Tidal Stirring Research Articles

Characteristics and Mechanisms of Spring Tidal Mixing and Sediment Transport in a Microtidal Funnel-Shaped Estuary

Information about estuarine mixing and its control of sediment transport is crucial to elucidating the dynamics and evolution of estuaries. Here, the microtidal and funnel-shaped Zhenhai Estuary, located in the southwestern Pearl River Delta of China, is used to investigate the characteristics and mechanisms of water mixing and sediment transport based on observations from three spring tides. The results reveal that the studied estuary remains well mixed during spring tides from 2013–2022 despite its microtidal regime. Tidal stirring, which is enhanced by tidal energy convergence and benefits from the funnel-shaped geometry and shallow bathymetry, favors vertical mixing, contributing to the formation of strong mixing in the estuary. Due to the well-mixed regime, sediment transport in the estuary is dominated by the advective term, followed by a moderate tidal pumping term and minor estuarine circulation term. Accordingly, sediments within the estuary tend to be transported landward owing to the regulation of the funnel-shaped geometry, and a gradual but slow infilling trend is predictable. This paper deepens our understanding of hydrodynamics and sediment transport in microtidal estuaries.

Dwarfs in Void Environments (DIVE): The Stellar Kinematics of Void Dwarf Galaxies Using the Keck Cosmic Web Imager

Dwarf galaxies located in extremely underdense cosmic voids are excellent test beds for disentangling the effects of large-scale environments on galaxy formation and evolution. We present the first results of the Dwarfs in Void Environments Survey, which has obtained integral field spectroscopy for low-mass galaxies (M ⋆ = 107–109 M ⊙) located inside (N = 21) and outside (N = 9) cosmic voids using the Keck Cosmic Web Imager. Using measurements of stellar line-of-sight rotational velocity v rot and velocity dispersion σ ⋆, we test the tidal stirring hypothesis, which posits that dwarf spheroidal galaxies are formed through tidal interactions with more massive host galaxies. We measure low values of v rot/σ ⋆ ≲ 2 for our sample of isolated dwarf galaxies, and we find no trend between v rot/σ ⋆ and the distance from a massive galaxy out to Mpc. These suggest that dwarf galaxies can become dispersion-supported, “puffy” systems even in the absence of environmental effects like tidal interactions. We also find indications of an upward trend between v rot/σ ⋆ and galaxy stellar mass, perhaps implying that stellar disk formation depends on mass rather than environment. Although some of our conclusions may be slightly modified by systematic effects, our main result still holds: that isolated low-mass galaxies may form and remain as puffy systems rather than the dynamically cold disks predicted by classical galaxy formation theory.

Detection of chemo-kinematical structures in Leo I

Context. A variety of formation models for dwarf spheroidal (dSph) galaxies have been proposed in the literature, but generally they have not been quantitatively compared with observations. Aims. We search for chemodynamical patterns in our observational data set and compare the results with mock galaxies consisting of pure random motions, and simulated dwarfs formed via the dissolving star cluster and tidal stirring models. Methods. We made use of a new spectroscopic data set for the Milky Way dSph Leo I, combining 288 stars observed with Magellan/IMACS and existing Keck/DEIMOS data, to provide velocity and metallicity measurements for 953 Leo I member stars. We used a specially developed algorithm called BEACON to detect chemo-kinematical patterns in the observed and simulated data. Results. After analysing the Leo I data, we report the detection of 14 candidate streams of stars that may have originated in disrupted star clusters. The angular momentum vectors of these streams are randomly oriented, consistent with the lack of rotation in Leo I. These results are consistent with the predictions of the dissolving cluster model. In contrast, we find fewer candidate stream signals in mock data sets that lack coherent motions ∼99% of the time. The chemodynamical analysis of the tidal stirring simulation produces streams that share a common orientation of their angular momenta, which is inconsistent with the Leo I data. Conclusions. Even though it is very difficult to distinguish which of the detected streams are real and which are only noise, we can be certain that there are more streams detected in the observational data of Leo I than expected in pure random data.

The Impact of Tidal Straining and Advection on the Stratification in a Partially Mixed Estuary

Stratification and mixing of the water column is an important dynamic process in the estuary, which plays a significant role in the estuarine circulation, mass transport and energy exchange. Based on the multi-station synchronous observation data from 26 February to 6 March in 2011 during dry season in the North Channel of the Changjiang Estuary, the Richardson number, the Simpson number and the potential energy anomaly of water were calculated to analyze the tidal variation of the mixing and stratification processes. The roles of the depth-mean straining, longitudinal advection, non-mean straining and tidal stirring in the processes of mixing and stratification of the water column were analyzed by calculating the contribution terms of the time-derivative of potential energy anomaly. The results show that the mixing and stratification of the water column in the North Channel have significant spatiotemporal variation. Stability of the stratification gradually decreases from neap tide to spring tide. In the reaches of salt wedge migration, permanent stratification develops during neap and mean tide, with stability increasing on the flood and decreasing on the ebb, which is dominated by longitudinal advection. During spring tide, periodic stratification develops, with development of stratification on the flood and its breakdown on the ebb, which is dominated by longitudinal advection and tidal stirring. In the main reaches of saltwater intrusion, permanent stratification develops during neap tide, with stability increasing on the ebb and decreasing on the flood, which is dominated by depth-mean tidal straining. During mean and spring tide, periodic stratification occurs, with development of stratification on the ebb and its breakdown on the flood, which is controlled by depth-mean tidal straining and assisted by tidal stirring. In the North Channel, tidal advection is the main stratifying agent in the salt wedge migration reaches, and tidal straining is the main stratifying agent in the main reaches of saltwater intrusion.

Investigating tidal river dynamics in a longitudinally varying channel geometry

Examining the flow dynamics in a tidal river, particularly downstream, is of great importance to consider the substantial interaction between river runoff and tidal waves. Previous studies have investigated flow dynamics in tidal rivers mainly concentrated on the lateral structure of the estuarine dynamics, with limited consideration of the impact of geometrical variation of the tidal channel, which has an enormous effect along the longitudinal direction. The aim of this study is to offer a comprehensive understanding of the dynamics of a tidal river with longitudinally varying channel geometry. To accomplish the aforementioned aims, this study provides a spatiotemporal analysis of water current and density for a fortnight period of a mesotidal estuarine channel that encompasses both curved and straight channels in the streamwise direction. Moreover, a quantitative analysis of the governing mechanisms for estuarine dynamics was performed to determine the alteration of driving mechanisms due to channel geometry variation. The longitudinal velocity profile indicates the existence of tidal asymmetry where the ebb currents are dominant over flood currents. The asymmetry, which is induced by the interaction between the channel curvature and density gradient, is observed to be more enhanced in the curved channel than in the straight channel. Moreover, the across-channel bathymetry difference between the curved and straight channels leads to different patterns of lateral flow in those channels. Additionally, the longitudinal and lateral velocities averaged for the spring/neap tides reveal that the channel with a higher curvature degree has greater residual circulation. On the other hand, the periodically varying density distribution denotes the existence of tidal straining, which is slightly modified by the variation in geometry. The notable findings of this study are as follows: (i) the variability in channel geometries introduces different responses of tidal straining and tidal stirring; (ii) the river effect (river-induced shear) in the curved channel substantially increases significantly compared to the straight channel over a certain threshold of river runoff. We believe that the findings of this study deepen our understanding of tidal river dynamics in a longitudinally varying channel geometry.

Tidal mitigation of offshore wind wake effects in coastal seas

With increasing offshore wind development, more and more marine environments are confronted with the effects of atmospheric wind farm wakes on hydrodynamic processes. Recent studies have highlighted the impact of the wind wakes on ocean circulation and stratification. In this context, however, previous studies indicated that wake effects appear to be attenuated in areas strongly determined by tidal energy. In this study, we therefore determine the role of tides in wake-induced hydrodynamic perturbations and assess the importance of the local hydrodynamic conditions on the magnitude of the emerging wake effects on hydrodynamics. By using an existing high-resolution model setup for the southern North Sea, we performed different scenario simulations to identify the tidal impact. The results show the impact of the alignment between wind and ocean currents in relation to the hydrodynamic changes that occur. In this regard, tidal currents can deflect emerging changes in horizontal surface currents and even mitigate the mean changes in horizontal flow due to periodic perturbations of wake signals. We identified that, particularly in shallower waters, tidal stirring influences how wind wake effects translate to changes in vertical transport and density stratification. In this context, tidal mixing fronts can serve as a natural indicator of the expected magnitude of stratification changes due to atmospheric wakes. Ultimately, tide-related hydrodynamic features, like periodic currents and mixing fronts, influence the development of wake effects in the coastal ocean. Our results provide important insights into the role of hydrodynamic conditions in the impact of atmospheric wake effects, which are essential for assessing the consequences of offshore wind farms in different marine environments.

Numerical Investigation of the Sediment Hyperpycnal Flow in the Yellow River Estuary

Sediment hyperpycnal flow is one of the most important processes for mass transport, which is essential to coastal morphodynamics. Herein, we studied the generation and maintenance of the sediment hyperpycnal flow in the Yellow River Estuary (YRE) using a three-dimensional finite volume coastal ocean model (FVCOM). The model considered the effect of sediment-laden water on density stratification, and was validated by field hydrodynamic and sediment data. Numerical results revealed that the hyperpycnal flow shows periodic characteristics with tidal cycles where the flow is weakened during flood tides and enhanced during ebb tides. A high suspended sediment concentration (SSC) of about 30–40 kg/m3 constitutes an important factor in the formation of hyperpycnal flows. High river discharge with high SSC is essential for maintaining the hyperpycnal flow in the YRE. The Simpson potential energy theory was applied to study the processes of estuarine circulation, tidal straining, and tidal stirring in the YRE. The tidal straining is the main control factor of the periodic stratification-mixing process of hyperpycnal flows in the YRE. Along the axis of the river mouth, the momentum balance is mainly dominated by the pressure gradient and advection.

On the tidal formation of dark matter-deficient galaxies

ABSTRACT Previous studies have shown that dark matter-deficient galaxies (DMDG) such as NGC 1052-DF2 (hereafter DF2) can result from tidal stripping. An important question, though, is whether such a stripping scenario can explain DF2’s large specific frequency of globular clusters (GCs). After all, tidal stripping and shocking preferentially remove matter from the outskirts. We examine this using idealized, high-resolution simulations of a regular dark matter-dominated galaxy that is accreted on to a massive halo. As long as the initial (pre-infall) dark matter halo of the satellite is cored, which is consistent with predictions of cosmological, hydrodynamical simulations, the tidal remnant can be made to resemble DF2 in all its properties, including its GC population. The required orbit has a pericentre at the 8.3 percentile of the distribution for subhaloes at infall, and thus is not particularly extreme. On this orbit the satellite loses 98.5 (30) per cent of its original dark matter (stellar) mass, and thus evolves into a DMDG. The fraction of GCs that is stripped off depends on the initial radial distribution. If, at infall, the median projected radius of the GC population is roughly two times that of the stars, consistent with observations of isolated galaxies, only ∼20 per cent of the GCs are stripped off. This is less than for the stars, which is due to dynamical friction counteracting the tidal stirring. We predict that, if indeed DF2 was crafted by strong tides, its stellar outskirts should have a very shallow metallicity gradient.

Characteristics and Driving Mechanisms of Mixing and Stratification in the North Passage of the Changjiang Estuary, China

Wang, Y.; Jiang, C.; Cheng, H.; Li, W., and Teng, L., 2022. Characteristics and driving mechanisms of mixing and stratification in the North Passage of the Changjiang estuary, China. Journal of Coastal Research, 38(1), 140–153. Coconut Creek (Florida), ISSN 0749-0208. Based on the measured hydrological and sediment data of spring and neap tides in the dry and flood seasons of 2016 in the North Passage of the Changjiang Estuary, the gradient Richardson number, Simpson number, and potential energy anomalies were calculated to analyze the tidal cycle, spring-neap variation, and seasonal variation of mixing and stratification. Taking into account the vertical difference of horizontal density gradient, the contribution terms of time derivative of potential energy anomalies was derived, including depth-mean straining, advection, nonmean straining, and tidal stirring. The driving mechanisms of mixing and stratification was investigated by calculating and comparing these four contribution terms. The results show that the seasonal variation of freshwater discharge mainly affected the distance of saltwater intrusion, the saline wedge moved upstream in dry season, while the tidal dynamics had a significant impact on mixing and stratification. In the reaches with saline wedge tip migration, the water column was periodically stratified, and the process was dominated by advection term. In the main reaches of saltwater intrusion, during spring tide the water column was periodically stratified, which was stratified during flood tidal current; it was well mixed during ebb tidal current, and the stratification during flood tidal current was promoted by advection and depth-mean straining. The mixing during ebb tidal current was promoted by tidal stirring and advection; during neap tide, the water column was permanently stratified. The stratification was enhanced during flood tidal current and weakened during ebb tidal current, and the stratification was mainly maintained by depth-mean straining.

Internal rotation of Milky Way dwarf spheroidal satellites with Gaia Early Data Release 3

ABSTRACT We present an analysis of the kinematics of 14 satellites of the Milky Way (MW). We use proper motions (PMs) from the Gaia Early Data Release 3 (EDR3) and line-of-sight velocities (vlos) available in the literature to derive the systemic 3D motion of these systems. For six of them, namely the Carina, Draco, Fornax, Sculptor, Sextans, and Ursa Minor dwarf spheroidal galaxies (dSph), we study the internal kinematics projecting the stellar PMs into radial, VR (expansion/contraction), and tangential, VT (rotation), velocity components with respect to the centre of mass. We find significant rotation in the Carina (|VT| = 9.6 ± 4.5 km s−1 ), Fornax (|VT| = 2.8 ± 1.3 km s−1 ), and Sculptor (|VT| = 3.0 ± 1.0 km s−1 ) dSphs. Besides the Sagittarius dSph, these are the first measurements of internal rotation in the plane of the sky in the MW’s classical dSphs. All galaxies except Carina show |VT|/σv < 1. We find that slower rotators tend to show, on average, larger sky-projected ellipticity (as expected for a sample with random viewing angles) and are located at smaller Galactocentric distances (as expected for tidal stirring scenarios in which rotation is transformed into random motions as satellites sink into the parent halo). However, these trends are small and not statistically significant, indicating that rotation has not played a dominant role in shaping the 3D structure of these galaxies. Either tidal stirring had a weak impact on the evolution of these systems or it perturbed them with similar efficiency regardless of their current Galactocentric distance.

Effect of burrows of Boleophthalmus pectinirostris (Linnaeus, 1758) (Class Actinopterygii Family Gobiidae) on total organic matter in the mangrove ecosystem of Pandansari Brebes, Central Java

B. pectinirostris constructs different shapes of burrows on the surface of the sediment because of different functions. This study was to determine the effect of burrows on total organic matter (TOM) on the sediment surface in the mangrove ecosystem in Pandansari Brebes, Central Java. Sampling was divided into mangrove areas and muddy beach areas. The shape of the burrows was visually observed in a year. TOM was taken from inside and outside the burrow in June and September using a syringe and analyzed by the LOI method. B. pectinirostris had circular burrow openings at the same height as the ground surface. At the mouth of the burrow, there were traces of fish coming in and out. The diameter of the burrows was around 3-4cm in the muddy beach area and 2-9cm in the mangrove area. TOM in the mangrove area inside the burrow was 9,034±0,851% and outside was 8,470%±0,967% and in the muddy beach area inside the burrow was 8,754%±0,476% and outside was 8,558%±0,924%. There was no difference in TOM inside and outside the burrow in the two areas (P>0.05). It is likely caused by the locomotion of fish and tidal stirring, so the burrows do not function as a TOM trap.

Tidal evolution of galaxies in the most massive cluster of IllustrisTNG-100

We study the tidal evolution of galaxies in the most massive cluster of the IllustrisTNG-100 simulation. For the purpose of this work, we selected 112 galaxies with the largest stellar masses at present and followed their properties over time. Using their orbital history, we divided the sample into unevolved (infalling), weakly evolved (with one pericenter passage), and strongly evolved (with multiple pericenters). The samples are clearly separated by the value of the integrated tidal force from the cluster the galaxies experienced during their entire evolution and their properties depend strongly on this quantity. As a result of tidal stripping, the galaxies of the weakly evolved sample lost between 10 and 80% of their dark mass and less than 10% of stars, while those in the strongly evolved one lost more than 70% of dark mass and between 10 and 55% of stellar mass, and are significantly less, or not at all dark-matter dominated. While 33% of the infalling galaxies do not contain any gas, this fraction increases to 67% for the weakly evolved sample, and to 100% for the strongly evolved sample. The strongly evolved galaxies lose their gas earlier and faster (within 2–6 Gyr), but the process can take up to 4 Gyr from the first pericenter passage. These galaxies are redder and more metal rich, and at redshift z = 0.5, the population of galaxies in the cluster becomes predominantly red. As a result of tidal stirring, the morphology of the galaxies evolves from oblate to prolate and their rotation is diminished, thus the morphology-density relation is reproduced in the simulated cluster. The strongly evolved sample contains at least six convincing examples of tidally induced bars and six more galaxies that had their bars enhanced by their interaction with the cluster.

Oceanographic Variability induced by Tides, the Intraseasonal Cycle and Warm Subsurface Water intrusions in Maxwell Bay, King George Island (West-Antarctica)

We examine the hydrographic variability induced by tides, winds, and the advance of the austral summer, in Maxwell Bay and tributary fjords, based on two recent oceanographic campaigns. We provide the first description in this area of the intrusion of relatively warm subsurface waters, which have led elsewhere in Antarctica to ice-shelf disintegration and tidewater glacier retreat. During flood tide, meltwater was found to accumulate toward the head of Maxwell Bay, freshening and warming the upper 70 m. Below 70 m, the flood tide enhances the intrusion and mixing of relatively warm modified Upper Circumpolar Deep Water (m-UCDW). Tidal stirring progressively erodes the remnants of Winter Waters found at the bottom of Marian Cove. There is a buoyancy gain through warming and freshening as the summer advances. In Maxwell Bay, the upper 105 m were 0.79 °C warmer and 0.039 PSU fresher in February than in December, changes that cannot be explained by tidal or wind-driven processes. The episodic intrusion of m-UCDW into Maxwell Bay leads to interleaving and eventually to warming, salinification and deoxygenation between 80 and 200 m, with important implications for biological productivity and for the mass balance of tidewater glaciers in the area.

Dark matter distribution and dynamics of dwarf spheroidal galaxies

AbstractI review the current status of dynamical modelling of dwarf spheroidal galaxies focusing on estimates of their dark matter content. Starting with the simplest methods using the velocity dispersion profiles I discuss the inherent issues of mass-anisotropy degeneracy and contamination by unbound stars. I then move on to methods of increasing complexity, aiming to break the degeneracy, up to recent applications of the Schwarzschild orbit superposition method. The dynamical modelling is placed in the context of possible scenarios for the formation of dwarf spheroidal galaxies, including the tidal stirring model and mergers of dwarf galaxies. The two scenarios are illustrated with examples from simulations: a comparison between the tidal evolution of dwarfs with cuspy and cored dark matter profiles and the formation of a dwarf spheroidal with prolate rotation.

Temporal and spatial evolution of surface sediments characteristics in the Dagu River estuary and their dynamic response mechanism

ABSTRACT Based on the 39 surface sediment samples collected in the flood season and the dry season in 2012 respectively and the measured hydrological data in October 2012, the sediment grain size characteristics has been analyzed and the response mechanism of surface sediments to estuarine hydrodynamics was revealed by calculating the range of waves and tidal currents. The results show that: (1) The grain size of the surface sediment samples decreased gradually from land to sea in the flood season. The fine sediment was redistributed under marine hydrodynamics in the dry season and the sediments showed coarser tendency ingeneral; (2) tidal current stirring sediment was very obvious in Dagu River estuary area, and wave stirring sediments mainly occurred in the tidal flat area and estuary sand bar area; (3) in the flood season, surface sediment sat the estuary were transported towards south and southeast. In the dry season, surface sediments were transported towards southwest at the north area of Jiaozhou Bay Bridge, and sediments were transported towards northeast area at the south of Jiaozhou Bay Bridge.

The major merger origin of the Andromeda II kinematics

AbstractProlate rotation (i.e. rotation around the long axis) has been reported for two Local Group dwarf galaxies: Andromeda II, a dwarf spheroidal satellite of M31, and Phoenix, a transition type dwarf galaxy. The prolate rotation may be an exceptional indicator of a past major merger between dwarf galaxies. We showed that this type of rotation cannot be obtained in the tidal stirring scenario, in which the satellite is transformed from disky to spheroidal by tidal forces of the host galaxy. However, we successfully reproduced the observed Andromeda II kinematics in controlled, self-consistent simulations of mergers between equal-mass disky dwarf galaxies on a radial or close-to-radial orbit. In simulations including gas dynamics, star formation and ram pressure stripping, we are able to reproduce more of the observed properties of Andromeda II: the unusual rotation, the bimodal star formation history and the spatial distribution of the two stellar populations, as well as the lack of gas. We support this scenario by demonstrating the merger origin of prolate rotation in the cosmological context for sufficiently resolved galaxies in the Illustris large-scale cosmological hydrodynamical simulation.

Shelf sea tidal currents and mixing fronts determined from ocean glider observations

Abstract. Tides and tidal mixing fronts are of fundamental importance to understanding shelf sea dynamics and ecosystems. Ocean gliders enable the observation of fronts and tide-dominated flows at high resolution. We use dive-average currents from a 2-month (12 October–2 December 2013) glider deployment along a zonal hydrographic section in the north-western North Sea to accurately determine M2 and S2 tidal velocities. The results of the glider-based method agree well with tidal velocities measured by current meters and with velocities extracted from the TPXO tide model. The method enhances the utility of gliders as an ocean-observing platform, particularly in regions where tide models are known to be limited. We then use the glider-derived tidal velocities to investigate tidal controls on the location of a front repeatedly observed by the glider. The front moves offshore at a rate of 0.51 km day−1. During the first part of the deployment (from mid-October until mid-November), results of a one-dimensional model suggest that the balance between surface heat fluxes and tidal stirring is the primary control on frontal location: as heat is lost to the atmosphere, full-depth mixing is able to occur in progressively deeper water. In the latter half of the deployment (mid-November to early December), a front controlled solely by heat fluxes and tidal stirring is not predicted to exist, yet a front persists in the observations. We analyse hydrographic observations collected by the glider to attribute the persistence of the front to the boundary between different water masses, in particular to the presence of cold, saline, Atlantic-origin water in the deeper portion of the section. We combine these results to propose that the front is a hybrid front: one controlled in summer by the local balance between heat fluxes and mixing and which in winter exists as the boundary between water masses advected to the north-western North Sea from diverse source regions. The glider observations capture the period when the front makes the transition from its summertime to wintertime state. Fronts in other shelf sea regions with oceanic influence may exhibit similar behaviour, with controlling processes and locations changing over an annual cycle. These results have implications for the thermohaline circulation of shelf seas.

Possible contribution of typhoon- and internal tide-induced offshore nutrient supply to the occurrence of a Chattonella bloom in the Yatsushiro Sea, Japan

The bloom dynamics of the raphidophycean flagellate, Chattonella antiqua during summer 2016 in the Yatsushiro Sea, Japan were studied using a hydrodynamic model and monitoring data after typhoon Namtheun passed. The bloom formed in the southern area of the Yatsushiro Sea. The bloom dynamics differed from the dynamics of the 2008–2010 Chattonella blooms, which initially formed in the northern area and subsequently expanded to the southern area by effects of tidal currents. In 2016, high DIN levels appeared in the period preceding the bloom after the passage of the typhoon and the bloom formed in the southern area. The high DIN levels did not originate from the deeper water within the Yatsushiro Sea by vertical mixing because of the low DIN in the water column prior to the typhoon. A hydrodynamic model was used in order to examine how the high DIN was supplied. Model results suggested that deep offshore water intruded onshore and through the Ushibuka Strait into the Yatsushiro Sea in the period preceding the bloom. This deep offshore water intrusion was controlled by an internal tide, typhoon-induced current and vertical mixing induced by tidal stirring. The internal tide and typhoon-induced current transported offshore water to the lower layer in the Ushibuka Strait, and subsequently tidal mixing transported nutrients to the euphotic zone in the Ushibuka Strait by vertical diffusion.

The Effects of Ram-pressure Stripping and Supernova Winds on the Tidal Stirring of Disky Dwarfs: Enhanced Transformation into Dwarf Spheroidals

Abstract A conclusive model for the formation of dwarf spheroidal (dSph) galaxies still remains elusive. Owing to their proximity to the massive spirals Milky Way (MW) and M31, various environmental processes have been invoked to explain their origin. In this context, the tidal stirring model postulates that interactions with MW-sized hosts can transform rotationally supported dwarfs, resembling present-day dwarf irregular (dIrr) galaxies, into systems with the kinematic and structural properties of dSphs. Using N-body+SPH simulations, we investigate the dependence of this transformation mechanism on the gas fraction, f gas, in the disk of the progenitor dwarf. Our numerical experiments incorporate for the first time the combined effects of radiative cooling, ram-pressure stripping, star formation, supernova (SN) winds, and a cosmic UV background. For a given orbit inside the primary galaxy, rotationally supported dwarfs with gas fractions akin to those of observed dIrrs (f gas ≳ 0.5), demonstrate a substantially enhanced likelihood and efficiency of transformation into dSphs relative to their collisionless (f gas = 0) counterparts. We argue that the combination of ram-pressure stripping and SN winds causes the gas-rich dwarfs to respond more impulsively to tides, augmenting their transformation. When f gas ≳ 0.5, disky dwarfs on previously unfavorable low-eccentricity or large-pericenter orbits are still able to transform. On the widest orbits, the transformation is incomplete; the dwarfs retain significant rotational support, a relatively flat shape, and some gas, naturally resembling transition-type systems. We conclude that tidal stirring constitutes a prevalent evolutionary mechanism for shaping the structure of dwarf galaxies within the currently favored CDM cosmological paradigm.

Drivers of Barotropic and Baroclinic Exchange through an Estuarine Navigation Channel in the Mississippi River Delta Plain

Estuarine navigation channels have long been recognized as conduits for saltwater intrusion into coastal wetlands. Salt flux decomposition and time series measurements of velocity and salinity were used to examine salt flux components and drivers of baroclinic and barotropic exchange in the Houma Navigation Channel, an estuarine channel located in the Mississippi River delta plain that receives substantial freshwater inputs from the Mississippi-Atchafalaya River system at its inland extent. Two modes of vertical current structure were identified from the time series data. The first mode, accounting for 90% of the total flow field variability, strongly resembled a barotropic current structure and was coherent with alongshelf wind stress over the coastal Gulf of Mexico. The second mode was indicative of gravitational circulation and was linked to variability in tidal stirring and the horizontal salinity gradient along the channel’s length. Tidal oscillatory salt flux was more important than gravitational circulation in transporting salt upestuary, except over equatorial phases of the fortnightly tidal cycle during times when river inflows were minimal. During all tidal cycles sampled, the advective flux, driven by a combination of freshwater discharge and wind-driven changes in storage, was the dominant transport term, and net flux of salt was always out of the estuary. These findings indicate that although human-made channels can effectively facilitate inland intrusion of saline water, this intrusion can be minimized or even reversed when they are subject to significant freshwater inputs.