Influence of Stratification and Bottom Boundary Layer on the Classical Ekman Model

A depth understanding of the different processes of water movements produced by the wind surface stress yields a better description and improvement of the marine food chain and ecosystem. The classical Ekman model proposes a hypothetical ocean, excluding the influence of continents and the Coriolis force. It also assumes infinite depth and a constant vertical eddy viscosity. The current study aims to understand how the vertical velocity profile is affected by the variation of the eddy viscosity coefficient (k(z)) and the consideration of a finite depth. The study uses an ideal analytical model with the Ekman classical model as a starting point. It has been demonstrated that, for a very stratified profile, when the depth is not considered infinity, the Ekman transport tends to a direction smaller than 80 degrees. It differs from the classical Ekman model, which proposes an approximated angle equal to 90 degrees. Considering the modified model, it was also found that the surface current deviation is smaller than 40 degrees, which differs from the 45 degrees proposed by the classical model. In addition, it was determined that for ocean depths smaller than 180 m, the maximum velocity does not occur at the water surface, as in the classical model, but at deeper levels.Escuela Superior Politecnica del Litoral (ESPOL

Observational evidences of strain-induced periodic stratification (SIPS) in the Gualdalquivir Estuary

http://isms.gal/wp-content/uploads/2018/06/eof-libro-abstracts-definitivo.pdfThe G&M14 stratification-circulation diagram allows to classify estuaries on the basis of river flow and tidal forcing in terms of the freshwater Froude number and a mixing parameter that accounts for the vertical structure of the water column [Geyer & MacCready, 2014]. In this study, instead of computing these parameters for the whole estuary as was originally proposed by these authors, both are computed locally, at different stretches along the Guadalquivir estuary. The analysis is based on the data collected at 21 stations in a 3 year continuous monitoring campaign (2008–2011), during which the estuary experienced a wide range of environmental conditions [Navarro et al., 2011]. Remarkably, the results reveal that conditions in the inner stretches fall most of the time in the Strain-Induced Periodic Stratification (SIPS) regime mapped in the G&M14 diagram [Simpson et al., 1990]. Near the mouth, the estuary is mostly partially-mixed. Freshwater discharges may shift the state of the lower stretches toward a salt-wedge structure whose location depends strongly on the fresh water volume discharged. Estimates of the Simpson number during the same period confirm this picture. The SIPS regime implies that the covariance between eddy viscosity and vertical shear of the longitudinal current drives part of the subtidal circulation, even more efficiently than the classical gravitational circulation does [Burchard & Hetland, 2010]. This may have a significant impact on the estuarine suspended matter distribution. Results further point out the limitations of estuarine classification of estuarine systems as a whole on the basis of stratification-circulation diagrams. Even spatially close stretches may transit different regions of the G&M14 diagram. A specific stretch of an estuary may change its structure and its state in the stratification-circulation diagram may undergo large excursions under changing environmental conditions

Phase separation of a driven granular gas in annular geometry

This work investigates phase separation of a monodisperse gas of inelastically colliding hard disks confined in a two-dimensional annulus, the inner circle of which represents a "thermal wall". When described by granular hydrodynamic equations, the basic steady state of this system is an azimuthally symmetric state of increased particle density at the exterior circle of the annulus. When the inelastic energy loss is sufficiently large, hydrodynamics predicts spontaneous symmetry breaking of the annular state, analogous to the van der Waals-like phase separation phenomenon previously found in a driven granular gas in rectangular geometry. At a fixed aspect ratio of the annulus, the phase separation involves a "spinodal interval" of particle area fractions, where the gas has negative compressibility in the azimuthal direction. The heat conduction in the azimuthal direction tends to suppress the instability, as corroborated by a marginal stability analysis of the basic steady state with respect to small perturbations. To test and complement our theoretical predictions we performed event-driven molecular dynamics (MD) simulations of this system. We clearly identify the transition to phase separated states in the MD simulations, despite large fluctuations present, by measuring the probability distribution of the amplitude of the fundamental Fourier mode of the azimuthal spectrum of the particle density. We find that the instability region, predicted from hydrodynamics, is always located within the phase separation region observed in the MD simulations. This implies the presence of a binodal (coexistence) region, where the annular state is metastable. The phase separation persists when the driving and elastic walls are interchanged, and also when the elastic wall is replaced by weakly inelastic one.Comment: 9 pages, 10 figures, to be published in PR

A Subtidal Box Model based on the Longitudinal Anomaly of Potential Energy for Narrow Estuaries. An Application to the Guadalquivir River Estuary (SW Spain).

The objective of the present study is to demonstrate the informative capacity of the longitudinal anomaly of potential energy (LAPE) in the analysis of the magnitude and spatiotemporal variability of estuarine processes. For this purpose, a LAPE balance equation is formulated. The LAPE integrates and varies with the vertical and longitudinal density distribution. The formulation is applied on a subtidal scale to each box or stretch of the Guadalquivir River estuary, a narrow, highly turbid, weakly stratified, and strongly anthropized estuary. Data recorded by a large network of monitoring stations in 2008 and 2009 are used to quantify advective transports as well as the transports associated with longitudinal dispersion and vertical turbulent mixing in different hydraulic regimes. In low-river flow conditions, (river flows Q < 40m3s−1), the magnitude of LAPE transports decreases upstream and varies locally, depending on neap-spring tidal cycles. The direction of the net LAPE transport creates convergence zones that are particularly consistent with maximum levels of estuarine turbidity. During high-river flows (Q > 400m3s−1), this convergence disappears and the maximum longitudinal density gradient moves towards the mouth. More specifically, tidal pumping -induced LAPE governs during these conditions and manages to compensate the sum of the mean nontidal and dispersive and differential advective LAPE transports. However, during the postriverflood period, the mechanisms controlling recovery downstream from the mouth are the longitudinal dispersive and differential advective LAPE transports. Furthermore, the convergence zone reappears with a longitudinal gradient of the net LAPE transport that is even greater than in low-river flow conditions.This research was partially funded by the Campus de Excelencia Internacional del Mar (Cei-MAR) and the Spanish Ministry of Economy and Competitiveness, Project PIRATES (CTM2017-89531-R). It was also supported by AQUACLEW. Project AQUACLEW is part of ERA4CS, an ERA- NET initiated by JPI Climate, and funded by FORMAS (SE), DLR (DE), BMWFW (AT), IFD (DK), MINECO (ES), ANR (FR) with co-funding by the European Commission.We would like to thank two anonymous reviewers for the thoughtful contribution that has signicantly improved the quality of the paper. Datasets for this research are available in http://doi.org/10.5281/ zenodo.3459610. This study is a tribute to the memory of RichardW. Garvine, whose research was the inspiration for our work

Non-equilibrium Anisotropic Phases, Nucleation and Critical Behavior in a Driven Lennard-Jones Fluid

We describe short-time kinetic and steady-state properties of the non--equilibrium phases, namely, solid, liquid and gas anisotropic phases in a driven Lennard-Jones fluid. This is a computationally-convenient two-dimensional model which exhibits a net current and striped structures at low temperature, thus resembling many situations in nature. We here focus on both critical behavior and details of the nucleation process. In spite of the anisotropy of the late--time spinodal decomposition process, earlier nucleation seems to proceed by Smoluchowski coagulation and Ostwald ripening, which are known to account for nucleation in equilibrium, isotropic lattice systems and actual fluids. On the other hand, a detailed analysis of the system critical behavior rises some intriguing questions on the role of symmetries; this concerns the computer and field-theoretical modeling of non-equilibrium fluids.Comment: 7 pages, 9 ps figures, to appear in PR

German to Spanish translation of Einstein’s work on the formation of meanders in rivers

In 1926 Albert Einstein gave a clear explanation of the physical processes involved in the meander formation and evolution in open channels (Einstein, 1926). Although this work is far from being recognized as one of his greatest achievements, such as his annus mirabilis papers in 1905, he shows a truly remarkable didactic skills that make it easy to understand even to the non- specialist. In particular, a brilliant explanation of the tea leaf paradox can be found in this paper of 1926, presented as a simple experiment for clarifying the role of Earth rotation and flow curvature in the differential river banks erosion. This work deserves to be considered as a pioneering work that has laid a basic knowledge in currently very active research fields in fluvial geomorphology, estuarine physics, and hydraulic engineering. In response to the curiosity aroused and transmitted to the authors over the years by undergraduates and MSc. students, and also due to its historical and scientific significance, we present here the Spanish translation of Einstein’s original work published in German in 1926 in Die Naturwissenschaften (Einstein, 1926). Einstein’s drawings have not been interpreted, but just updated preserving their original spirit

Bridge-piling modifications on tidal flows in an estuary

This paper investigates the impact of the bridge-pilling modifications on the tidal flow at the strait of a human-altered mesotidal estuary (Cádiz Bay, Spain). The analysis was accomplished by combining (1) field data of current velocities in a cross-section close to the pile of a recently built bridge, and (2) hydrodynamics results from the calibrated and tested DELFT3D numerical model on the study site. The analysis was focused on bridge piling effects at intratidal scale during neap and spring tides. Semidiurnal (D2), quarti-diurnal (D4) and sixth-diurnal (D6) bands were examined. The ratio of quarter-diurnal to semidiurnal amplitudes (D4/D2) is largest (¿1) near the bottom of the channel due to friction, and the ratio D6/D2 is largest (¿1.5) close to the pile and in the shallowest part of the cross-section, due to internal asymmetry and tidal advective accelerations. Comparing neap and spring tides, the most marked asymmetries are found in the latter. These asymmetries occur around the wake of the pile, alternating from flood to ebb. The location of the bridge pile causes a decrease in the amplitude of D2 (50%), and a phase lag in the tidal wave of 30 min. The D6 amplitude increases up to almost 100% due to distortion of the tidal wave induce by the wake created at the pile. Moreover, the pile reduces about 1 m the depth where the pycnocline is observed. Additionally, the pile can be considered as an obstacle for the sediment flux. These effects can be extrapolated to similar infrastructures, such as wind or wave energy farms, that can potentially modify the estuarine dynamics.Ministerio de Economía y Competitividad CTM2017-89531-RJunta de Andalucía - Transformación Económica, Industria, Conocimiento y Universidades A-TEP-88-UGR2

Estuarine Salinity Response to Freshwater Pulses

Freshwater pulses (during which river discharge is much higher than average) occur in many estuaries and strongly impact estuarine functioning. To gain insight into the estuarine salinity response to freshwater pulses, an idealized model is presented. With respect to earlier models on the spatiotemporal behavior of salinity in estuaries, it includes additional processes that provide a more detailed vertical structure of salinity. Simulation of an observed salinity response to a freshwater pulse in the Guadalquivir Estuary (Spain) shows that this is important to adequately simulate the salinity structure. The model is used to determine the dependency of the estuarine salinity response to freshwater pulses for different background discharge, tides, and different intensities and durations of the pulses. Results indicate that the change in salt intrusion length due to a freshwater pulse is proportional to the ratio between peak and background river discharge and depends linearly on the duration of the pulse if there is no equilibration during the pulse. The adjustment time, which is the time it takes for the estuary to reach equilibrium after an increase in river discharge, scales with the ratio of the change in salt intrusion length and the peak river discharge. The recovery time, that is, the time it takes for the estuary to reach equilibrium after a decrease in river discharge, does not depend on the amount of decrease in salt intrusion length caused by the pulse. The strength of the tides is of minor importance to the salt dynamics during and after the pulse

Distribución espacial de salinidad y turbidez en el estuario del Guadalquivir

En esta contribución se analiza la variabilidad espacio-temporal a escala promediomareal de salinidad y la turbidez en el estuario del Guadalquivir. Se han determinado los flujos de concentración de sal y sólidos en suspensión a lo largo de todo el estuario