Mean Current Direction Research Articles

Antarctic Krill Biomass and Flux Measured Using Wideband Echosounders and Acoustic Doppler Current Profilers on Submerged Moorings

During austral summer 2018/2019, we deployed an array of six submerged moorings equipped with Nortek Signature100 integrated wideband echosounder and acoustic Doppler current profilers (ADCPs) on the continental shelf of the northern Antarctic Peninsula. Acoustic data from these instruments were used to classify targets, estimate water flow and the biomass of Antarctic krill (Euphausia superba), and quantify krill flux (biomass transport). We differentiated krill from other target aggregations using a supervised classification of data from the echosounder representing five wideband frequency bins spanning 68–113 kHz and two narrowband frequencies at 70 and 120 kHz. We estimated krill biomass using echosounder data collected at 120 kHz and water flow using the ADCP data. We estimated the biomass flux from the product of mean volumetric krill density and flow speed over a depth-integration range of 150 m. The overall mean krill areal biomass density based on hourly averages was 174 g⋅m–2 during the austral summer (December–March). Mean daily biomass decreased by an order of magnitude, from 300 to 31 g⋅m–2, over the sampling period, and fluctuated by nearly a factor of 4 above and below the local trend within weekly intervals. Mean current direction was along-shelf toward the west, and mean flow speed increased from ∼0.10 to 0.14 m⋅s–1 during the season. Krill flux was correlated with biomass variation, and the grand mean flux was 0.13 g⋅m–2⋅s–1. During the study period and in our approximately 1,300 km2 study area, average total biomass of krill was 116265 metric tons, and total cumulative krill biomass was 2.5 million tons. Our results demonstrate the utility of integrated echosounder-ADCP systems for quantifying krill flux in an important foraging area used by krill-dependent predators (seabirds and marine mammals) that breed nearby and highlight the scales of variability in a key prey resource required by these predators.

Chemical Fluxes From a Recently Erupted Shallow Submarine Volcano on the Mariana Arc

AbstractHydrothermal discharge from submarine arc volcanoes is thought to be an important contributor to global hydrothermal budgets, but quantitative flux measurements are scarce. Ahyi Seamount, a shallow (<100 m) submarine intraoceanic arc volcano located in the Commonwealth of the Northern Mariana Islands, erupted in May 2014. In May and December 2014, we sampled the hydrothermal plume created by the eruption and estimated chemical fluxes from Ahyi by combining shipboard hull‐mounted Acoustic Doppler Current Profile current vector measurements with continuous and discrete Conductivity, Temperature, and Depth (CTD) data. Towed CTD sections were conducted perpendicular to the mean current direction: a sampling strategy that optimized chemical flux calculations by reducing complexities introduced by temporal variability in the speed and direction of plume dispersion. The Ahyi plume had an elevated optical backscatter signal accompanied by evidence of reduced chemical species and a lowered pH. We found enriched concentrations of H2, 3He, CH4, particulate S, Mn, and Fe, observations consistent with a highly active hydrothermal system. The fluxes of magmatic 3He and Fe from Ahyi were similar to that measured at three slow‐spreading ridge‐crest sites, whereas CH4 and Mn were 100–1,000 times lower. This is the first study to constrain export fluxes of a shallow submarine arc volcano into the euphotic zone. However, our data were collected soon after an eruption and thus may not be fully representative of the longer‐term chemical inputs from Ahyi.

Current and wave effects around windfarm monopile foundations

Laboratory measurements were undertaken to investigate wave and current velocities in the vicinity of a wind turbine monopile foundation, in order to inform environmental impact assessments and to quantify flow variability in the region of the power take off cable. Flow measurements were made up to 15.5 pile diameters (D) downstream of the pile. Measurements were also taken around the perimeter of the pile (~0.75 D from the pile centre) at the approximate representative height of the power cable.In current-only tests, the mean flow was reduced immediately downstream of the pile, but returned to within 5% of background levels by 8.3 D downstream of the pile centre in representative conditions. A new parameterisation of the velocity recovery is given. The turbulent eddy shedding frequency was well predicted by the Strouhal number. Turbulence peaked at 1.5 D from the pile centre, and the subsequent decay was parameterised. Velocity magnitudes at the side of the pile were up to 1.35 times greater than background flow rates, in line with potential flow theory. Velocities in the wake region were much less than predicted by potential flow theory, corresponding with increased turbulence.Tests with waves indicated that oscillatory velocities reduced immediately down-wave of the pile, but returned quickly to background levels (by 1.65 to 3.5 D of the pile centre). The general near-pile distribution of the orbital velocity maximum was well represented by potential flow theory. Orbital velocities were reduced immediately up-wave and down-wave of the pile. At the side of the pile in wind sea conditions, the velocity increased up to 1.66 times the background level. This increased to 1.85 times in swell conditions.For orthogonal currents and waves, a velocity parameter was calculated as the mean current plus wave orbital velocity, resolved. With the mean current direction as a reference, the maximum flow was observed at the side of the pile. At 0.75 D from the pile centre, the flow was enhanced by up to 1.2 times the no–pile case. Spectral peaks in the velocity were evident at both wave frequency and at the Strouhal frequency, immediately down current from the pile.

Interannual variability of alongshore spring bloom dynamics in a coastal sea caused by the differential influence of hydrodynamics and light climate

Abstract. Timing and spatial distribution of phytoplankton blooms in coastal oceans are highly variable. The interactions of various biological and physical factors leading to the observed variability are complex and remain poorly understood. We present an example for distinct differences in the spatio-temporal chlorophyll a (CHL-a) distribution on an interannual scale, integrating high-frequency data from an autonomous measuring device (FerryBox), which operated on an alongshore route in the coastal German Bight (North Sea). While in one year the distribution of CHL-a was spatially homogeneous (2004), a bloom only developed in one part of the transect in the following spring period (2005). We use a one-dimensional Lagrangian particle tracking model, which operates along the mean current direction, combined with a NPZ-model to identify the mechanisms controlling the observed interannual bloom variability on the alongshore transect. Our results clearly indicate that in 2004 the local light climate determined the spatial and temporal dynamics of the spring bloom. In contrast, the import of a water mass with elevated CHL-a concentrations from the adjacent Southern Bight triggered the spring bloom in 2005. The inflow event did, however, not last long enough to spread the bloom into the eastern part of the study area, where high turbidity prevented local phytoplankton growth. The model identifies two interacting mechanisms, light climate and hydrodynamics, that controlled the alongshore dynamics. Especially the occurrence of a pronounced spring bloom despite unfavourable light conditions in 2005 underlines the need to carefully consider hydrodynamics to understand the dynamics of the plankton community in coastal environments.

Distribution of Grain Size and Clay Minerals in Sediments from the INDEX Area, Central Indian Basin

The granulometry of 21 box core sediments from five selected sites in the Central Indian Basin (CIB) have been studied to understand the sediment size characteristics in the Pioneer area before the benthic disturbance experiment. The sediments are predominantly clayey silt. Statistical parameters were determined to understand the control and variations in the grain size. The mean grain size (Mzφ) ranged from 7.0 to 8.6 for the surface and from 6.6 to 8.6 for the subsurface sediments, indicating deposition of very fine fractions during the Neogene period. The sediments were poorly sorted as revealed from the higher ρ1 values (2.7 to 2.9). Higher and negative values of skewness (SK1) indicated a winnowing action of prevailing currents in the area. Average kurtosis (KG) values for the surface (0.4 to 0.8) and subsurface (0.5 to 1.2) showed that sediments were very platykurtic and leptokurtic. Clay mineralogical studies showed that smectite had a wider range (from 6% to 55%) and decreased southward because of its terrigenous origin. The terrigenous influence of the sediments could be documented as far as 12°S. Smectite and kaolinite increased eastward and chlorite decreased southward. This observation positively correlated with the mean current direction in the area. However, the influence of Antarctic Bottom Water (AABW) in the area could be predicted from the southward increase in chlorite content.

Measurement of surface currents using sequential synthetic aperture radar images of slick patterns near the edge of the Gulf Stream

Two‐dimensional surface currents are estimated over an area of ∼100 km2 near the inshore edge of the Gulf Stream by correlating the surface slick patterns observed on two synthetic aperture radar (SAR) images collected about 20 min apart. The currents obtained from this analysis are found to agree well with shipboard acoustic Doppler current profiler (ADCP) measurements at 10 to 20‐m depths. The orientations of the long, linear slicks observed in this region differ from the mean current direction by about 16° but coincide with the direction of maximum radial deformation or strain, as calculated from the observed current field. The calculated current gradients indicate a mean surface convergence of about 2×10−5 s−1 and a cyclonic vorticity of about 1×10−4 s−1, values which fall within the range of previous estimates in the study area. These results suggest that slick patterns may be used to infer surface current magnitude and direction from a time sequence of SAR images but that care must be exercised in inferring current directions from the orientations of slick patterns within a single image.

Seasonal transport variability of the Deep Western Boundary Current in the equatorial Atlantic

A total of 21 about year‐long current meter records in the depth range of the upper and middle North Atlantic Deep Water (NADW) were analyzed to determine the mean and the fluctuations of the upper Deep Western Boundary Current (DWBC) in the equatorial Atlantic. The investigation was based on moored arrays at 44°W from three different deployment periods, 1989/1990, 1990/1991 and 1992/1994, and was supplemented by current profiling along 44°W and 35°W. The approximately 100‐km‐wide DWBC at 44°W, just north of the equator, was attached to the topography with the current maximum exceeding 70 cm s−1. Currents within the DWBC core followed the topography, and the close agreement between the mean current direction and the direction of maximum variance indicated that the major contribution to the DWBC variability near the equator was due to pulsing rather than meandering. For mean transports of upper and middle NADW, the current meter records were averaged over their deployment duration yielding a best estimate of 13 Sv in the depth range 1000 to 3100 m. The mean transport appeared robust, as subsets of the data from two different years yielded about the same mean transport, namely, 12.4 and 13.6 Sv. The DWBC transport time series showed a definite seasonal cycle, ranging from less than 7 Sv during September/October to about 25 Sv during January/February. Annual and semiannual transport harmonics had similar amplitudes, at about 6 Sv each, and together they explained about two thirds of the total transport variability. After crossing the equator, the DWBC splits into two cores with the major flow along a chain of seamounts near 3.5°S, near 35°W. Magnitudes and phases of the transport variability at 35°W, south of approximately 1.5°S, were similar to that at 44°W. Further, for the flow of lower NADW which was detached from the upper DWBC core, similar periodicity and phases were observed in the deep records at 44°W.

Anticyclonic eddies trapped on the continental shelf by topographic irregularities

Nonlinear effects produced during the scattering of a barotropic shelf wave (BSW) by a spatially varying mean current are studied using a primitive equation numerical model. Both the BSW phase and the mean current propagate in the same (positive) direction along shelf/slope topography which is uniform everywhere except for a localized topographic irregularity, e.g., a submarine canyon. The mean current is specified at the upstream boundary and adjusts to the topography, closely following isobaths through the model domain. The incident BSW signal is then introduced at the upstream boundary either as a harmonic wave or as a pulse of finite duration. The BSW signal scatters its energy into other available wave modes when it encounters the topographic irregularity. The scattered wave field is dominated by evanescent modes which are trapped at the topographic irregularity and appear as intense mesoscale flows between the coast and the mean current. Nonlinear dynamics transform these large‐amplitude evanescent modes into persistent eddy‐like features on the shelf. The nonlinear interaction is much stronger when the current on the shelf associated with the BSW is opposite to the mean current direction (i.e., negative), so anticyclonic eddies are preferentially generated at the topographic irregularity. For a harmonic BSW, an anticyclonic eddy periodically appears when the negative current phase passes and disappears when the positive current phase passes. A BSW pulse with negative velocity at the coast produces a strong anticyclonic eddy which persists, after the pulse has passed, for a time period substantially longer than the pulse duration. A pulse with positive velocity at the coast does not generate any persistent features on the shelf. The anticyclonic eddies produce mass exchange between the shelf and the mean current and could contribute significantly to cross‐shelf exchange on continental shelves.

Effect of flow velocity and body size on swimming trajectories of sea scallops, Placopecten magellanicus (Gmelin): a comparison of laboratory and field measurements

The effect of flow velocity and body size on swimming trajectories of Placopecten magellanicus (Gmelin) was studied in a 5 m, flow-through laboratory flume and in a shallow (7 m depth) tidal channel in Lunenburg Bay, Nova Scotia, Canada. In the flume, scallops of 4 size classes, 11–20, 21–30, 31–50 and 51–80 mm shell height, (SH) were induced to swim at mean free-stream flow speeds of 0, 5, 9, 14 and 21 cm s −1. Swimming (active) and post-swimming (passive sinking) phases of individual trajectories were quantified in terms of swimming height, net displacement and direction, swimming time and swimming speed. Smaller scallops (≤ 30 mm) were adverted by the flow during both phases while larger scallops (> 30 mm) were advected only during the passive phase. Net horizontal displacement (active and passive stages combined) was positively related to flow speed for all size classes but the proportion of the variance explained by flow speed decreased with increasing scallop size. Most scallops swimming under flow had a downstream net swimming direction (finished their swimming phase downstream of the start point). For small scallops (≤ 30 mm SH), the proportion swimming downstream was independent of flow speed at flows of 0–5 cm s ( p > 0.30), but not independent of the flow speed at higher flows ( p < 0.01). For large scallops (> 30 mm SH), the proportion swimming downstream was independent of flow speed. In the field study, scallops ranging from 17–72 mm were induced to swim at current speeds ranging from 7–55 cm s −1. Model II regression and circular-linear correlation analysis showed a positive relationship between net horizontal displacement and current speed for scallops landing downstream and a negative relationship for scallops landing upstream, indicating the effect of advection. For small scallops that swam at medium (~ 18 cm s −1) current speeds, the distribution of displacement directions was significantly non-random and the mean direction did not differ significantly from the mean current direction. For large scallops that swam at both medium and high (~ 38 cm s −1) current speeds, the distribution of displacement directions was not significantly different from random. However, at high current speeds the mean size of scallops that landed upstream was significantly greater and the mean swimming height was significantly lower than for those that landed downstream. Flume and field measurements of swimming trajectories demonstrated that predictability of dispersion patterns of P. magellanicus released in bottom culture based on near-bed current data will be poor, especially when scallops larger than 30 mm SH are used.

Characteristics of deep currents off Cape Shiono-misaki before and after formation of the large meander of the Kuroshio in 1981

Deep currents measured by moored current meters over the shelf-slope off Cape Shiono-misaki, Kii Peninsula during the period from 28 April, 1981 to 4 May, 1982 are analyzed to determine characteristics of the deep current before and after the large meander of the Kuroshio formed. The observed deep currents show some different characteristics between the periods before and after the formation of the large meander of the Kuroshio,i.e.: 1. The mean current direction over the shelf slope changed to westward after the meander was formed, though it was eastward at two offshore stations before the meander was formed. 2. The eddy kinetic energy,\(ke((\overline {u'^2 } + \overline {\upsilon '^2 } )/2)\) became large at all stations after the meander formed. 3. It appears that there were current variations in the period band shorter than 10 days which propagated offshore before the meander formed but inshore after the meander formed. 4. After the meander formed, the current variations with a period of O(25 days) were amplified at two of the three stations. The current variations in this period band showed high coherence among the three stations.

Turbulent Boundary Layer in Wave‐current Motion

The mean current velocity profile in the combined wave‐current motion is calculated by use of the depth‐integrated momentum equation. The velocity distribution is assumed to be logarithmic inside as well as outside the wave boundary layer, but with different slopes. Hereby, it is possible to describe the flow in the whole range from the pure wave motion (without any mean current) to the pure current motion (without any waves). The theory covers an arbitrary angle between wave propagation direction and mean current direction.

Origin of longitudinal triangular ripples on the Nova Scotian continental rise

Since the recognition of current-produced bedforms in the deep ocean, attempts have been made to predict direction, intensity and variability of abyssal flow from the nature of sea floor irregularities (see refs 1–3). Longitudinal triangular ripples (LTRs) are inter mediate-scale bedforms (<20 cm high, metres long) that are becoming recognized as important and geographically widespread predictors of bottom-current flow. I describe here recent sea floor photographs of LTR fields on the lower continental rise off Nova Scotia which show that the bedforms develop in fine-grained, cohesive sediment with an alignment parallel to the regional contours and to mean current direction. An observational model for LTR formation is proposed, wherein LTRs are deposited initially as ‘tails’ behind large sea floor biological mounds and are subsequently constructed and propagate in the direction of mean current by sediment transport oblique to the bedform axis. This transport is induced by short-term currents that deviate from mean current direction and cause formation of a separation bubble on one side or the other of the LTR crest.

Sedimentologie des roches detritiques de la Formation de Keşan (Paleogene): Un facies a turbidites au Sud-Ouest de la Thrace turque

Two thousand meters of Paleogene clastic sediments (mainly epiclastics) exposed in the Keşan area, southwestern Turkish Thrace (southwestern Alpine Ergene basin) have been studied and the stratigraphic sequence divided into two major lithostratigraphic units: (1) the Keşan Formation (sandy); (2) the Yenimuhacir Formation (muddy). The Keşan Formation consists of non-cyclic Çinarlidere (Upper Eocene) and cyclic Şaplidere (Lower Oligocene) members. Volcanic rocks occur at three different localities of the Çinarlidere member and are interbedded (contemporaneously) with clastics. Clastics beds are generally gently tilted (at 15°–25° towards the north) and present a long outcrop along the strike. Two key horizons were followed along the strike approximately 10 km from west to east. On the basis of certain sedimentary structures, the Keşan Formation has been defined as a “sandy turbidite sequence” and statistical treatment of paleocurrent directions shows that the mean current direction of two members of this formation are in opposite directions. The quantitative analysis of structural and textural features makes it possible to distinguish two groups of turbidites: “proximal” and “distal”. These two groups of sandstones are designated as “sedimentary facies” and their occurrence is explained on the basis of the flow regime of the currents. The compositional analysis of light and heavy fractions of both member sandstones of the Keşan Formation showed marked petrological variations. The existence of two distinct “compositional facies” was established by computational statistics. Analysis of composition and current-direction data suggests that two main sources have provided materials for the formation of the Keşan sandstones. These are: (1) the Biga massif to the south-southeast, for the Çinarlidere sediments; and (2) the Rodop massif to the west-northwest, for the Şaplidere sediments. Finally, the sedimentation history of the Paleogene Keşan Beds in relation to volcanic activity and the facies development in the Keşan basin have been examined and the origin of the upper member turbidite cyclicity has been discussed. Conclusions are as follows: 1. (1) Although the direction of the axis of the depositional basin, (the Keşan as well as the Ergene basin) remained constant during the Paleogene, its axial plunge has changed its direction twice; once west-northwest (in Çinarlidere) and once south-southeast (in Şaplidere). 2. (2) The epiclastic and volcanic sedimentary (“pyroclastics”) deposits of the Keşan Formation have not been produced by the contemporaneous volcanic activity in the area: their source appears to be outside the basin of deposition. 3. (3) The tectonic structures of the Keşan area show that north—south oriented major stresses have been dominant and here tectonic evolution was younger than deposition. 4. (4) It can be postulated that the Ergene basin started to become a “stratigraphic basin” either at the beginning of or possibly earlier than the Paleogene.

Investigation of the discharge structure in a rare gas alkali MHD generator plasma. II.

The discharge structures in a noble gas alkali MHD generator were investigated under stationary conditions by means of image converter pictures and probe measurements. The current concentrations already known from short-time discharges and referred to as streamers were observed. The nature of these streamers was investigated by considering the relationship between the streamers and the ionization instabilities usually present in noble gas MHD generators. The measurements showed that the streamers are directed parallel to the mean current direction and constitute regions of elevated current density in which ionization instabilities can develop.

A description of the upper 100 m at Ocean Weather Station Juliett (52°30′N, 20°00′W)

A detailed study of the upper 100 m at Ocean Weather Station Juliett (O.W.S. J), obtained through simultaneous measurements from two ships, reveals an abundance of small-scale temperature and salinity structures with a density field that is relatively more homogeneous than either the temperature or salinity fields. It is deduced that the temperature and salinity structure is orientated with a banded nature along the mean current direction.