

The shortwave radiation constitutes a major driving force for the oceanic thermohaline circulation and must be accurately parameterized. An extensive comparison between the monthly mean solar radiation measured at numerous meteorological stations along the Mediterranean coast and the estimated solar radiation from a widely used bulk formula leads to two important conclusions: first, there is systematic overestimation of the shortwave radiation from the bulk formula at the station sites of about 25 W m−2 (averaged over a 30 year period), and second, there is significant interannual variability in the observed solar radiation that cannot be explained either by the variability in the available data for the cloudiness or by the absorption due to water vapor content in the atmospheric column. As the overestimation occurs during summer cloud‐free months, we assume that cloud attenuation is adequately parameterized and the discrepancy between the observations and the estimates comes from the formula for clear‐sky insolation. A correction to the clear‐sky insolation formula is attempted on the basis of recent satellite data on aerosol optical thickness index. Results from our analysis indicate that aerosols may provide an explanation for both the observed weaker shortwave radiation and its interannual variations. This implies that a different parameterization scheme must be sought for the aerosol attenuation in the shortwave radiation formula.
Origin of shore‐normal channels from the shoreface of Sable Island, Canada
Authors: Amos C. L. , Li M. Z. , Chiocci F. L. , La Monica G. B. , Cappucci S. , King E. H. , Corbani F.
Published: May 2003

A suite of modern, shore‐normal channels was discovered on the southern shoreface of Sable Island, Canada. Repetitive multibeam and side scan surveys of these features, as well as box coring and sampling, showed that the depressions were perennial features of the shoreface. They occurred in fine and medium sand and were most abundant in‐depths less than 20 m. They were up to 1 m deep and 50 m wide, increased in size seawards, and generally followed the seabed slope. The channels dominated the inner shoreface becoming wider and deeper seawards to depths in excess of 40 m. There is a continuum of scales in width and depth of these channels, the smallest of which are equated with gutters preserved in the geological record. The internal structure of the shoreface sediments where the gutters are found is characterized by normally graded tempestites composed of cut‐and‐fill structures overlying an erosion surface. Thus, the environmental setting, internal structure, and scale are similar to ancient counterparts. Gutters have been interpreted to result from downwelling over palaeo‐shorefaces during ancient storms [Myrow, 1992b]. The composition of gutter infill, as well as the stability, evolution, associated bed forms, and hydrodynamic conditions under which they formed, were examined to verify genetic interpretations of the ancient counterparts. The multisensor benthic lander RALPH was deployed within a field of shore normal channels for 17 days during late winter 1998. RALPH burst‐sampled flow at hourly intervals while imaging the seabed. Resulting sequential imagery revealed the genesis of a group of five gutters during storm spin‐down that was coincident with the formation of large‐scale wave ripples. This was followed by infilling of the gutters and wave ripple destruction within 20 hours by longshore sand transport. The gutters were located within, and parallel to, a larger channel (50 m wide and 0.50 m deep). They were formed by coastal set‐up and subsequent downwelling that was triggered by the rotation of storm winds from seawards (onshore wind) to landwards (offshore wind). The gutters formed within 2–3 hours during spin‐down of a winter storm under “live‐bed” conditions of sand transport. Wave height and mean tidal flows were well below those of the storm peak and so they were not directly responsible for creating the gutters. The mechanism we propose for the evolution of gutters supports the geological interpretations; that is, a storm‐driven, downwelling event as described by Swift and Niedoroda [1985]. However, the downwelling event appeared to take place suddenly (rapid acceleration in flow), briefly (few hours duration), and within narrow corridors guided by wider and deeper channels of the shoreface. High turbidity during the time of gutter formation leads us to believe that the phenomenon of inner shelf turbidity currents [as suggested by Walker, 1985] exists and may be instrumental in channel formation.
Arctic warming: Evolution and spreading of the 1990s warm event in the Nordic seas and the Arctic Ocean
Authors: Karcher Michael J. , Gerdes Rüdiger , Kauker Frank , Köberle Cornelia
Published: May 2003

Observations in the Arctic Ocean revealed changes in oceanic temperature, salinity and ice cover of the 1990s as compared with earlier data. With a numerical model, we favorably reproduce the development and subsequent propagation of temperature anomalies in water of Atlantic origin in the 1980s and 1990s. These propagated into the Arctic Ocean via the Barents Sea and the Fram Strait. Two warm anomalies entered the Arctic Ocean through these passages. While the first smaller anomaly only warmed up the western Eurasian Basin, the second large anomaly spread far into the eastern Eurasian Basin and across the Lomonossov Ridge into the western Arctic basins. Intensified boundary currents during the high NAO state in the first half of the 1990s significantly influenced the amplitude and speed of propagation of the temperature anomalies inside the Arctic Ocean. In contrast to the notion of a continuous warming process during the 1990s, our model results suggest the warming of the Atlantic Layer in the Arctic Ocean occurred in the form of events. The event with the largest anomalous heat input during the modeled period entered the Arctic between 1989 and 1994. It is possible to trace back the additional heat input into the Arctic to an increased volume inflow via the Faroer‐Scotland passage and reduced heat loss to the atmosphere in the early 1990s. After a weaker warm inflow in the second half of the 1990s, the most recent observations and the model results point to a recurring warm anomaly in the inflow from 1999 onward.
Southern Ocean productivity in relation to spatial and temporal variation in the physical environment
Authors: Constable Andrew J. , Nicol Stephen , Strutton Peter G.
Published: May 2003

The physical factors that have been reported to affect primary and secondary production in the Southern Ocean are examined and critically reviewed. Long time series of physical measurements from the Southern Ocean are available and there is a theoretical base from which models can be constructed. In contrast, there are few large‐scale measurements of biological parameters and a paucity of long‐term biological data sets for the Antarctic region. The absence of predictive models for the biological systems of the region is underpinned by the absence of theoretical understanding of the variations in the physical environment and their effects on primary, secondary, or tertiary production. To further this understanding, we have examined some of the major seasonal and interannual physical data available for the region (sea ice extent and retreat rate, wind stress, and surface ocean circulation patterns) and have examined their relationship to spatial and temporal variation in satellite‐derived proxies of primary productivity (Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data). The results indicate that there are regional differences in the dominant physical forcings and that simple models will fail to replicate the observed patterns of primary production. We have also used the dynamics of Antarctic krill in the South Atlantic as an example to develop a model and explore the various hypotheses that have been put forward to explain interannual variability in this region. Results from this model indicate that the physical system may change in ways that cause periodic shifts in the relative importance of the factors that affect secondary production. The implications for the design of future research programs are explored.
On two‐phase sediment transport: Dilute flow
Authors: Hsu Tian‐Jian , Jenkins James T. , Liu Philip L.‐F.
Published: May 2003

In this paper we present a model describing dilute sediment transport that involves two‐phase mass and momentum equations and k − ϵ equations for the energy of turbulent flow. We apply the model to a steady, uniform, open channel flow and calculate sediment concentration and velocity profiles in two ways. In a semianalytical approach we adopt boundary layer approximations and obtain concentration profiles that improve upon the single‐phase Rouse formula [Rouse, 1937]. The improvement is due to the incorporation of dissipation of the flow turbulent energy due to the presence of sediment. In a numerical approach we integrate the time‐dependent equations of the model to obtain steady state solutions. The numerical solutions are tested against experimental data and used to check the validity of the boundary layer approximations in the semianalytical approach.
Implications of cabbeling on the formation and transformation mechanism of North Pacific Intermediate Water
Authors: You Yuzhu
Published: May 2003

The World Ocean Atlas 1994 is used to study the formation and transformation of North Pacific Intermediate Water (NPIW), paying attention to the broad subarctic‐tropical frontal zone (SATFZ). Four neutral density surfaces, σN = 26.5, 26.9, 27.2, and 27.4, are mapped to span the NPIW layer with the σN = 26.9 surface identified with the salinity minimum of NPIW. Various dianeutral motion processes, cabbeling, thermobaricity, vertical turbulent mixing and double‐diffusion, and water‐mass property conversion are quantified; cabbeling appears to be the most important. Spatially, dianeutral transport is strongest in the SATFZ. Within the SATFZ subdomain 36°N–44°N, 142°E–140°W, with assumed epineutral and dianeutral diffusivities of K = 103 m2 s−1 and D = 10−5 m2 s−1 cabbeling contributes to a maximum dianeutral downwelling transport of 0.9 Sv on the σN = 26.5 neutral density surface (the upper boundary of NPIW) and 0.7 Sv on the σN = 26.9 surface (at the NPIW core). In meridional sections, cabbeling shows epineutral extension from the SATFZ towards the subtropical gyre, most apparent along the σN = 26.9 surface. Dianeutral transport is found to be sensitive to the epineutral diffusivity K on the upper two surfaces σN = 26.5 and 26.9 where the transport is dominated by cabbeling. With only half an order of magnitude increase in diffusivity K = 103.5m2 s−1, the total NPIW (about 3 Sv) can be accomplished by cabbeling alone. These results suggest that cabbeling plays a crucial role in the NPIW transformation. Since cabbeling describes physically the dianeutral downwelling resulted from epineutral mixing, strong cabbeling in the SATFZ implies that NPIW sources are first transformed by cabbeling from the upper to the lower neutral density surfaces, and subsequently, NPIW is carried by epineutral advection and gyre circulation into the subtropical interior. Thus cabbeling is revealed as the principle mechanism responsible for transforming subpolar source waters into subtropical NPIW water via the SATFZ.
Effects of wave‐current interaction on shear instabilities of longshore currents
Authors: Özkan‐Haller H. Tuba , Li Ye
Published: May 2003

We examine the effects of wave‐current interaction on the dynamics of instabilities of the surf zone longshore current. We utilize coupled models for the simulation of the incident waves and the wave‐induced nearshore circulation. The coupling between the models occurs through radiation stress gradient terms (accounting for the generation of nearshore circulation) and through wave‐current interaction terms (leading to the modification of the wave field by the generated circulation field). Simulations are carried out with a realistic barred beach configuration and obliquely incident waves for two frictional regimes. The results show that the shear instabilities of the longshore current have a significantly altered finite amplitude behavior when wave‐current interaction effects are included for beaches with relatively high frictional damping. The primary effects are a reduction of the offshore extent of the motions and a delay of the onset of instabilities. In addition, the energy content of the motions within two surf zone widths is reduced, the propagation speed increases, and tendency to form offshore directed jets is reduced. The horizontal mixing induced by the instabilities is also reduced when wave‐current interaction is considered, leading to a larger peak mean longshore current and a larger offshore current shear. These effects appear to be primarily linked to a feedback mechanism, whereby the incident wave field gains energy at locations of offshore directed currents. For more energetic shear instability fields that occur when frictional damping is small, this feedback affects the propagation speed and energy content of the instabilities near and onshore of the current peak only minimally. However, the offshore extent of the motions and the tendency to shed vortices offshore are still reduced. A reduction in the mixing due to the instabilities is evident offshore of the current peak, hence the mean longshore current profile is only affected offshore of the current peak. The inclusion of wave‐current interaction significantly affects the shear instability signature observed in the shoreline runup for either frictional regime. These results indicate that the energy content and frequency extent of the shoreline response is increased markedly due to the wave‐current interaction process. This effect appears to be related to variations in the forcing of the circulation that arise due to the refraction of the incident waves around offshore directed features of the circulation.
1DV bottom boundary layer modeling under combined wave and current: Turbulent separation and phase lag effects
Authors: Guizien Katell , Dohmen‐Janssen Marjolein , Vittori Giovanna
Published: May 2003

On the basis of the Wilcox [1992] transitional k‐ω turbulence model, we propose a new k‐ω turbulence model for one‐dimension vertical (1DV) oscillating bottom boundary layer in which a separation condition under a strong, adverse pressure gradient has been introduced and the diffusion and transition constants have been modified. This new turbulence model agrees better than the Wilcox original model with both a direct numerical simulation (DNS) of a pure oscillatory flow over a smooth bottom in the intermittently turbulent regime and with experimental data from Jensen et al. [1989], who attained the fully turbulent regime for pure oscillatory flows. The new turbulence model is also found to agree better than the original one with experimental data of an oscillatory flow with current over a rough bottom by Dohmen‐Janssen [1999]. In particular, the proposed model reproduces the secondary humps in the Reynolds stresses during the decelerating part of the wave cycle and the vertical phase lagging of the Reynolds stresses, two shortcomings of all previous modeling attempts. In addition, the model predicts suspension ejection events in the decelerating part of the wave cycle when it is coupled with a sediment concentration equation. Concentration measurements in the sheet flow layer give indication that these suspension ejection events do occur in practice.
Observations of nearshore circulation: Alongshore uniformity
Authors: Feddersen Falk , Guza R. T.
Published: May 2003

Nearshore circulation, observed for 4 months on a 200‐m‐long stretch of natural beach during the SandyDuck field experiment, is shown to be alongshore uniform. An alongshore momentum balance between (wind and wave) forcing and bottom stress, cross‐shore integrated between the shoreline and approximately 4 m water depth, holds on each of five instrumented cross‐shore transects (skill ≥ 0.87). The corresponding five best fit drag coefficients are similar, consistent with the assumption that terms in the momentum balance associated with alongshore nonuniformity are negligible. In addition, the alongshore nonuniformity of the circulation and bathymetry were examined at five cross‐shore locations. Except near the shoreline, the circulation and bathymetry were rarely strongly alongshore nonuniform, and the circulation nonuniformities were usually no larger than expected from current‐meter noise alone. Near the shoreline, the bathymetry was more irregular and the circulation was often detectably nonuniform, although no relationship between bathymetric and circulation nonuniformities was found. The closure of the alongshore momentum balances on cross‐shore transects, and the observed alongshore uniformity of the circulation on four of five alongshore transects, demonstrates that the simplified dynamics of alongshore uniform circulation are valid during the experiment.
Characteristic patterns of shelf circulation at the boundary between central and southern California
Authors: Winant C. D. , Dever E. P. , Hendershott M. C.
Published: May 2003

The coastal circulation in the Santa Barbara Channel (SBC) and the southern central California shelf is described in terms of three characteristic flow patterns. The upwelling pattern consists of a prevailing equatorward flow at the surface and at 45 m depth, except in the area immediately adjacent to the mainland coast in the SBC where the prevailing cyclonic circulation is strong enough to reverse the equatorward tendency and the flow is toward the west. In the surface convergent pattern, north of Point Conception, the surface flow is equatorward while the flow at 45 m depth is poleward. East of Point Conception, along the mainland coast, the flow is westward at all depths and there results a convergence at the surface between Point Conception and Point Arguello, with offshore transport over a distance on the order of 100 km. Beneath the surface layer the direction of the flow is consistently poleward. The relaxation pattern is almost the reverse of the upwelling pattern, with the exception that in the SBC the cyclonic circulation is such that the flow north of the Channel Islands remains eastward, although weak. The upwelling pattern is more likely to occur in March and April, after the spring transition, when the winds first become upwelling favorable and while the surface pressure is uniform. The surface convergent pattern tends to occur in summer, when the wind is still strong and persistently upwelling favorable, and the alongshore variable upwelling has build up alongshore surface pressure gradients. The relaxation pattern occurs in late fall and early winter, after the end of the period of persistent upwelling favorable winds.
Seasonal and decadal patterns of soil carbon uptake and emission along an age sequence of burned black spruce stands in interior Alaska
Authors: O'Neill Katherine P. , Kasischke Eric S. , Richter Daniel D.
Published: May 2003

Postfire changes in the local energy balance and soil chemistry may significantly alter rates of carbon turnover in organic‐rich soils of boreal forests. This study combines field measurements of soil carbon uptake and emission along a 140‐year chronosequence of burned black spruce stands to evaluate the timescales over which these disturbance effects operate. Soil CO2 efflux increased as a function of stand age at a mean rate of 0.12 Mg C ha−1 yr−2 up to a maximum of 2.2 Mg C ha−1 yr−1 in the 140‐year‐old stand. During this same time period, organic soil horizons sequestered carbon and nitrogen at rates of 0.28–0.54 and 0.0076 Mg N ha−1 yr−1, respectively. A mass balance model based on field measurements suggests that postfire changes in root and microbial respiration caused these soils to function as a net source of carbon for 7–15 years after fire, releasing between 1.8 and 11.0 Mg C ha−1 to the atmosphere (12.4–12.6% of total soil organic matter). These estimates are on the same order of magnitude as carbon losses during combustion and suggest that current models may underestimate the effect of fire on carbon emissions by a factor of 2.
OH formation by HONO photolysis during the BERLIOZ experiment
Authors: Alicke B. , Geyer A. , Hofzumahaus A. , Holland F. , Konrad S. , Pätz H. W. , Schäfer J. , Stutz J. , Volz‐Thomas A. , Platt U.
Published: May 2003

The photolysis of nitrous acid (HONO) in the early morning hours is an important source of OH radicals, the most important daytime oxidizing species. Although the importance of this mechanism has been recognized for many years, no accurate quantification of this OH source is available, and the role of HONO photolysis is often underestimated. We present measurements of HONO and its precursor NO2 by Differential Optical Absorption Spectroscopy (DOAS) during the Berliner Ozonexperiment (BERLIOZ) field campaign in July/August 1998 at Pabstthum near Berlin, Germany. HONO concentrations, formation rates, and simultaneously measured HONO photolysis frequencies are used to calculate the total amount of OH formed by HONO photolysis during a full diurnal cycle. A comparison with the OH formation by photolysis of O3 and HCHO and by the reaction of alkenes with ozone shows that HONO photolysis contributed up to 20% of the total OH formed in a 24 hour period during this campaign. In the morning hours, HONO photolysis was by far the most important OH source during BERLIOZ.
Nighttime formation of peroxy and hydroxyl radicals during the BERLIOZ campaign: Observations and modeling studies
Authors: Geyer Andreas , Bächmann Kurt , Hofzumahaus Andreas , Holland Frank , Konrad Stefan , Klüpfel Thomas , Pätz Hans‐Werner , Perner Dieter , Mihelcic Djuro , Schäfer Hans‐Jürgen , Volz‐Thomas Andreas , Platt Ulrich
Published: May 2003

Traditionally, tropospheric radical chemistry is discussed in terms of the daytime photochemically produced hydroxyl radical (OH). Radicals, however, are also important during nighttime: this is especially true for ozone and the nitrate radical (NO3), which both act as key initiators of the degradation of alkenes such as biogenic monoterpenes. These reactions lead to the formation of peroxy radicals (HO2 and RO2) and hydroxyl radicals at night. We present recent observations of nighttime concentrations of NO3, RO2, HO2, and OH by differential optical absorption spectroscopy (DOAS), matrix isolation electron spin resonance (MIESR), laser‐induced fluorescence (LIF), and a chemical amplifier (CA) in the framework of the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum, Germany, together with modeling studies of nocturnal radical chemistry. Modeled RO2 mixing ratios reached 40 ppt while the measured ROx level went up to 22 ppt at the same time. Modeled and measured HO2 mixing ratios were up to 6 and 4 ppt, respectively. In the case of OH, a nocturnal concentration of (1.85 ± 0.82) × 105 cm−3 was measured during one night. At this time, the model yielded an OH level of (4.1 ± 0.7) × 105 cm−3. This overestimation by the model could point to a missing nocturnal sink of OH. Nitrate radical reactions with terpenes were found responsible for producing 77% of the RO2 radicals, 53% of the HO2, and 36% of the OH radicals during night. Nighttime ozonolysis formed 12% of the RO2, 47% of the HO2, and 64% of the OH radicals. Another 11% of the RO2 radicals were formed by OH–volatile organic compound (VOC) reactions. A positive linear correlation of RO2 and NO3 was observed and could be reproduced in model calculations originating from the loss of both radicals by reaction with NO and the NO3‐initiated RO2 production. The contribution of nighttime OH to the atmosphere's oxidation capacity (oxidation rate of VOCs, CO, and CH4) was found negligible (<0.5%).