Haline stratification in the Rhine-Meuse freshwater plume: a three-dimensional model sensitivity analysis

The parameterization for transilient turbulence coefficients suggested by Stull and Driedonks (SD. 1987) is tested against the large-eddy simulations (LES) of Ebert et al. (ESS, 1989) for the special case of an idealized convective boundary layer. The SD parameterization is based on a nonlocal approximation to the turbulence kinetic energy (TKE) equation, and requires turbulent exchange (i.e., the matrix of transilient mixing coefficients is assumed to be symmetric) and dominance of the smaller eddies (i.e., elements closer to the main diagonal of the matrix are greater). Measurements from the LES model, however, show that the transilient matrix is asymmetric in convective situations, with larger eddies dominating. Mean-state conditions such as the deep convective mixing and mixed-layer growth are satisfactorily described by the parameterization, but the surface layer is too deep and the entrainment zone thickness is poorly defined. Turbulence properties such as skewed vertical velocity distributions are not possible within the constraint of a symmetric matrix, and partial convective overturning is also not possible because of the restriction that small eddies dominate. Future improved parametcrizations might continue to be based on the TKE equation, but should allow transilient matrix asymmetry.

Rotation and Vertical Mixing Effects on Volume Exchange in Eastern Long Island Sound

A physical formulation of the problem is considered. A mathematical model and a numerical algorithm of the turbulence model as part of the ocean circulation model for simulations for decades are formulated. The model is based on the evolution equations for turbulent kinetic energy (TKE) and the frequency of its viscous dissipation. A numerical solution algorithm for both the circulation model and the turbulence model is based on implicit schemes of splitting with respect to physical processes and geometric coordinates. For the turbulence model, this provided analytical solutions at a splitting step related to TKE generation and dissipation. Numerical experiments have been performed with a model of the joint circulation of the North Atlantic, the Arctic Ocean, and the Bering Sea to reproduce the annual cycle and synoptic disturbances of ocean characteristics. The model has a resolution of 0.25° in latitude and longitude and 40 levels in the vertical, which are compressed toward the surface to reproduce the process of developed turbulence better. The results are compared with observations and with the results of simulations using traditional parameterizations of the upper ocean mixing. It is shown that the model reproduces ocean characteristics correctly, only slightly increasing the computation time in comparison with simple parameterizations. Spatial and temporal characteristics of turbulence are analyzed.

The results of a numerical study of Delaware Bay using the Regional Ocean Modeling System (ROMS) are presented. The simulations are run over a range of steady river inputs and used M2 and S2 tidal components to capture the spring–neap variability. Results provide a description of the spatial and temporal structure of the estuarine exchange flow and the salinity field, as well the along-channel salt flux in the estuary. The along-channel salt flux is decomposed into an advective term associated with the river flow, a steady shear dispersion Fe associated with the estuarine exchange flow, and a tidal oscillatory salt flux Ft. Time series of Fe and Ft show that both are larger during neap tide than during spring. This time variability of Ft, which is contrary to existing scalings, is caused by the lateral flows that bring velocity and salinity out of quadrature and the stronger stratification during neap tide, which causes Ft to be enhanced relative to spring tide. A fit for the salt intrusion length L with river discharge Q for a number of isohalines is performed. The functional dependences of L with Q are significantly weaker than Q−1/3 scaling. It is concluded that the response of the salt field with river discharge is due to the dependence of Fe and Ft with Q and the relative importance of Ft to the total upstream salt flux: as river discharge increases, Fe becomes the dominant mechanism. Once Fe dominates, the salt field stiffens because of a reduction of the vertical eddy viscosity with increasing Q.

The intratidal behaviour of the vertical shear dispersion transport for salt over three cross-sections in the upper reaches of the Tamar Estuary is presented for spring and neap tides. A variable, ψ, is defined which is proportional to the vertical shear salt flux at each instant and is equal to the negative salt flux due to vertical shear divided by the tidally-averaged longitudinal salinity gradient. The tidal average of ψ can be compared with dispersion coefficients deduced from tidally-averaged or long-term data from other estuaries. Temporal variability in ψ is discussed in terms of the root-mean-square deviations (about the depth-mean values) of the vertical profiles of salinity and longitudinal current over each tidal cycle. During spring tides ψ maximizes on the ebb within 3 h of high water and, depending on the section, this maximum lies in the range of 100–310 m2/s (compared with 140–470 m2/s at neap tides), with large values occurring for 2–3 h periods during the ebb. At the end of the ebb and during the flood ψ is very small and can be negative. Similar behaviour is exhibited by ψ during neap tides at the cross-section nearest the head. However, at the most seaward cross-section the velocity shear, stratification and ψ maximize at low water rather than the early ebb. A one-dimensional model of the vertical current structure which uses prescribed salinity fields and which calculates ψ is able to reproduce this behaviour. The model is also used to provide insight into the effects of stratification and its ebb-flood asymmetry on the intratidal and tidally-averaged (residual) currents.

Temperature and salinity data from Long Island Sound (LIS) show that the pycnocline rises seaward of Mattituck sill and that the magnitude of the longitudinal density gradient over the area of the sill remains relatively constant throughout the year. This reflects the influence of internal hydraulic processes and vertical mixing on hydrography and volume exchange. Records of moored instruments indicate intratidal and fortnightly variations of water column stratification and current velocity in eastern LIS. During spring tides, thick bottom mixed layers reveal vertically homogeneous temporal changes in temperature, salinity, and sigma‐t, which arise from the oscillatory advection of a longitudinal density gradient. Intratidal variations translate into barotropic rectification of the residual flow and vertical homogeneity of low‐passed temperature, salinity, and sigma‐t. During neap tides the flow may be subject to hydraulic control. Near‐surface temperature and salinity oscillations of higher amplitude than near‐bottom fluctuations are a reflection of vertical excursions of the halocline. The halocline is raised in flood periods and depressed in ebb periods. Intratidal stratification changes reflect the corresponding low‐passed temperature, salinity, and sigma‐t behavior with depth. Near‐bottom residual flow favors salt intrusion as it accelerates down the salinity gradient due to the adjustment of the longitudinal density gradient established on spring tides. The residual density and flow fields reflect a change in dynamical balances from spring periods, dominated by frictional stresses, to neap periods, when inertial and rotation effects increase.

Frontiers Events is a rapidly growing calendar management system dedicated to the scheduling of academic events. This includes announcements and invitations, participant listings and search functionality, abstract handling and publication, related events and post-event exchanges. Whether an organizer or participant, make your event a Frontiers Event!

The long-term salinity field in San Francisco Bay

The feeding ecology of wintering Little Grebes was studied on a brackish tidal river in central West Wales during the winters of 1981/2–1983/4. Peak numbers occurred in late November and mid-February. Increased wind strength reduced the likelihood of birds being present in the study area. The frequency of their occurrence was highest at low and high tide, when flood conditions made shallow water (< 2 m) available for feeding. Little Grebes foraged for 85–¸89% of daylight hours, and evidence is provided to suggest that feeding only occurs during the day at this site. Inter-dive recovery time increased exponentially with dive duration, suggesting increased energetic costs of prolonged dives. Dive time increased with water depth, hence feeding at greater depths is probably disproportionately more costly than in shallow water. Since feeding success (based on prey handling on the water surface) also declined with depth of the water column, it would appear that the observed patterns of use of shallow water in t...

Ocean circulations closely associated with the changes of wind system are investigated using a three-dimensional tide-circulated coupling model. Based on the coupling model, we apply different wind datasets, to some extent reflecting wind system variations associated with global climate change, but constant monthly temperature and salinity fields to assess the effect of differing wind strength stresses in the East China Sea region, leading to significant influences on hydrographic fields. In the model based on the Hellman-Rosenstein wind dataset, more distant from the mouth of the Changjiang River (Yangtze River), the Yellow Sea Warm Current strengthens because of the wind stress; weaker wind stresses may reduce the strength of the Yellow Sea Warm Current, resulting in it retreating along its path. The modeled results suggest that there is at least a 1.5°C anomaly in temperature and a 2 psu anomaly in salinity between the Changjiang River mouth and the Korean Peninsula when different wind fields are imposed in the model. The major differences in the sea surface temperature (SST) and the sea surface salinity (SSS) fields usually occur between latitudes 30°N and 35°N, which coincides with the regions having the steepest north-south temperature gradient.