Some observations of internal wave current fluctuations at the shelf-edge and their implications for sediment transport

Observations of internal wave current fluctuations at a site on the European continental shelf are described. These have revealed current ‘pulses’ of regular tidal (M₂) phase which may be associated with internal tides generated at the shelf-edge. Current ‘pulses’ have been observed with amplitudes of 30 to 40 cm s^?1 superimposed on peak spring tidal currents of the order 60 to 70 cm s^?1. The measurements have shown that these fluctuations extended throughout the bottom mixed layer to within at least 2 m of the sea bed where they may play an important role in modifying sediment transport rates.     

Nearshore currents on the southern Namaqua shelf of the Benguela upwelling system

Nearshore currents of the southern Namaqua shelf were investigated using data from a mooring situated three and a half kilometres offshore of Lambert's Bay, downstream of the Cape Columbine upwelling cell, on the west coast of South Africa. This area is susceptible to harmful algal blooms (HABs) and wind-forced variations in currents and water column structure are critical in determining the development, transport and dissipation of blooms. Time series of local wind data, and current and temperature profile data are described for three periods, considered to be representative of the latter part of the upwelling season (27 January–22 February), winter conditions (5–29 May) and the early part of the upwelling season (10 November–12 December) in 2005. Differences observed in mean wind strength and direction between data sets are indicative of seasonal changes in synoptic meteorological conditions. These quasi-seasonal variations in wind forcing affect nearshore current flow, leading to mean northward flow in surface waters early in the upwelling season when equatorward, upwelling-favourable winds are persistent. Mean near-surface currents are southward during the latter part of the upwelling season, consistent with more prolonged periods of relaxation from equatorward winds, and under winter conditions when winds were predominantly poleward. Within these seasonal variations in mean near-surface current direction, two scales of current variability were evident within all data sets: strong inertial oscillations were driven by diurnal winds and introduced vertical shear into the water column enhancing mixing across the thermocline, while sub-inertial current variability was driven by north–south wind reversals at periods of 2–5 days. Sub-inertial currents were found to lag wind reversals by approximately 12 h, with a tendency for near-surface currents to flow poleward in the absence of wind forcing. Consistent with similar sites along the Californian and Iberian coasts, the headland at Cape Columbine is considered to influence currents and circulation patterns during periods of relaxation from upwelling-favourable winds, favouring the development of a nearshore poleward current, leading to poleward advection of warm water, the development of stratification, and the creation of potentially favourable conditions for HAB development.     

Winter variability of physical processes and sediment-transport events on the Eel River shelf,northern California

A 5-yr data set of near-bed current and suspended-sediment concentration measured within 2 m of the seabed in 60-m water depth has been analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River continental shelf, northern California. This data set encompasses a wide range of shelf conditions with winter events characterized as: Major Flood (1996/97), strong El Niño (1997/98), strong La Niña (1998/99), and Major Storm (1999/00). Data were collected at a site located 25 km north of the Eel River mouth, on the landward edge of the mid-shelf mud deposit. During the winter months sediment resuspension is forced primarily by near-bed oscillatory flows, and sediment transport occurs both as suspended load and as gravity-driven (fluid-mud) flows. Winter conditions that caused periods of increased sediment transport existed on average for 142 d yr⁻¹ over the total record, ranging between 89 d in the Major Flood year (1996/97) and 171 d in the La Niña year (1998/99). Hourly averaged values of significant wave height varied between 0.5 and 10.7 m and hourly averaged values of near-bed orbital velocities ranged between 0 and 125 cm s⁻¹. During the five winters, sediment threshold conditions were exceeded an average of 35% of the time, ranging from 19% in the Major Flood year (1996/97) to 52% in the La Niña year (1998/99). Mean concentration of suspended sediment, measured at 30 cmab, ranged from values close to 0–8 g l⁻¹. Among winters, major sediment flux events exhibited different patterns due to varying combinations of physical processes including river floods, waves, and shelf circulation. Within winters, the major period of sediment flux varied from a 3-d fluid mud event (Major Flood winter) to a 50-d period of persistent southerlies (El Niño winter) and a winter of continuous storm cycles (La Niña winter). Winter-averaged suspended-sediment concentration appeared to vary in response to river discharge, while total sediment flux responded to storm intensity. The net sediment flux appeared to depend on timing of river discharge and shelf conditions. On the Eel River shelf, the mid-shelf mud deposit apparently is not emplaced by deposition from the river plume, but by secondary processes from the inner shelf including off-shelf transport of sediment suspensions and gravity-driven fluid-mud flows. Thus, these inner-shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that forms and nourishes the mid-shelf deposit. Large-scale shelf circulation patterns and interannual variability of the physical forcing are also important in determining the locus of the mid-shelf deposit, and both are influenced by climate variations. Post-depositional alteration of the deposit also depends on the subsequent shelf conditions following major floods.     

Dynamics,sediment transport,and morphology in a tide-dominated embayment

Western Port, Victoria, Australia is a tide-dominated embayment with an unusual and complex shape. Bottom currents and circulation and their effects on sediment processes were examined using instrumented tripods to measure currents, tides and wave activity, and to estimate sediment transport at nine locations in the bay. Overall bottom water movement patterns were determined by use of seabed drifters. The characteristics of the bay reflect a small catchment and low freshwater and sediment input. A complex system of channels is flanked by extensive intertidal areas. Tidal range varies up to more than 3 m, generating bottom currents up to 70cm s^?1. Flow directions generally conform to channel alignments but major deviations are important. Net circulation in the bay is clockwise around the large central island. The ratio of tidal range to half-tidal period (ΔH/ΔT) when compared with measured currents gave a method of prediction of the annual frequency distribution of maximum bottom current velocities. Determination of threshold current velocities enabled prediction of annual frequency of bedload movement (generally 50–100 per cent of tide cycles). Bedload mass transport for all observed tide cycles was calculated, and estimates of annual mass transport capacity (between 10² and 10⁶ g cm^?1 a^?1( were obtained from a relationship between predicted tidal conditions and mass transport. Seabed drifters delineated the major bottom water movements in the bay and adjacent Bass Strait, and also detailed circulation patterns. This linked the data from the tripod stations, in particular patterns of ebb- and flood-dominance. A coherent picture of the processes operating in Western Port is presented by integrating these studies with corroborative studies of sediment distribution and morphology, hydrochemistry, and mathematical modelling. Some morphological characteristics related to tide-dominance are discussed.     

Assessing the importance of tropical cyclones on continental margin sedimentation in the Mississippi delta region

Recent research on the Mississippi margin indicates notable seasonal variation in seabed dynamics. During years with minimal tropical-system activity, sediments initially deposited from late spring to early fall are remobilized by wind-driven currents and wave energy during extra-tropical weather systems in the winter. This research reveals the profound significance of tropical cyclones on Louisiana Shelf sedimentation. The amount of material delivered to and advected across the shelf by recent tropical cyclones is considerably larger than that related to winter storm systems. In Fall 2004, the river-dominated shelf of Louisiana was impacted by three tropical systems in less than a month, including Hurricane Ivan. Ivan, with maximum sustained winds in excess of 74 m s⁻¹ (144 knots) and a minimum measured central pressure of 910 mbar, was the eighth most intense Atlantic hurricane on record at the time. In order to assess the impact these tropical systems had on the continental margin west of the Mississippi delta, seabed samples were collected from box cores in October 2004 and analyzed for particle-reactive radionuclides ²³⁴Th, ⁷Be, and ²¹⁰Pb. Radiochemical data and observations from X-radiographs indicate event-driven sediment deposits ranged from 4 to 30 cm on the shelf and 2–6 cm in the Mississippi Canyon. These deposits exhibit distinct radiochemical signatures and differ visually and texturally from the underlying sediment. The well-developed physical stratification and graded nature of the deposits observed in core X-radiographs suggests that the sediment could have been deposited from sediment-gravity flows. Inventories of ⁷Be and ⁷Be/²³⁴Th_xs ratios reveal this series of cyclones transported considerably more material to the outer shelf and slope than periods of minimal tropical-system activity. When compared to seasonal depositional rates created by winter storms, tropical-cyclone-related event deposits on the middle and outer shelf are up to an order of magnitude greater in thickness. The number and thickness of these event deposits decrease with distance from the delta and suggest that only the most severe tropical systems are likely capable of redistributing significant quantities of sediment to more distal portions of the shelf and slope. These severe-event-driven deposits may account for as much as 75% of the sediment burial budget on decadal time scales within Mississippi Canyon. Higher than average tropical cyclone activity, predicted by the National Hurricane Center over the next decade, may be the major mechanism controlling sediment transport and deposition on the Mississippi River continental shelf and in Mississippi Canyon.     

Hydrologic and biogeochemical controls on the spatial and temporal patterns of nitrogen and phosphorus in the Kuparuk River,arctic Alaska

Nitrogen (N) and phosphorus (P) dynamics in the Kuparuk River in arctic Alaska were characterized in a 3‐year study using routine samples near the mouth of the river at the Arctic Ocean, synoptic whole‐river surveys, and temporally intense sampling during storms in three headwater basins. The Lower Kuparuk River has low nitrate concentrations (mean [NO₃]‐N] = 17 µg l^?1 ± 1·6 SE) and dissolved inorganic N (DIN, mean [N] = 31 µg l^?1 ± 1·2 SE) compared with rivers in more temperate environments. Organic forms constituted on average 90% of the N exported to the Arctic Ocean, and high ratios of dissolved organic N (DON) to total dissolved N (TDN) concentrations (mean 0·92) likely result from waterlogged soils formed by reduced infiltration due to permafrost and low hydrologic gradients. Annual export of TDN, DON, and particulate N averaged 52 kg km^?2, 48 kg km^?2, and 4·1 kg km^?2 respectively. During snowmelt, the high volume of runoff typically results in the highest nutrient loads of the year, although high discharge during summer storms can result in substantial nutrient loading over short periods of time. Differences in seasonal flow regime (snowmelt versus rain) and storm‐driven variation in discharge appear to be more important for determining nutrient concentrations than is the spatial variation in processes along the transect from headwaters towards the ocean. Both the temporal variation in nitrate:DIN ratios of headwater streams and the spatial variation in nitrate:DIN between larger sub‐basins and smaller headwater catchments is likely controlled by shifts in nitrification and soil anoxia. Copyright © 2008 John Wiley & Sons, Ltd.     

Suspended sediment discharge in subsurface flow from the head hollow of a small forested watershed,northern Japan

Surface flow and suspended sediment discharge from the head hollow of the Jozankei Experimental Watershed in Hokkaido, northern Japan, were measured to clarify the implications of subsurface hydrology for soil movement. Subsurface discharges during the extremely large storms of 1993 to 1994 were measured in a V-notch weir installed at a natural spring near the bottom of the head hollow, and shallow groundwater levels were observed in the wells excavated in the hollow. Sediment samples whose particle size range from 0·001 to 0·1 mm were manually and automatically collected at 15 to 60 min intervals, by use of 1 or 21 polyethylene bottles. Maximum concentration and flux of suspended sediment during the storms preceded the peak discharge of subsurface flow by several hours. Neither the changes in concentration (mg l⁻¹) nor flux (mg s⁻¹) of suspended sediment coincided with those in subsurface discharge (l s⁻¹). Furthermore, sediment concentration was poorly correlated with the rate of change in subsurface discharge (l s⁻²) during the rising limb of the hydrograph. Suspended sediment flux during the acceleratory limb, however, was closely correlated with the rate of change in subsurface discharge. The relationship between suspended sediment flux and rate of change in subsurface discharge were in inverse proportion to initial subsurface discharge before the storm runoff and they represented rare seasonal variation. Subsurface hydraulic erosion and transport of suspended sediment resulting from changes in rate of change in subsurface discharge actively occur during the acceleratory rising limb of the hydrograph. Accordingly, subsurface hydraulic erosion during the acceleratory rising limb of the hydrograph can be physically understood by analysing suspended sediment flux associated with rate of change in subsurface discharge and initial subsurface discharge. © 1997 John Wiley & Sons, Ltd.     

Rill erosion along the thalweg of a hillslope hollow: A case study from the cotswold hills,Central England

Rill erosion is an important erosional form on agricultural soils in England, causing large losses of soil, particularly on cultivated slopes. This paper describes a rill system that developed in a small agricultural catchment in north Oxfordshire during the winter of 1992–93. The rill system comprised two components: a system of ‘feeder rills’ along the valley-side slopes, which were the result of flow concentration and erosion along wheelings, and a thalweg rill, which formed along a dry valley bottom as a result of surface runoff concentration from the feeder rills. Total volumetric soil loss from the rill system was 32·28 m³, equivalent to 3·01 m³, ha^?1 for the rill catchment area, or 3·91 t ha^?1. Mean discharge for the thalweg rill and feeder rills, calculated during a storm event, was 31·101s^?1 and 1·171s^?1, respectively. All flows were fully turbulent and supercritical. We emphasize the need for a spatially distributed approach to the study of runoff and erosion at the catchment scale.     

Tidal and residual currents over abrupt deep-sea topography based on shipboard ADCP data and tidal model solutions for three popular bathymetry grids

The response of tidal and residual currents to small-scale morphological differences over abrupt deep-sea topography (Seine Seamount) was estimated for bathymetry grids of different spatial resolution. Local barotropic tidal model solutions were obtained for three popular and publicly available bathymetry grids (Smith and Sandwell TOPO8.2, ETOPO1, and GEBCO08) to calculate residual currents from vessel-mounted acoustic Doppler current profiler (VM-ADCP) measurements. Currents from each tidal solution were interpolated to match the VM-ADCP ensemble times and locations. Root mean square (RMS) differences of tidal and residual current speeds largely follow topographic deviations and were largest for TOPO8.2-based solutions (up to 2.8 cm?s^?1) in seamount areas shallower than 1,000 m. Maximum RMS differences of currents obtained from higher resolution bathymetry did not exceed 1.7 cm?s^?1. Single depth-dependent maximum residual flow speed differences were up to 8 cm?s^?1 in all cases. Seine Seamount is located within a strong mean flow environment, and RMS residual current speed differences varied between 5 % and 20 % of observed peak velocities of the ambient flow. Residual flow estimates from shipboard ADCP data might be even more sensitive to the choice of bathymetry grids if barotropic tidal models are used to remove tides over deep oceanic topographic features where the mean flow is weak compared to the magnitude of barotropic tidal, or baroclinic currents. Realistic topography and associated flow complexity are also important factors for understanding sedimentary and ecological processes driven and maintained by flow–topography interaction.     

Assessing the fidelity of surface currents from a coastal ocean model and HF radar using drifting buoys in the Middle Atlantic Bight

The rapid expansion of urbanization along the world’s coastal areas requires a more comprehensive and accurate understanding of the coastal ocean. Over the past several decades, numerical ocean circulation models have tried to provide such insight, based on our developing understanding of physical ocean processes. The systematic establishment of coastal ocean observation systems adopting cutting-edge technology, such as high frequency (HF) radar, satellite sensing, and gliders, has put such ocean model predictions to the test, by providing comprehensive observational datasets for the validation of numerical model forecasts. The New York Harbor Observing and Prediction System (NYHOPS) is a comprehensive system for understanding coastal ocean processes on the continental shelf waters of New York and New Jersey. To increase confidence in the system’s ocean circulation predictions in that area, a detailed validation exercise was carried out using HF radar and Lagrangian drifter-derived surface currents from three drifters obtained between March and October 2010. During that period, the root mean square (RMS) differences of both the east–west and north–south currents between NYHOPS and HF radar were approximately 15 cm s^?1. Harmonic analysis of NYHOPS and HF radar surface currents shows similar tidal ellipse parameters for the dominant M₂ tide, with a mean difference of 2.4 cm s^?1 in the semi-major axis and 1.4 cm s^?1 in the semi-minor axis and 3° in orientation and 10° in phase. Surface currents derived independently from drifters along their trajectories showed that NYHOPS and HF radar yielded similarly accurate results. RMS errors when compared to currents derived along the trajectory of the three drifters were approximately 10 cm s^?1. Overall, the analysis suggests that NYHOPS and HF radar had similar skill in estimating the currents over the continental shelf waters of the Middle Atlantic Bight during this time period. An ensemble-based set of particle tracking simulations using one drifter which was tracked for 11 days showed that the ensemble mean separation generally increases with time in a linear fashion. The separation distance is not dominated by high frequency or short spatial scale wavelengths suggesting that both the NYHOPS and HF radar currents are representing tidal and inertial time scales correctly and resolving some of the smaller scale eddies. The growing ensemble mean separation distance is dominated by errors in the mean flow causing the drifters to slowly diverge from their observed positions. The separation distance for both HF radar and NYHOPS stays below 30 km after 5 days, and the two technologies have similar tracking skill at the 95 % level. For comparison, the ensemble mean distance of a drifter from its initial release location (persistence assumption) is estimated to be greater than 70 km in 5 days.     

Drifter observations of the Hebrides slope current and nearby circulation patterns

The mean flow at and around the Hebrides and Shetland Shelf slope is measured with ARGOS tracked drifters. Forty-two drifters drogued at 50 m were deployed in three circles over the Hebrides slope at 56.15°N in two releases, one on 5th December, 1995 and the second on 5–9th May, 1996. The circles span a distance of some 20 km from water depths of 200 m to 1200 m. Drifters are initially advected poleward along-slope by the Hebrides slope current at between 0.05 and 0.70 m s^–1 in a laterally constrained (25–50 km wide) jet-like flow. Drifters released in winter remained in the slope current for over 2000 km whilst summer drifters were lost from the slope current beyond the Wyville-Thomson Ridge, a major topographic feature at 60°N. Dispersion from the slope region into deeper waters occurs at bathymetric irregularities, particularly at the Anton Dohrn Seamount close to which the slope current is found to bifurcate, both in summer and winter, and at the Wyville-Thomson Ridge where drifters move into the Faeroe Shetland Channel. Dispersion onto the continental shelf occurs sporadically along the Hebrides slope. The initial dispersion around the Hebrides slope is remarkably sensitive to initial position, most of the drifters released in shallower water moving onto the shelf, whilst those in 1000 m or more are mostly carried away from the slope into deeper water near the Anton Dohrn Seamount. The dispersion coefficients estimated in directions parallel and normal to the local direction of the 500 m contour, approximately the position of the slope current core, are approximately 8.8 × 10³ m² s^–1 and 0.36 × 10³ m² s^–1, respectively, during winter, and 11.4 × 10³ m² s^–1 and 0.36 x 10³ m² s^–1, respectively, during summer. At the slope there is a minimum in across-slope mean velocity, Reynolds stress, and across-slope eddy correlations. The mean across-slope velocity associated with mass flux is about 4 × 10^–3 m s^–1 shelfward across the shelf break during winter and 2 × 10^–3 m s^–1 during summer. The drifters also sampled local patterns of circulation, and indicate that the source of water for the seasonal Fair Isle and East Shetland currents are the same, and drawn from Atlantic overflows at the Hebrides shelf.     

Residual vorticity induced by the action of tidal currents in combination with bottom topography in the Southern Bight of the North Sea

Abstract

Results are presented of calculations on the generation of residual vorticity by tidal currents over the bottom topography of the Southern Bight of the North Sea. A typical order of magnitude is 10^?6 to 10^?7 s ^?1. This is compared with current measurements on calm days, when similar magnitudes are found. At windspeeds less than about 5 m/s tidal generation of residual vorticity is important; at higher windspeeds wind effects begin to dominate. Our results are relevant in understanding the spatial variability of residual currents, because a non-zero vorticity implies the existence of horizontal gradients in the residual current field.     

Transfer of carbon dioxide and methane through the soil‐water‐atmosphere system at Mer Bleue peatland,Canada

Surface waters associated with peatlands, supersaturated with CO₂ and CH₄ with respect to the atmosphere, act as important pathways linking a large and potentially unstable global repository of C to the atmosphere. Understanding the drivers and mechanisms which control C release from peatland systems to the atmosphere will contribute to better management and modelling of terrestrial C pools. We used non‐dispersive infra‐red (NDIR) CO₂ sensors to continuously measure gas concentrations in a beaver pond at Mer Bleue peatland (Canada); measurements were made between July and August 2007. Concentrations of CO₂ in the surface water (10 cm) reached 13 mg C l^?1 (epCO₂ 72), and 26 mg C l^?1 (epCO₂ 133) at depth (60 cm). The study also showed large diurnal fluctuations in dissolved CO₂ which ranged in amplitude from ～1·6 mg C l^?1 at 10 cm to ～0·2 mg C l^?1 at 60 cm depth. CH₄ concentration and supersaturation (epCH₄) measured using headspace analysis averaged 1·47 mg C l^?1 and 3252, respectively; diurnal cycling was also evident in CH₄ concentrations. Mean estimated evasion rates of CO₂ and CH₄ over the summer period were 44·92 ± 7·86 and 0·44 ± 0·25 µg C m^?2s^?1, respectively. Open water at Mer Bleue is a significant summer hotspot for greenhouse gas emissions within the catchment. Our results suggest that CO₂ concentrations during the summer in beaver ponds at Mer Bleue are strongly influenced by biological processes within the water column involving aquatic plants and algae (in situ photosynthesis and respiration). In terms of carbon cycling, soil‐stream connectivity at this time of year is therefore relatively weak. Copyright © 2008 John Wiley & Sons, Ltd.     

Aeolian sediment transport on a recovering storm‐eroded foredune with sand fences

Studies of sediment transport on developed coasts provide perspective on how human adjustments alter natural processes. Deployment of sand‐trapping fences is a common adjustment that changes the characteristics of the dune ramp and its role in linking sediment transfers from the backshore to the foredune. Fence effects were evaluated in the field using anemometer arrays and vertical sediment traps placed across a beach and dune at Seaside Park, New Jersey, USA during onshore and longshore winds. The foredune is 18 m wide and 4.5 m above the backshore. The mean speed of onshore winds at 0.5 m elevation decreased by 17% from the berm crest to the upper ramp and 36% in the lee of a fence there. Sediment transport during mean wind speeds up to 8.0 m s^?1 at 0.5 m elevation was < 0.06 kg m^?1 h^?1 on the berm crest and backshore where fetch distances were < 45 m and surface sediment was relatively coarse (0.74–0.85 mm) but increased to 5.63 kg m^?1 h^?1 on the upper ramp aided by the longer fetch distances (up to 82 m) and finer grain size of the source sediment there (0.52 mm). Sediment transport along the berm crest and backshore during longshore winds, where fetch distances were > 200 m, was up to 58.69 kg m^?1 h^?1, about three orders of magnitude greater than during the onshore winds. Fences can displace the toe of the ramp farther seaward than would occur under natural conditions. They can create a gentler slope and change the shape of the ramp to a more convex form. A fence on the ramp can cut off a portion of sediment supply to the upper slope. Decisions about fence placement thus should consider these morphologic changes in addition to the effects on dune volume. Copyright © 2017 John Wiley & Sons, Ltd.     

Streamflow and suspended sediment yield following the 2000 Bobcat fire,Colorado

Postfire runoff and erosion are a concern, and more data are needed on the effects of wildfire at the watershed‐scale, especially in the Colorado Front Range. The goal of this study was to characterize and compare the streamflow and suspended sediment yield response of two watersheds (Bobcat Gulch and Jug Gulch) after the 2000 Bobcat fire. Bobcat Gulch had several erosion control treatments applied after the fire, including aerial seeding, contour log felling, mulching, and straw wattles. Jug Gulch was partially seeded. Study objectives were to: (1) measure precipitation, streamflow, and sediment yields; (2) assess the effect of rainfall intensity on peak discharges, storm runoff, and sediment yields; (3) evaluate short‐term hydrologic recovery. Two months after the fire, a storm with a maximum 30 min rainfall intensity I₃₀ of 42 mm h^?1 generated a peak discharge of 3900 l s^?1 km^?2 in Bobcat Gulch. The same storm produced less than 5 l s^?1 km^?2 in Jug Gulch, due to less rainfall and the low watershed response. In the second summer, storms with, I₃₀ of 23 mm h^?1 and 32 mm h^?1 generated peak discharges of 1100 l s^?1 km^?2 and 1700 l s^?1 km^?2 in the treated and untreated watersheds respectively. Maximum water yield efficiencies were 10% and 17% respectively, but 18 of the 23 storms returned ≤2% of the rainfall as runoff, effectively obscuring interpretation of the erosion control treatments. I₃₀ explained 86% of the variability in peak discharges, 74% of the variability in storm runoff, and >80% of the variability in sediment yields. Maximum single‐storm sediment yields in the second summer were 370 kg ha^?1 in the treated watershed and 950 kg ha^?1 in the untreated watershed. Copyright © 2005 John Wiley & Sons, Ltd.     

Excess bottom222Rn profiles and their implications in the northwestern Pacific Ocean

Vertical profiles of excess bottom²²²Rn and potential temperature were measured at 23 stations in the northwestern Pacific Ocean. The Rn profiles were classified into the following three types: quasi-exponential (type E), benthic boundary layer (type B), and horizontally disturbed (type H). The ratio among types E, B and H was approximately 2 : 3 : 1.An apparent vertical eddy diffusivity (K) was calculated by applying a one-dimensional diffusion model to the Rn profiles of types E and B. Type E had K values ranging from 15 to 180 cm² s^?1 (average: 70 cm² s^?1). As to type B, K values for the benthic boundary layer (4.5–260 cm² s^?1, average: 120 cm² s^?1) were always more than an order of magnitude larger than those for the upper layer (0.2–35 cm² s^?1, average: 7 cm² s^?1), indicating more active vertical mixing in the benthic boundary layer than in the upper layer.Rn profiles were measured in regions where the bottom topography is known. It was verified that the occurrence of type H related closely with local bottom topographic features accompanied by lateral transient supply of Rn-rich or Rn-poor water.A couple of Rn profiles at the same location, measured at time intervals of several years, were compared with each other for three locations. The general characteristics of Rn profiles were shown to remain unaltered with time, while the fine structure of Rn profiles may have short-term variations caused by local bottom topography and fluctuations of bottom current as indicated in type H.     

Impact of eddies on ocean diapycnal mixing in Gulf Stream region

Based on high-resolution,Array for Real-time Geostrophic Oceanography(Argo)profiles and Sea Level Anomaly(SLA)data,this study statistically analyzes and compares turbulent diapycnal mixing profiles inside and outside mesoscale eddies in the Gulf Stream region.The result indicates that average diapycnal diffusivity at 300–540 m depths in anticyclonic eddies reaches4.0×10-5 m2 s-1.This is significantly higher than the 1.6×10-5 m2 s-1 outside eddies and 0.8×10-5 m2 s-1 in cyclonic eddies.Probabilities of diapycnal diffusivity greater than 10-4 m2 s-1 within anticyclonic and cyclonic eddies and outside eddies are29%,5%and 12%,respectively.However,magnitudes of average diapycnal diffusivity at 540–900 m depths in these three cases are of the same order,10-5 m2 s-1.Twenty-four of a total 38 anticyclonic eddies had enhanced mixing in the ocean interior,and 22 were observed during or shortly after strong winds.The coincidence between enhanced mixing and strong wind stress indicates that more wind-induced,near-inertial wave energy propagates downward in anticyclonic eddies.The deeper part of 12 profiles(below 540 m)in anticyclonic eddies had vertical overturns with Thorpe scale exceeding 5 m,among which three profiles had overturns reaching 20 m.Enhanced mixing may have occurred in deep layers of some profiles,although it was not evident in average conditions.     

Flow convergence over a broad,flat continental shelf

The broad (～500 km) southeastern Bering Sea continental shelf contains three fronts; outer (shelf break, ～170-m depth), middle (～100-m depth), and inner (～50-m depth). The shelf break and inner fronts appear to be analogous to similar fronts reported from other mid-latitude continental shelves; extensively studied examples are from the mid-Atlantic bight, off Nova Scotia, and around the British Isles. The middle front may have counterparts on the broad North Sea and East China Sea shelves.One-month current and temperature records from either side of the middle front, ～150 km landward from the shelf break, showed convergence in the layers deeper than 30 m in both the cross-shelf flow field and heat flux. The convergence was ～3 cm s^?1, so an average upwelling at ～1 × 10^?3cm s^?1 and divergence in the surface layer were required to maintain continuity. Variations in the degree of convergence arose primarily from 1 to 5-day fluctuations in sub-tidal flow across the outer shelf domain seaward of the front.Diffusive landward heat flux was dominated by tidal scales. Horizontal eddy conductivities describing the flux were ～1 ? 10⁶ on the landward side and ～5 × 10⁶cm² s^?1 on the seaward side, and were less in the layers above the bottom layer. Advective flux by the mean flow was the same order as diffusive flux, but landward in the bottom layer and seaward in the mid-water column layers, in agreement with deductions from water mass analyses. Frontal effects reduced the net cross-shelf heat flux beneath ～30 m by about 50%. The observation of a flow convergence in the middle of a broad, flat continental shelf poses an important question of dynamics.     

Currents and upwelling along the Latium coasts in the Central Tyrrhenian Sea (western Mediterranean)

In this study, springtime coastal currents along the Latium coast and their relation to external forcings, mainly wind stress curl and atmospheric pressure, are analysed. As a main result, we find that hydrographical measurements reveal currents parallel to the bottom isobaths, but with isopycnal sloping upwards towards the coast, suggesting the importance of upwellings in determining the coastal currents. This is confirmed by thermal satellite data showing the presence of a \sim10-km-wide patch of cold water east of Mount Argentario, i.e. a cyclonic vortex. The current meter data give rather small values of the time-averaged alongshore velocities (\sim2 cm s⁻¹ for most current meters and \sim3 cm s⁻¹ for the current meter placed immediately off the Argentario) and also smaller values for the offshore velocities. The correlation between these two types of currents is rather poor; this is also due to the variability characteristic of a wind-induced upwelling. Finally, we obtain a value of 0.74 for the correlation between the alongshore current (first mode of Empirical Orthogonal Functions decomposition) and the wind stress if a 23-h time lag is assumed.     

Winter methane dynamics beneath ice and in snow in a temperate poor fen

The influence of winter on methane (CH₄) stored in pore water and emitted through snow was investigated in a temperate poor fen in New Hampshire over two winters. Methane accumulated beneath ice layers (1 cm) deposited by freezing rain, resulting in snow-pore air mixing ratios as high as 140 ppmv during the first winter and 600 ppmv during the second. An early winter snow crust of 300 kg m^?3 caused no discontinuity in a linear mixing ratio profile and therefore was not observed to retard snowpack emissions. Methane concentration-depth profiles in pore water steepened and concentrations increased by as much as 400 μM at the 10 and 20 cm depths as the ice cover formed. This suggests that the peat-ice cover plays an important part in CH₄ build-up in pore water by limiting the transport of gases between the peat and the atmosphere. Pore water concentrations gradually declined through late winter. The seasonality of dissolved CH₄ in pore water over two winters and one summer showed an average annual amplitude of 1.3 gCH₄m^?2 (25–75cm depth range), with a winter maximum of 4.7gCH₄m^?2. Emissions during the winter with average snowfall accounted for a larger percentage (9.2% in 1993–1994) of total annual emission than the winter with below-average snowfall and warmer air temperature (2% in 1994–1995). Emissions averaged 56 and 26mg m^?2 day^?1 during the first and second winter (December, January and February), respectively.