Weak Stratification Research Articles

A novel (2+1)-dimensional nonlinear evolution equation for weakly stratified free-surface boundary layers

To get an insight into the dynamics of the oceanic surface boundary layer we develop an asymptotic model of the nonlinear dynamics of linearly decaying three-dimensional long-wave perturbations in weakly stratified boundary-layer flows. Although in nature the free-surface boundary layers in the ocean are often weakly stratified due to solar radiation and air entrainment caused by wave breaking, weak stratification has been invariably ignored. Here, we consider an idealized hydrodynamic model, where finite-amplitude three-dimensional perturbations propagate in a horizontally uniform unidirectional weakly stratified shear flow confined to a boundary layer adjacent to the water surface. Perturbations satisfy the no-stress boundary condition at the surface. They are assumed to be long compared with the boundary-layer thickness. Such perturbations have not been studied even in a linear setting. By exploiting the assumed smallness of nonlinearity, wavenumber, viscosity and the Richardson number, on applying triple-deck asymptotic scheme and multiple-scale expansion, we derive in the distinguished limit a novel essentially two-dimensional nonlinear evolution equation, which is the main result of the work. The equation represents a generalization of the two-dimensional Benjamin–Ono equation modified by the explicit account of viscous effects and new dispersion due to weak stratification. It describes perturbation dependence on horizontal coordinates and time, while its vertical structure, to leading order, is given by an explicit analytical solution of the linear boundary value problem. It shows the principal importance of weak stratification for three-dimensional perturbations.

Initial Limnology of Laguna Pozo Verde, Costa Rica: Bathymetry, Water, Sediments, and Diatoms

Introducción : Costa Rica tiene cientos de lagos, muchos de los cuales nunca han sido estudiados científicamente. Objetivo: Conocer limnológia de la Laguna Pozo Verde en el Parque Nacional Juan Castro Blanco, Costa Rica (~1935 m de elevación), para proporcionar datos de referencia para estudiar cambios futuros. Métodos: Medimos la profundidad y temperatura del agua y la profundidad de Secchi; sedimentos superficiales analizados; y examinó mapas e imágenes de satélite. Resultados: Aunque algunos la describen como formada por procesos volcánicos, la Laguna Pozo Verde probablemente se formó en un deslizamiento de tierra, que ocurre con frecuencia en esta zona lluviosa en la empinada ladera sur del inactivo volcán Porvenir. Nuestros sondeos mostraron una profundidad máxima de 9,25 m cerca del centro del lago. El agua era moderadamente transparente (profundidad de Secchi 2,6 m), con pH circunneutro y temperaturas de 15,9 a 18,1 °C, con estratificación débil a 0,5 m. Los sedimentos superficiales contenían 27% de materia orgánica y tenían relaciones C/N y valores de isótopos de carbono estables consistentes con algas lacustres y plantas C 3 ; Surirella angusta compuso más del 90% de las diatomeas. Conclusión: La laguna es significativamente más omera de lo informado y los sedimentos superficiales albergar una combinación de diatomeas única entre los 88 lagos examinados en Costa Rica.

The Sensitivity of an Idealized Weddell Gyre to Horizontal Resolution

AbstractEstimates of the Weddell Gyre transport vary widely between climate simulations. Here, we investigate if inter‐model variability can originate from differences in the horizontal resolution of the ocean model. We run an idealized model of the Weddell Gyre at eddy‐parameterized, eddy‐permitting, and eddy‐rich resolutions and find that the gyre is strongly sensitive to horizontal resolution. The gyre transport is largest at eddy‐permitting resolutions (45 Sv with a noisy bathymetry) and smallest at eddy‐parameterized resolutions (12 Sv). The eddy‐permitting simulations have the largest horizontal density gradients and the weakest stratification over the gyre basin. The large horizontal density gradients induce a significant thermal wind transport and increase the mean available potential energy for mesoscale eddies. The distribution of eddy kinetic energy indicates that explicit eddies in simulations intensify the bottom circulation of the gyre via non‐linear dynamics. If climate models adopt horizontal resolutions that the Weddell Gyre is most sensitive to, then simulations of the Weddell Gyre could become more disparate.

On the dynamics of gravity current motion in a stratified ambient

This study presents LES results of lock-release propagating gravity current (GC) at the base of a linearly stratified ambient. The focus of the study is to investigate the effect of ambient stratification on GC motion for both subcritical and supercritical regimes. The effects of Reynolds number and the ratio of the fluid depth in the lock to the total ambient fluid depth on GC front velocity are also examined. Also, the effect of stratification strength on mixing is evaluated. It is found that both subcritical and supercritical GCs have a constant-velocity phase. This phase terminates due to the current interaction with the internal waves (IWs) for the former, while for the latter, it is due to the decay of the head buoyancy. The duration of the constant-velocity phase increases with increasing stratification strength when the current is supercritical. For weak stratification the turbulence is considerable, while it is suppressed with stronger stratification. Mixing is more intense for supercritical GCs; however, mixing efficiency is much higher for subcritical GCs.

Large-scale coherent structure and canopy-level turbulence in the convective boundary layer over urban areas

Thermally unstable stratification alters the winds over urban areas and excites organized, large-scale motions (LSMs) in the atmospheric boundary layer (ABL). The current understanding of the interaction between urban turbulence and buoyancy-induced coherent structures is rather limited due to their multiscale, multiphysics nature. Nine sets of large-eddy simulation (LES) are performed to critically examine the influence of atmospheric instability on turbulent boundary layers (TBLs) interacting with idealized urban roughness elements. Varying the stratification from neutral to almost free convective conditions enables the coherent structures to transit from horizontal rolls to open cellular patterns. The aerodynamic roughness parameters tend to decrease in more unstable stratification, potentially a result of buoyancy effects that might enhance organized ejections and suppress energetic sweeps within the roughness sublayers (RSLs). Unexpectedly, roughness-induced sweeps dominate the vertical momentum fluxes within urban canopies even under the most convective condition. However, decoupling the momentum and heat transports in highly convective states results in a subtle influence of urban roughness on the vertical heat fluxes therein. The spatial distribution of canopy-level turbulent momentum fluxes is found to be strongly modulated by the overlying large-scale coherent structures. The modulation is enhanced with increasing instability. In addition, urban-like surfaces elevate the large-scale rolls and assist them in forming under a weaker stratification than in canonical settings.

The interannual variability of the Indian Ocean subtropical mode water based on the Argo data

Gaining insight into the interannual variability of the Indian Ocean Subtropical Mode Water (IOSTMW) is essential for understanding ocean dynamics in the Southwest Indian Ocean, since it carries the signal of winter mixing and transports it into the ocean interior. As the number of Argo profiles in the Southwest Indian Ocean increases, it has become possible to study temporal variations in IOSTMW using observation data. We used Argo products to examine the interannual variability of the IOSTMW from 2005 to 2020. We examined various definitions to determine the most suitable definition for IOSTMW in this study, choosing to define the IOSTMW as a layer with a vertical temperature gradient of less than 1°C per 100 meters (dT/dz< 1°C/100 m) and a temperature range of 16°C–18°C because this correlates strongly with winter heat loss in the same year. This method is particularly useful for investigating how mode water captures anomalous winter mixing signals and advects them to the ocean interior via subduction. Furthermore, we found that summer stratification can play a role in either facilitating or hindering the formation of thick IOSTMW layers. Our study indicates that thin IOSTMW layers are primarily caused by extremely weak winter heat loss associated with anomalously weak latent heat, whereas thick IOSTMW formation is aided by weak summer stratification.

Identification and relative contributions of environmental driving factors for abundant and rare bacterial taxa to thermal stratification evolution

The thermal stratification of reservoir affects water quality, and water quality evolution is largely driven by microorganisms. However, few studies have been conducted on the response of abundant taxa (AT) and rare taxa (RT) to thermal stratification evolution in reservoirs. Here, using high-throughput absolute quantitative techniques, we examined the classification, phylogenetic diversity patterns, and assembly mechanisms of different subcommunities during different periods and investigated the key environmental factors driving community construction and composition. The results showed that community and phylogenic distances of RT were higher than AT (P < 0.001), and community and phylogenic distances of the different subcommunities were significantly positively correlated with the dissimilarity of environmental factors (P < 0.001). Nitrate (NO3−-N) was the main driving factor of AT and RT in the water stratification period, and Mn was the main driving factor in the water mixing period (MP) based on redundancy analysis (RDA) and random forest analysis (RF). The interpretation rate of key environmental factors based on the selected indicator species in RT by RF was higher than that of AT, and Xylophilus (10.5%) and Prosthecobacter (0.1%) had the highest average absolute abundance in AT and RT during the water stable stratification period (SSP), whereas Unassigned had the highest abundance during the MP and weak stratification period (WSP). The network of RT and environmental factors was more stable than that of AT, and stratification made the network more complex. NO3−-N was the main node of the network during the SSP, and manganese (Mn) was the main node during the MP. Dispersal limitation dominated community aggregation, the proportion of AT was higher than that of RT. Structural Equation Model (SEM) showed that NO3−-N and temperature (T) had the highest direct and total effects on β-diversity of AT and RT for the SP and MP, respectively.

Effects of phase boundary and shear on diffusive instability

We study the diffusive instability subject to a solid–liquid interface and a Kolmogorov flow using modal, non-modal analyses and energy analysis. It is found that the phase boundary has different effects on the threshold of diffusive convection for weak and strong salinity stratification. In the context of oceanography where the salinity Rayleigh number RS is very high, the ice–water interface has negligible influence on the onset of diffusive convection. In the presence of shear, the diffusive convection for RS &lt; 106 tends to be inhibited and with the increase of shear intensity, the oscillatory, steady convective and Kelvin–Helmholtz instabilities will be successively dominant. For RS &gt; 106, the shear has a destabilizing effect on the diffusive convection, due to the generation of thermohaline-sheared instability found by Radko (J. Fluid Mech., vol. 805, 2016, pp. 147–170). Non-modal analysis indicates that for realistic parameters of high-latitude oceans, with the transition of the ultimate energy source of instability from the density gradient to background current, the thermohaline-shear instability is expected to transition from oscillatory to steady instability. The initial transient amplification due to double diffusion has also been studied, which is due to the generation of overstable instability at the initial phase. For RS &lt; 106, the optimal initial condition to achieve the maximum transient growth favours longitudinal rolls. For thermohaline-shear instability, however, it favours transverse rolls and specifically, in oscillatory thermohaline-shear instability, the transient amplification can be enhanced by the shear by one order of magnitude, thus having important influence on the stability of the system.

Bolgiano-Obukhov spectrum and mixing efficiency in stably stratified turbulence.

In this paper, using a shell model, we simulate highly turbulent stably stratified flow for weak to moderate stratification at unitary Prandtl number. We investigate the energy spectra and fluxes of velocity and density fields. We observe that for moderate stratification, in the inertial range, the kinetic energy spectrum E_{u}(k) and the potential energy spectrum E_{b}(k) show dual scaling-Bolgiano-Obukhov scaling [E_{u}(k)∼k^{-11/5} and E_{b}(k)∼k^{-7/5}] for k<k_{B}, where k_{B} is the Bolgiano wave number, and Kolmogorov scaling (∼k^{-5/3}) for k>k_{B}. In addition, we find that the mixing efficiency η_{mix} varies as η_{mix}∼Ri for weak stratification, whereas η_{mix}∼Ri^{1/3} for moderate stratification, where Ri is the Richardson number.

Ocean Eddies in the Drake Passage: Decoding Their Three-Dimensional Structure and Evolution

The Drake Passage is known for its abundant mesoscale eddies, but little is known about their three-dimensional characteristics, which hinders our understanding of their impact on eddy-induced transport and deep-sea circulation. A 10-year study was conducted using GLORYS12 Mercator Ocean reanalysis data from 2009 to 2018. The study analyzed the statistical characteristics of eddies in the Drake Passage, spanning from the surface down to a depth of 2000 m in three dimensions. The findings indicate that the mean radius of the eddies is 35.5 km, with a mean lifespan of 12.3 weeks and mean vorticity of 2.2 × 10−5 s−1. The eddies are most active and energetic near the three main fronts and propagate north-eastward at an average distance of 97.8 km. The eddy parameters vary with water depth, with more anticyclones detected from the surface to 400 m, displaying a larger radius and longer propagation distance. Cyclones have longer lifespans and greater vorticity. However, beyond 400 m, there is not much difference between anticyclones and cyclones. Approximately 23.3% of the eddies reach a depth of 2000 m, with larger eddies tending to penetrate deeper. The eddies come in three different shapes, bowl-shaped (52.7%), lens-shaped (27.1%) and cone-shaped (20.2%). They exhibit annual and monthly distribution patterns. Due to its high latitude location, the Drake Passage has strong rotation and weak stratification, resulting in the generation of small and deep-reaching eddies. These eddies contribute to the formation of Antarctic intermediate water and lead to modulation of turbulent dissipation.

Sediment resuspension due to internal solitary waves of elevation in the Messina Strait (Mediterranean Sea)

By combining real-field observations and theoretical predictions, we describe role and relationships among north-propagating internal solitary waves (ISWs) generated by tidal currents in the Messina Strait (Mediterranean Sea), buoyancy deformation, sediment resuspension, and mixing effects. In particular, our results show that the presence of ISWs traveling along the Gioia Basin (north of the Strait) is not strictly related to seasonality. During winter, when the remote observation of ISWs from satellite is particularly rare due to the weak water column stratification, we observe elevation-type ISWs from hydrographic data. This finding reveals a different scenario with respect to the summer one, when the high stratified water column gives rise to depression-type north-propagating ISWs and the subsequent sea surface manifestations, detectable from satellite imagery. Moreover, our beam transmission observations and theoretical predictions of the induced near-bottom horizontal velocity suggest that these elevation-type ISWs induce sediment resuspension over the seafloor, as well as mixing effects as they break on the frontal slope nearby Capo Vaticano.

Experimental study on combustion and emission characteristics of ethanol-gasoline blends in a high compression ratio SI engine

The utilization of spark induced compression ignition (SICI) is a proven method to extend the operation load range, improve thermal efficiency as well as ensure robust control of the combustion process of homogeneous charge compression ignition (HCCI). However, negative valve overlap (NVO), intake-charge heating, and other methods of achieving SICI and the knock occurrence limit its load range and industrialization. In this paper, the effects of ethanol blending with different excess air ratios (λ) on the knock and combustion characteristics, output power, indicated thermal efficiency (ITE), and emissions were investigated in a high compression ratio (CR), single-cylinder, four-stroke SICI engine. The experimental results show that weak stratification increases the knock intensity, while strong stratification suppresses knock tendency at the stoichiometric ratio. Ethanol addition can suppress knock tendency, reduce the randomness of knock and improve the combustion stability. In comparison with pure gasoline, the IMEP and ITE of E100 increase by 6.3–15.1% and 6.8–9.1% at different λ, respectively. For conditions with low ethanol blending ratio (such as 0% or 10%) and λ with 1.0 or 1.2, knock is the limitation to further improve its thermal efficiency, while fuel economy is more affected by the overall combustion phase with an increase in ethanol blending ratio and λ. There exists a non-monotonic correlation between the NOx, THC, and CO emissions and the ethanol blending ratio due to the competing effects of oxygen-containing properties, higher latent heat of evaporation (LHE), intake-charge cooling effect, and higher combustion temperature. Ethanol blending, except for E100, does not exhibit the PN reduction tendency. Knock deteriorates PN emissions, especially in nucleation mode, and ethanol blending shifts its size distribution towards nucleation mode.

Numerical study on the energy extraction performance by flapping foils in a density stratified flow

An energy extractor based on fully-activated flapping foils operating in a density stratified flow is studied numerically. The effect of stratification strength characterizing as the internal Froude number on the energy extraction performance is first investigated by fixing a typical set of parameters. The hydrodynamic characteristics of flapping foils with different kinematics in the stratified regime are then examined by varying the pitching amplitude. Our results suggest that the stratification effect decreases the energy harvesting efficiency owing to resulting poor synchronization between forces and flapping motion. Within the considered parametric space, the minimum energy extraction performance is identified at the Froude number around Fr=2, giving rise to an attached boundary layer without the favorable flow separation and leading edge vortex shedding. Nevertheless, a sudden increase of extracted power is observed when the stratification level is enhanced to Fr=1, which is attributed to the distinct flow structures generated by the induced internal waves. For the varying kinematics, the Froude number corresponding to the minimum performance is found to increase as the pitching amplitude becomes higher. It is indicated that the induced internal waves tend to dominate the flow patterns at a highly strong stratification, whereas the kinematics of flapping foils still play a crucial role in determining the vortex dynamics for the moderate and weak stratification strength.

Turbulent Kinetic Energy Production in a Far‐Field River Plume Under Upwelling‐Favorable Winds

AbstractAn idealized three‐dimensional numerical model is used to investigate turbulent kinetic energy (TKE) production in a far‐field river plume under upwelling‐favorable winds. TKE production decreases over longer length scales as the river plume thickens. Maximum TKE production appears in the surface layer and is mainly generated by the alongshore component of the velocity shear. The large velocity shear and weak stratification in the surface layer result in a gradient Richardson number (Ri) of &lt;0.25, which corresponds to the locally high TKE production. We find that asymmetrical TKE production occurs at the two edges of the river plume, due to the opposite nonlinear interaction of the Ekman and geostrophic effects at the shoreward and seaward edges of the river plume. This asymmetrical TKE production combines with secondary upwelling circulation in the river plume under upwelling‐favorable winds, which may generate more intense biological activity at the shoreward edge of the river plume than at the seaward edge. Numerical model experiments are performed to examine the effects of wind, river discharge, and stratified conditions on TKE production in the river plume. Finally, we propose a conceptual model in which the depth‐averaged TKE production in the river plume is proportional to the alongshore wind stress () and inversely proportional to the cube of the surface boundary layer thickness (D3), which is consistent with the results of numerical experiments.

Terrigenous dissolved organic matter input and nutrient-light-limited conditions on the winter microbial food web of the Beagle Channel

The viral shunt and microbial loop are trophic pathways in the microbial food web, recognized as major drivers of carbon and nutrient cycles in the sea. The Beagle Channel, located at the southernmost extreme of South America, is a subantarctic marine environment influenced by terrestrial runoff and glacial meltwater with high temporal and spatial variability. There is a knowledge gap on the trophic pathways and the microbial food web in the eastern part of the Channel. We herein present for the first time a comprehensive and spatially wide study of the effects of physicochemical and bio-optical parameters on the microbial food web along the Eastern Beagle Channel. The study took place under winter hydrographic conditions, featuring weak vertical stratification of the water column and low chlorophyll a concentrations (0.13 ± 0.07 mg m−3). Biogeochemical gradients were notable between the inner and outer (easternmost) sectors of the Channel, separated by a shallow sill at Mackinlay Strait. As a chemical water mass tracer, the presence of the highest concentrations in bottom waters of NO3 + NO2 in the inner channel and the difference in the T-S diagram indicate the Subantarctic Pacific origin of this water mass. Structural equation models showed that the specific UV absorbance of water (SUVA254, a proxy for the aromatic carbon content) and NO3 + NO2 were the main parameters affecting the microbial food web in surface/subsurface waters suggesting that prokaryotes biomass was partially supported by allochthonous dissolved organic matter (DOM) derived from river discharges. Our results show a major role of the viral shunt in surface/subsurface waters retaining nutrients in the basal levels of the food web. We suggest that the viral shunt in surface/subsurface waters and the water residence time west of Mackinlay Strait allows infected cells and viral lysis products to reach bottom waters propelling the production of recalcitrant and labile DOM there.

Analysis of Internal Solitary Waves in the Gulf of Oman and Sources Responsible for Their Generation

A combination of multiple data sources has been used to study the characteristics of internal solitary waves (ISWs) in the Gulf of Oman (GoO). Water column stratification in the Gulf has been examined using field observations and World Ocean Atlas 2018 datasets. The spatiotemporal distribution of ISWs has been obtained from satellite images obtained by means of Synthetic Aperture Radar (SAR) and optical sensors taken from 2018 to 2020. The mechanisms of ISW generation in the GoO have been studied using the data revealed from different available sources. The results show that there are annually two major typical stratifications in the GoO throughout the year, strong stratification in May through September and weak stratification during other months. Dispersion relations corresponding to these types of stratification have been obtained with acceptable accuracy for both deep and shallow regions. The spatiotemporal distribution of ISWs demonstrates that the western and southern regions of the GoO are the hotspots for generation of ISWs in this basin. Several mechanisms of ISW generation in the GoO are discussed including tide, eddies, lee waves, and atmospheric perturbation; the latter one is, apparently, responsible for the appearance of large-amplitude ISWs.

Controlling factors of phytoplankton distribution in the river–lake transition zone of a large lake

River–lake transition zones have been identified as major drivers of phytoplankton growth. With climate change reducing the frequency of complete lake overturns, it is expected that the Rhône River, the main tributary to Lake Geneva (France/Switzerland), will become the major source of nutrients for the lake euphotic zone. The river–lake transition zone was hence examined at the mouth of the Rhône River with the aim of understanding the complexities and controls of phytoplankton distribution in this specific deltaic ecosystem. Two field campaigns were carried out in which water samples were collected from longitudinal and transversal transects across the transition zone. These samples were analyzed for both nutrient and phytoplankton concentrations, while the fraction of Rhône River water in a lake sample was determined by the stable isotope composition of the water. The results indicate contributions in P and Si related to the Rhône intrusion into the lake. Furthermore, this river–lake transition zone appears to be a dynamic area that can locally present optimal conditions for phytoplankton growth. In early spring, a wind event broke the early and weak stratification of the lake, forcing the Rhône River and its turbidity plume to intrude deeper. Thus, this sharp drop of the turbidity within the euphotic zone allowed an increase in the phytoplankton biovolume of 44%. In early fall, outside of the turbid near field of the river mouth, the Rhône interflow, located just below the thermocline, promoted a local deep chlorophyll maximum.

Analytical determination of the water thermal accumulator capacity in a consumption mode with a constant debit and a fixed temperature

The water thermal accumulators for heat and cold storage are an important part of various engineering installations. This article examines a tank with volume of 1000 [l] in a consumption mode with a constant debit and a fixed temperature. The output volumes of hot water in two regimes – weak stratification and strong stratification are calculated analytically and are compared with the results from an analogical simulation in ANSYS. The ANSYS simulations are unconditionally more accurate, but require specialized knowledge and software. The solutions without simulation procedures are suggested. The output volume of hot water with fixed temperature - as a function of initial and final temperatures in accumulator, flow debit, tank construction and external conditions - can be calculated fast and accurate enough for the engineering practice. Relative differences between analytic and simulations results for all regimes are below 10.5%.

Novel Air2water Model Variant for Lake Surface Temperature Modeling With Detailed Analysis of Calibration Methods

The air2water model is a simple and efficient tool for modeling surface water temperature in lakes based solely on the air temperature. In this article, we propose to modify the air2water model in such a way that different parameters would be associated with lake stratification of cold waters than with lake stratification of warm waters. The situation of a mix of both cold water and warm water is also considered. The model is tested on 22 lowland Polish lakes against two classical air2water variants. As the new air2water model variant is slightly more complicated than the basic versions, we focus on the importance of the choice of the calibration method. Each variant of the air2water model is calibrated with eight different optimization methods, which are also compared on various benchmark problems. We show that the proposed variant is superior to the classical air2water models on about 90% of tested lakes, but only if the calibration approach is properly selected, which confirms the importance of the links between the model and appropriate optimization procedures. The proposed air2water variant performs well on various lowland lakes, with exception of large but shallow ones, probably due to the weak stratification of the shallow lakes.

GENERATION OF INTERNAL WAVES BY A SUB-MESOSCALE EDDY AT CAPE SVYATOY NOS IN THE BARENTS SEA

Data on generation of soliton-like internal waves by a mesoscale eddy in the region of the Barents Sea near Cape Svyatoy Nos, widely known for its dynamic activity, are presented here. The results are based on the analysis of satellite optical (Sentinel-2 MSI) and radar (Sentinel-1 SAR) images obtained sequentially on August 8 and 9, 2021. A detailed analysis of the dynamics of the eddy and internal waves was made possible due, among other things, to the phytoplankton bloom, which created the necessary optical contrasts that make it possible to monitor the dynamics of currents in the near-surface layer of the sea. Significant parameters of recorded internal waves were measured using satellite methods and analyzed using the TPXO9.0 model and numerical solution of the internal wave equation. It has been shown, that internal waves were caused by a submesoscale eddy formed near the cape. Waves had quite low velocity (0.10–0.16 m/s) what was due to weak stratification of the water environment.