Horizontal Density Gradients Research Articles

Seasonal Variation in Residence Times of Two Neighboring Bays with Contrasting Topography

Residence time is an important parameter for evaluating the physical and biochemical characteristics of estuaries and bays. The residence times of two neighboring South Korean bays with contrasting topographies, Gwangyang Bay and Jinju Bay, were calculated for winter and summer using a three-dimensional model. Gwangyang Bay, with considerable industrial development nearby, has a relatively large volume and depth, whereas Jinju Bay is shallow and is primarily used for aquaculture. River discharge in the two bays is high in summer and low in winter, in part due to the influence of the East Asian Monsoon. Temperature and salinity in both bays are vertically homogeneous during winter but stratified in summer. The mean residence time in Gwangyang Bay is more than 20 days in winter due to its large volume, but less than 5 days in summer due to increased gravitational circulation driven by large volumes of river discharge and a horizontal density gradient. However, the mean residence time of Jinju Bay is less than 5 days in both seasons due to its small volume and shallow depth. These results reveal that Gwangyang Bay is vulnerable to industrial pollution during winter, but that it may remove pollutants efficiently in summer. The shallower Jinju Bay is likely to remain free from anthropogenic eutrophication caused by nutrient inputs from aquaculture due to its short residence time throughout the year.

Morphologies of the topside ionosphere observed by COSMIC at high-latitudes during the 17 March 2013 magnetic storm

The present study considers effects on the topside ionosphere at high-latitude (|MLat| ≥ 60°) during the magnetic storm on 17 March 2013. We divide the day into two segments, with magnetic conditions quiet during the first 6 h and disturbed thereafter, thus allowing comparisons of differences between these two states. The COSMIC data source consisted of 272 electron density profiles (EDPs) measured by COSMIC satellites using Abel inversions that assume the absence of horizontal plasma density gradients. We compared resultant EDPs with predictions of the International; Reference Ionosphere (IRI) model for the same altitudes along individual tangent-point lines. As conditions allowed, we also compared EDPs with densities measured by nearby DMSP satellites. During the pre-storm quiet period, 65 of the sampled 68 EDPs were highly correlated with IRI predictions. The remaining 3 EDPs showed significant irregularities along parts of the 800–200 km altitude range. We divided stormtime 204 COSMIC EDPs into three morphological categories. In the first group, 130 EDPs exhibit shapes similar to IRI-predictions. Of these, 72 (9) had larger (smaller) than predicted magnitudes. The second group consisted of 37 EDPs that showed little variation in magnitude over the designated altitude range. The remaining group had another 37 EDPs marked by significant irregularities. We found that the deepest EDP irregularity resulted from GPS signals passing through a deep plasma density trough observed by DMSP. The trough extended along the equatorward boundary of the evening-side, auroral oval. We suggest that EDP irregularities reflect density gradients along or near EDP tangent point lines.

A hydrodynamic model for Galveston Bay and the shelf in the northern Gulf of Mexico

Abstract. A 3-D unstructured-grid hydrodynamic model for the northern Gulf of Mexico was developed, with a hybrid s–z vertical grid and high-resolution horizontal grid for the main estuarine systems along the Texas–Louisiana coast. This model, based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), is driven by the observed river discharge, reanalysis atmospheric forcing, and open boundary conditions from global HYCOM output. The model reproduces the temporal and spatial variation of observed water level, salinity, temperature, and current velocity in Galveston Bay and on the shelf. The validated model was applied to examine the remote influence of neighboring large rivers, specifically the Mississippi–Atchafalaya River (MAR) system, on salinity, stratification, vertical mixing, and longshore transport along the Texas coast. Numerical experiments reveal that the MAR discharge could significantly decrease the salinity and change the stratification and vertical mixing on the inner Texas shelf. It would take about 25 and 50 d for the MAR discharge to reach the mouth of Galveston Bay and Port Aransas, respectively. The influence of the MAR discharge is sensitive to the wind field. Winter wind constrains the MAR freshwater to form a narrow lower-salinity band against the shore from the Mississippi Delta all the way to the southwestern Texas coast, while summer wind reduces the downcoast longshore transport significantly, weakening the influence of the MAR discharge on surface salinity along Texas coast. However, summer wind causes a much stronger stratification on the Texas shelf, leading to a weaker vertical mixing. The decrease in salinity of up to 10 psu at the mouth of Galveston Bay due to the MAR discharge results in a decrease in horizontal density gradient, a decrease in the salt flux, and a weakened estuarine circulation and estuarine–ocean exchange. We highlight the flexibility of the model and its capability to simulate not only estuarine dynamics and shelf-wide transport, but also the interactions between them.

Coastal summer eddies in the Peter the Great Bay of the Japan sea: In situ data, numerical modeling and Lagrangian analysis

Coastal mesoscale anticyclonic eddies are regularly generated by the Primorsky Current flowing past the capes in the largest bay of the Japan Sea, the Peter the Great Bay. They were observed in 18 of 21 CTD summer surveys performed from 2001 to 2017. The regional ROMS model is used to study the evolution of the eddy that has been sampled in the oceanographic survey in August 2009. The model is able to reproduce the observed eddy on the days of sampling including its location, size and vertical structure. The Lagrangian maps are computed to investigate and document the generation, deformation and decay of the eddy, to identify the vortex contour and origin of water masses inside it. Such coastal eddies are typical for the summer period in the Bay regulating water exchange between the continental shelf and open sea. They are shown to be generated after intensification of the eastern wind which is sufficiently strong to produce downwelling and increased horizontal density gradients and to generate an anticyclonic vorticity past the capes.

A description of the Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE)

The Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE) was an extensive experiment undertaken by the Defence Science and Technology (DST) Group that focused on collecting ionospheric sensor data from multiple overlapping ionospheric paths in the Australian region in order to improve understanding of the characteristics of ionospheric variability and its effect on HF radio propagation. The experiment ran from July to October 2015 and included a period of three weeks of increased sample density and three days of dedicated over-the-horizon (OTH) radar operations. It was anticipated that ELOISE would sample a wide range of environmental conditions and present an opportunity to characterise periods of “normal variability” and periods of “exceptional variability” in the ionosphere.This report is a general description of the aims of this experiment and the types of data collected. Particular interest focused on observing and measuring variability in ionospheric electron density gradients and their effect on oblique HF propagation. To this end, ELOISE established a pair of two dimensional HF receiver arrays to directly measure the oblique angle of arrival (AoA) on many overlapping oblique paths. ELOISE also established a dense sub-network of spatially separated quasi-vertical incidence soundings in the vicinity of Alice Springs in central Australia. This enabled a comparison of gradients observed in a dense network of vertical sounders with gradient effects observed in oblique propagation passing overhead. Several additional ionospheric observing systems were also used to give complementary pictures of the fine scale characteristics of ionospheric variability in the region. Plain Language SummaryThis paper is an overview of the 2015 Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE), an experiment designed to observe and characterise mid-latitude ionospheric disturbances in the Australian region and understand their impact on high frequency (HF) signal propagation.ELOISE involved the simultaneous operation of a large collection of ionospheric sounders enabling ionospheric variability to be characterised on a finely sampled large scale.Particular efforts were made to provide direct high fidelity measurements of the angle of arrival (AoA) on many oblique HF propagation paths. These direct AoA measurements imply horizontal electron density gradients that can be compared to ionospheric gradients estimated from conventional models of the ionosphere derived from the spatial network of sounder sites.The size and scope of the experiment are detailed in this paper and some preliminary results are presented.

Spatial and Temporal Variability of Dense Shelf Water Cascades along the Rottnest Continental Shelf in Southwest Australia

Along the majority of Australian shallow coastal regions, summer evaporation increases the salinity of shallow waters, and subsequently in autumn/winter, the nearshore waters become cooler due to heat loss. This results in the formation of horizontal density gradients with density increasing toward the coast that generates gravity currents known as dense shelf water cascades (DSWCs) flowing offshore along the sea bed. DSWCs play important role in ecological and biogeochemical processes in Australian waters through the transport of dissolved and suspended materials offshore. In this study a numerical ocean circulation model of Rottnest continental shelf, validated using simultaneous ocean glider and mooring data, indicated that the passage of cold fronts associated with winter storms resulted in rapid heat loss through evaporative cooling. These conditions resulted in enhancement of the DSWCs due to modifications of the cross-shelf density gradient and wind effects. Specifically, onshore (offshore) directed winds resulted in an enhancement (inhibition) of DSWCs due to downwelling (vertical mixing). Consequently, the largest DSWC events occurred during the cold fronts when atmospheric temperatures reinforced density gradients and onshore winds promoted downwelling that enhanced DSWCs. Advection of DSWCs was also strongly influenced by the wind conditions, with significantly more transport occurring along-shelf compared to cross-shelf.

The influence of the spatial and temporal collocation windows on the comparisons of the ionospheric characteristic parameters derived from COSMIC radio occultation and digisondes

GPS radio occultation (RO) ionospheric products obtained by Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission during the year of 2014 and the observations from 3 digisonde stations which are located at different latitudes are used to study the influence of different time and space collocation windows on the comparisons of the ionospheric characteristic parameters (ICPs), including the peak density and peak height, derived from the two techniques. The results show that the correlation coefficients (CC) and the standard deviation of the absolute biases (SDAB) between the ICPs derived from the two techniques vary distinctly under different spatial and time collocation windows. Generally, the CC (SDAB) of the ICPs decrease (increase) as the size of the collocation window increases in time dimension or in space dimension. The rate of change of the statistic parameters with the increase in the size of the collocation window in time dimension and space dimension is analyzed for each digisonde station. It is found that within the collocation window of 60min,20°,20°, the influence of the increase of 1° in the space window on the statistical comparison is much more significant than that of the increase of 1 min in the time window, and it is suggested that there can be appropriate relaxation on the time window within the threshold of 60 min to get a balance between the quality of the comparison results and the number of the matched pairs. In addition, it is found that the same variations in the longitude window and in the latitude window may have different influences on the comparison results when the horizontal gradients in electron density are distinctly different along different directions at the digisonde station, and strict space collocation window is preferred when comparing the observations from COSMIC RO with those from the digisonde station in such cases.

Directional Droplet Propulsion on Gradient Boron Nitride Nanosheet Grid Surface Lubricated with a Vapor Film below the Leidenfrost Temperature.

Controlled propulsion of liquid droplets on a solid surface offers important applications in various fields, including fog harvesting, heat transfer, microfluidics, and microdevice technologies. The propulsion of the liquid droplet is realized only if the driven force exceeds the resistance force. Sometimes the directional propulsion of droplets only takes place at the Leidenfrost state to achieve enough lubrication for a vapor cushion. The thick vapor cushions levitate liquid droplets to reduce resistance force. However, it is still challenging to reduce the vapor cushion thickness and simultaneously realize the directional droplet's motion, especially below the Leidenfrost temperature. Here, a structurally hydrophobic boron nitride nanosheet (BNNS) grid surface was constructed with a two-direction topographical gradient, i. e., the perpendicular altitude gradient and the horizontal density gradient. The polar nature of the B-N bonds results in intrinsic hydrophilicity of the boron nitride layer, which increases the Leidenfrost point and facilitates wetting even at high temperature. Much thinner vapor-lubricating layers are competent in the droplet's directional motion below the Leidenfrost temperature of the BNNS grid surface because the air gap trapped within boron nitride nanosheet grids acts as a part of the lubrication layer.

Ground-based Observations of the Polar Region Space Environment at the Jang Bogo Station, Antarctica

Jang Bogo Station (JBS), the second Korean Antarctic research station, was established in Terra Nova Bay, Antarctica (74.62°S 164.22°E) in February 2014 in order to expand the Korea Polar Research Institute (KOPRI) research capabilities. One of the main research areas at JBS is space environmental research. The goal of the research is to better understand the general characteristics of the polar region ionosphere and thermosphere and their responses to solar wind and the magnetosphere. Ground-based observations at JBS for upper atmospheric wind and temperature measurements using the Fabry-Perot Interferometer (FPI) began in March 2014. Ionospheric radar (VIPIR) measurements have been collected since 2015 to monitor the state of the polar ionosphere for electron density height profiles, horizontal density gradients, and ion drifts. To investigate the magnetosphere and geomagnetic field variations, a search-coil magnetometer and vector magnetometer were installed in 2017 and 2018, respectively. Since JBS is positioned in an ideal location for auroral observations, we installed an auroral all-sky imager with a color sensor in January 2018 to study substorms as well as auroras. In addition to these observations, we are also operating a proton auroral imager, airglow imager, global positioning system total electron content (GPS TEC)/scintillation monitor, and neutron monitor in collaboration with other institutes. In this article, we briefly introduce the observational activities performed at JBS and the preliminary results of these observations.

Sustenance of Phytoplankton in the Subpolar North Atlantic During Winter

AbstractWe consider two factors that affect the mixed layer depth (MLD) and potentially contribute to phytoplankton sustenance over winter—variability of air‐sea fluxes and three‐dimensional processes arising from horizontal density gradients (fronts). The role of these two factors is addressed using several three‐dimensional idealized numerical simulations in a process study ocean model forced with air‐sea fluxes at different temporal averaging frequencies. Results show that in winter, when the average mixed layer is much deeper than the euphotic layer and the period of daylight is short, phytoplankton production is relatively insensitive to high‐frequency variability in air‐sea fluxes. Short‐lived stratification events during light‐limited conditions have very little impact on phytoplankton production. On the other hand, the slumping of fronts shallows the mixed layer in a patchy manner and the associated restratification persists considerably longer than that caused by changes in air‐sea fluxes. Simulations with fronts show that in winter, the average MLD is about 600 m shallower than simulations without fronts. Prior to spring warming, the depth‐integrated phytoplankton concentration in the model with fronts is about twice as large as the case without fronts. Hence, even in winter, restratification by fronts is important for setting the MLD; it increases the residence time of phytoplankton in the euphotic layer and contributes to phytoplankton growth, thereby sustaining phytoplankton populations in winter. Higher model resolution intensifies submesoscale dynamics, leading to stronger restratification, shallower mixed layers, greater variability in the MLD, and more production of phytoplankton.

The effect of a density gradient in groundwater on ATES system efficiency and subsurface space use

Abstract. A heat pump combined with Aquifer Thermal Energy Storage (ATES) has high potential in efficiently and sustainably providing thermal energy for space heating and cooling. This makes the subsurface, including its groundwater, of crucial importance for primary energy savings. ATES systems are often placed in aquifers in which salinity increases with depth. This is the case in coastal areas where also the demand for ATES application is high due to high degrees of urbanization in those areas. The seasonally alternating extraction and re-injection between ATES wells disturbs the preexisting ambient salinity gradient causing horizontal density gradients, which trigger buoyancy flow, which in turn affects the recovery efficiency of the stored thermal energy. This section uses analytical and numerical methods to understand and explain the impact of buoyancy flow on the efficiency of ATES in such situations, and to quantify the magnitude of this impact relative to other thermal energy losses. The results of this research show that losses due to buoyancy flow may become considerable at (a relatively large) ambient density gradients of over 0.5 kg m−3 m−1 in combination with a vertical hydraulic conductivity of more than 5 m day−1. Monowell systems suffer more from buoyancy losses than do doublet systems under similar conditions.

Langmuir turbulence in horizontal salinity gradient

Langmuir circulation (LC) is believed to be one of the leading causes of turbulent mixing in the upper ocean. Large eddy simulation (LES) models that solve the Craik–Leibovich equations are used to study LC in the upper ocean, yielding new insights that could not be obtained from field observations or turbulent closure models alone. The present study expands our previous LES modeling investigations of LC to real ocean conditions with large-scale environmental motion due to strong horizontal density gradient, which is introduced to the LES model through scale separation analysis. The model is applied to field observations in the Gulf of Mexico when a measurement site was impacted by fresh water inflow. Model results suggest that LC can enhance turbulence in the water column and deepen the mixed layer (ML) with or without the large scale motions, being consistent with previous studies. The strong salinity gradient is shown to be able to reduce the mean flow in the ML, align Langmuir cells with the pressure gradient direction and inhibit turbulence in the ocean surface boundary layer.

Seasonality in the cross-shelf physical structure of a temperate shelf sea and the implications for nitrate supply

We address a long-standing problem of how nutrients are transported from the shelf edge and from rivers to support regular, seasonal primary production in the interior of a wide, temperate, shelf sea. Cross-shelf sections of hydrography and nutrients, from a series of cruises between March 2014 and August 2015, along with time series of river discharge and river nutrient load are used to assess the seasonality of cross-shelf transports. Riverine nitrogen inputs are estimated to account for 30% of the nitrate available for the spring bloom on the inner shelf, and 10% in the mid- to outer-shelf. In the bottom layer in summer, high salinity, nutrient-rich waters are transported on-shelf as a result of wind-driven Ekman transport, cross-shelf pressure gradients and/or internal tidal wave Stoke’s drift. In the centre of the shelf this advection is responsible for 25% of the increase in bottom water nitrate seen between April and November 2014. The remaining nitrate increase suggests that about 50–62% of the nitrogen fixed into organic material during spring, summer and autumn phytoplankton growth is recycled in the bottom water over the 12 months between March 2014 and March 2015. In winter, when the water column is vertically mixed, there is a weak net off-shelf transport of about 1 m2 s−1, possibly driven by a reversal of the horizontal density gradient caused by excess cooling of shallower shelf waters. Overall, shelf nitrate concentrations are maintained by a combination of riverine supply, recycling of organic material, and summer on-shelf transports. We suggest that the main driver of inter-annual variability in pre-spring nitrate concentrations is variability in the depth of the winter mixed layer over the shelf slope.

Role of Baroclinic Processes on Flushing Characteristics in a Highly Stratified Estuarine System, Mobile Bay, Alabama

AbstractFlushing of an estuary quantifies the overall water exchange between the estuary and coastal ocean and is crucially important for water quality as well as biological and geochemical processes within the system. Flushing times and freshwater age in Mobile Bay were numerically calculated under realistic and various controlled forcing conditions. Their responses to external forcing were explained by the three‐dimensional characteristics of general circulation in the system. The flushing time ranges from 10 to 33 days under the 25th–75th percentile river discharges, nearly half of the previous estimates based on barotropic processes only, suggesting the important contribution of baroclinic processes. Their influence, quantified as the “new ocean influx,” is on the same order of the river discharge under low to moderate river discharge conditions. The baroclinic influence increases and then decreases with increasing river discharge, aligning with the response of horizontal density gradient. By enhancing the net influx from the ocean mainly through density‐driven circulation, baroclinic processes contribute to reduce flushing times. The three‐dimensional circulation, which differs greatly between the wet and dry seasons, explains the temporal and spatial variations of the flushing characteristics. Wind forcing influences the three‐dimensional circulation in the system with easterly and northerly winds tending to reduce the flushing time, while southerly and westerly winds the opposite.

Estimating the Characteristics of Traveling Ionospheric Disturbances from Vertical Incidence Ionograms within a Compound Parabolic Layer Model

The characteristic U-shaped traces (cusps) in ionograms are associated with the passage of traveling ionospheric disturbances (TIDs), which lead to horizontal electron density gradients in the ionosphere and, therefore, to off-angle reflections in radio sounding. A new form of representation is considered for daily ionospheric sounding data. A compound parabolic layer model is proposed, which allows analytical calculation of ray paths to speed up the “homing-in” of the rays. Changes in the shape of the trace in the ionogram due to varying the TID characteristics are examined. A discussion is given of the possibilities for estimating TID characteristics from digitized vertical sounding ionograms.

The physical limnology of a permanently ice‐covered and chemically stratified Antarctic lake using high resolution spatial data from an autonomous underwater vehicle

AbstractWe used an Environmentally Non‐Disturbing Under‐ice Robotic ANtarctic Explorer to make measurements of conductivity and temperature in Lake Bonney, a chemically stratified, permanently ice‐covered Antarctic lake that abuts Taylor Glacier, an outlet glacier from the Polar Plateau. The lake is divided into two lobes – East Lobe Bonney (ELB) and West Lobe Bonney (WLB), each with unique temperature and salinity profiles. Most of our data were collected in November 2009 from WLB to examine the influence of the Taylor Glacier on the structure of the water column. Temperatures adjacent to the glacier face between 20 m and 22 m were 3°C colder than in the rest of WLB, due to latent heat transfer associated with melting of the submerged glacier face and inflow of cold brines that originate beneath the glacier. Melting of the glacier face into the salinity gradient below the chemocline generates a series of nearly horizontal intrusions into WLB that were previously documented in profiles measured with 3 cm vertical resolution in 1990–1991. WLB and ELB are connected by a narrow channel through which water can be exchanged over a shallow sill that controls the position of the chemocline in WLB. A complex exchange flow appears to exist through the narrows, driven by horizontal density gradients and melting at the glacier face. Superimposed on the exchange is a net west‐to‐east flow generated by the higher volume of meltwater inflows to WLB. Both of these processes can be expected to be enhanced in the future as more meltwater is produced.

Subtidal hydrodynamics in a tropical lagoon: A dimensionless numbers approach

Observations in a tropical lagoon of the Yucatan peninsula motivated a non-dimensional number analysis to examine the relative influence of tidal stress, density gradients and wind stress on subtidal hydrodynamics. A two-month observation period in Chelem Lagoon covered the transition from the dry to the wet season. Chelem Lagoon is influenced by groundwater inputs and exhibits a main sub-basin (central sub-basin), a west sub-basin and an east sub-basin. Subtidal hydrodynamics were associated with horizontal density gradients that were modified seasonally by evaporation, precipitation, and groundwater discharge. A tidal Froude number (Fr0), a Wedderburn number (W), and a Stress ratio (S0) were used to diagnose the relative importance of dominant subtidal driving forces. The Froude number (Fr0) compares tidal forcing and baroclinic forcing through the ratio of tidal stress to longitudinal baroclinic pressure gradient. The Wedderburn number (W) relates wind stress to baroclinicity. The stress ratio (S0) sizes tidal stress and wind stress. S0 is a new diagnostic tool for systems influenced by tides and winds, and represents the main contribution of this research. Results show that spring-tide subtidal flows in the tropical lagoon had log(Fr0)≫0 and log(S0)>0, i.e., driven mainly by tidal stresses (advective accelerations). Neap tides showed log(Fr0)≪0 and log(S0)<0), i.e., flows driven by baroclinicity, especially at the lagoon heads of the east and west sub-basins. However, when the wind stress intensified over the lagoon, the relative importance of baroclinicity decreased and the wind stress controlled the dynamics (log(W)≫0). Each sub-basin exhibited a different subtidal response, according to the dimensionless numbers. The response depended on the fortnightly tidal cycle, the location and magnitude of groundwater input, and the direction and magnitude of the wind stress.

Submesoscale Coherent Structures on the Continental Shelf

AbstractDiscovery and analysis of submesoscale variability O(0.3–30) km on the continental shelf is made possible by a high-resolution (Δx = 75 m) Regional Oceanic Modeling System (ROMS) simulation of the Southern California Bight (SCB). This variability is manifest in ubiquitous yet ephemeral coherent structures: fronts, filaments, and vortices. Similar to their open-ocean counterparts, fronts and filaments on the shelf are identified by their strong vertical velocity, surface convergence, cyclonic vorticity, and horizontal density gradient. Life cycles of these features typically last 3–5 days, with the formation dominated by a horizontal advective tendency that increases density and velocity gradients (i.e., frontogenesis). The shape of the coastline and depth of the water column both influence the abundance and spatial orientation of shallow-water fronts and filaments. Closer to shore, fronts and filaments often align themselves parallel to isobaths, and headlands often act as sites of intense vorticity generation through bottom stress. A quasi-steady, approximate momentum balance among rotation, pressure gradient, and vertical mixing—known as turbulent thermal wind (TTW)—often is valid in the strong secondary circulations local to fronts and filaments. However, front and filament circulations subject to strong diurnal variation in surface heating and vertical mixing are inconsistent with steady-state TTW balance. The secondary circulations can induce ephemeral material trapping and substantial vertical heat fluxes on the shelf.

Observations of Tidal Straining Within Two Different Ocean Environments in the East China Sea: Stratification and Near‐Bottom Turbulence

AbstractTidal straining describes the straining effect induced by the vertical shear of oscillatory tidal currents that act on horizontal density gradients. It tends to create tidal periodic stratification and modulate the turbulence in the bottom boundary layer (BBL). Here, we present observations of current, hydrology and turbulence obtained at two mooring stations that are characterized by two typical hydrological environments in the East China Sea (ECS). One is located adjacent to the Changjiang River's mouth, and the other is located over a sloping shelf which is far from the freshwater sources. Tidal straining induces a semidiurnal switching between stable and unstable stratification at both stations. Near‐bottom high‐frequency velocity measurements further reveal that the dissipation rate of turbulent kinetic energy (TKE) is highly elevated during periods when unstable stratification occurs. A comparison between the TKE dissipation rate ( ) and the shear production ( ) further reveals that the near‐bottom mixing is locally shear‐induced most of the time except during the unstable stratification period. Within this period, the magnitude of dissipation exceeds the expected value based on the law of the wall by an order of magnitude. The buoyancy flux that calculated by the balance method is too small to compensate for the existing discrepancy between the dissipation and shear production. Another plausible candidate is the advection of TKE, which may play an important role in the TKE budget during the unstable stratification period.

Phytoplankton community structure in relation to hydrographic features along a coast-to-offshore transect on the SW Atlantic Continental Shelf

The continental shelf in Southern Brazil is characterized by high biological productivity associated with horizontal and vertical density gradients due to the mixing of distinct water masses. Phytoplankton biomass and composition were evaluated in summer 2013 along an on-offshore transect off the mouth of the Patos Lagoon (Lat. 32°12S). Photosynthetic active radiation, temperature, salinity and fluorescence vertical profiles were carried out and Brünt-Väisäla frequency was estimated. Three water bodies were identified: the Subtropical Shelf Water along the entire transect, the Plata Plume Water on the middle shelf surface and the Tropical Water farther offshore. The water was sampled (N = 40) for the analyses of dissolved inorganic nutrients, phytoplankton cell density and composition. Phytoplankton present in the water was identified and quantified by the classical microscope sedimentation technique, complemented with CHEMTAX analysis of high-performance liquid chromatography (HPLC) pigment data. From the results obtained, chlorophyll a concentration was higher at both coastal stations (1.6–2.0mgm−3) where the water column was homogeneous and diatoms dominated the stations. This group was replaced by dinoflagellates in stratified conditions on the shelf and farther offshore. Along the onshore-offshore gradient, two types of dinoflagellates were found: the peridinin-containing dinoflagellates Prorocentrum and Scrippsiella with a small contribution at the coastal stations, and the fucoxantin-containing small Gymnodiniales cells (< 15µm) with more than 50% of the total chlorophyll a at the stations on the continental shelf, especially associated with the chlorophyll maximum at the base of the euphotic zone. The positive (negative) relationship between the biomass of dinoflagellates (diatoms) with the Brünt-Väisäla frequency, respectively, support the hypothesis that stratification is the most important environmental factor that determines the biomass of phytoplankton communities and distribution on the shelf and in coastal waters off Southern Brazil in summer. Picoplankton cells (Prochlorococcus and Synechococcus), recorded for the first time in the region under study, were predominant in the nutrient-poor and well-lit surface layers along the transect, indicating the importance of their low sedimentation rates (small size) and photo-adaptive strategies to survive on the upper layers of the water column.