Stephen G. Monismith

Measurements of Reynolds stress profiles in unstratified tidal flow

In this paper we present a method for measuring profiles of turbulence quantities using a broadband acoustic doppler current profiler (ADCP). The method follows previous work on the continental shelf and extends the analysis to develop estimates of the errors associated with the estimation methods. ADCP data was collected in an unstratified channel and the results of the analysis are compared to theory. This comparison shows that the method provides an estimate of the Reynolds stresses, which is unbiased by Doppler noise, and an estimate of the turbulent kinetic energy (TKE) which is biased by an amount proportional to the Doppler noise. The noise in each of these quantities as well as the bias in the TKE match well with the theoretical values produced by the error analysis. The quantification of profiles of Reynolds stresses simultaneous with the measurement of mean velocity profiles allows for extensive analysis of the turbulence of the flow. In this paper, we examine the relation between the turbulence and the mean flow through the calculation of u * , the friction velocity, and C d , the coefficient of drag. Finally, we calculate quantities of particular interest in turbulence modeling and analysis, the characteristic lengthscales, including a lengthscale which represents the stream-wise scale of the eddies which dominate the Reynolds stresses.

Structure and Flow-Induced Variability of the Subtidal Salinity Field in Northern San Francisco Bay

Abstract The structure of the salinity field in northern San Francisco Bay and how it is affected by freshwater flow are discussed. Two datasets are examined: the first is 23 years of daily salinity data taken by the U.S. Bureau of Reclamation along the axis of northern San Francisco Bay; the second is a set of salinity transects taken by the U.S. Geological Survey between 1988 and 1993. Central to this paper is a measure of salinity intrusion, X2: the distance from the Golden Gate Bridge to where the bottom salinity is 2 psu. Using X2 to scale distance, the authors find that for most flow conditions, the mean salinity distribution of the estuary is nearly self-similar with a salinity gradient in the center 70% of the region between the Golden Gate and X2 that is proportional to X−12. Analysis of covariability of Q and X2 showed a characteristic timescale of adjustment of the salinity field of approximately 2 weeks. The steady-state response deduced from the X2 time series implies that X2 is proportional ...

Observations of Turbulence in a Partially Stratified Estuary

The authors present a field study of estuarine turbulence in which profiles of Reynolds stresses were directly measured using an ADCP throughout a 25-h tidal day. The dataset that is discussed quantifies turbulent mixing for a water column in northern San Francisco Bay that experiences a sequence of states that includes a weak ebb and flood that are stratified, followed by a strong, and eventually unstratified, ebb and flood. These measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification. Examination of individual Reynolds stress profiles along with profiles of Richardson number and turbulent Froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects. Using the measured turbulence production rate P, the dissipation rate e is estimated. The observed turbulence had values of e/nN 2 . 20 all of the time and e/nN 2 . 200 most of the time, suggesting that the observed motions were buoyancy affected turbulence rather than internal waves. However, at times, turbulent Froude numbers in much of the upper-water column were less than one, indicating important stratification effects. Taken as a whole, the data show that stratification affects the turbulent velocity variance q2 most severely; that is, observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes. Finally, the dataset is compared to predictions made using the popular Mellor‐Yamada level 2.5 closure. These comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions. The length scale does not appear to compensate for these errors, and, as a result, similar errors are seen in the eddy viscosity predictions. It is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self-transport from the near-bed region to the overlying stratification.

Creation of residual flows in a partially stratified estuary

The creation of residual flows in estuaries is examined using acoustic Doppler current profiler data sets from northern San Francisco Bay. The data sets are analyzed using principal component analysis to examine the temporal variability of the flows which create the residual circulation. It is seen that in this periodically and partially stratified estuary the residual flows are created through a series of pulses with strong variability at the 24-hour timescale, through the interaction of shear, stratification and mixing. This interaction is captured through the use of a dimensionless number, the horizontal Richardson number (Rix), which is developed to examine the local balance between the stratifying and destratifying forces at the tidal timescale. It is seen that Rix is a valuable parameter in predicting the onset of the residual-creating events, with a threshold value of ≈3 on ebb tides. This critical value is argued to be a threshold, above which the stratification and shear flow create a feedback effect, each further intensifying the other. This feedback results in a highly variable exchange flow which creates the estuarine residual in intermittent pulses rather than as a steady flow. Although typically attributed to baroclinic forcing, an argument is made that these pulses of residual-creating exchange flow could be created by barotropic forcing in the presence of variable stratification which is asymmetric between flood and ebb tides. This result poses a great challenge for turbulence modeling, as the timing and magnitude of stratification and shear must be correctly simulated on the tidal timescale in order to reproduce the effects seen in the data sets presented.

An experimental study of the upwelling response of stratified reservoirs to surface shear stress

An experimental study of the dynamic response of non-rotating, stratified reservoirs to the application of surface shear stresses is presented. The experiments were made using two-layered and linear stratifications; a moving belt was used to supply a shear stress to the fluid. Detailed measurements of the density field show that upwelling occurs at all values of the Wedderburn number, W, rather than only occurring when W < 1. Differences between the two-layer theory and the present observations are attributed to the fact that the experimental stratifications were continuous, rather than layered. Shearflow dispersion is observed to be an important mechanism for distributing the effects of localized upwelling over the entire length of the mixed layer. A model of mixed-layer deepening based on upwelling and shearflow dispersion is presented and is compared to the observations of this and other experimental studies.

Wave-Driven Circulation of a Coastal Reef–Lagoon System

Abstract The response of the circulation of a coral reef system in Kaneohe Bay, Hawaii, to incident wave forcing was investigated using field data collected during a 10-month experiment. Results from the study revealed that wave forcing was the dominant mechanism driving the circulation over much of Kaneohe Bay. As predicted theoretically, wave setup generated near the reef crest resulting from wave breaking established a pressure gradient that drove flow over the reef and out of the two reef channels. Maximum reef setup was found to be roughly proportional to the offshore wave energy flux above a threshold root-mean-square wave height of 0.7 m (at which height setup was negligible). On the reef flat, the wave-driven currents increased approximately linearly with incident wave height; however, the magnitude of these currents was relatively weak (typically <20 cm s−1) because of (i) the mild fore-reef slope of Kaneohe Bay that reduced setup resulting from a combination of frictional wave damping and its re...

Does the Sverdrup critical depth model explain bloom dynamics in estuaries

In this paper we use numerical models of coupled biological-hydrodynamic processes to search for general principles of bloom regulation in estuarine waters. We address three questions: What are the dynamics of stratie cation in coastal systems as ine uenced by variable freshwater input and tidal stirring? How does phytoplankton growth respond to these dynamics? Can the classical Sverdrup Critical Depth Model (SCDM) be used to predict the timing of bloom events in shallow coastal domains such as estuaries? We present results of simulation experiments which assume that vertical transport and net phytoplankton growth rates are horizontally homogeneous. In the present approach the temporally and spatially varying turbulent diffusivities for various stratie cation scenarios are calculated using a hydrodynamic code that includes the Mellor-Yamada 2.5 turbulence closure model. These diffusivities are then used in a time- and depth-dependent advection-diffusion equation, incorporating sources and sinks, for the phytoplankton biomass. Our modeling results show that, whereas persistent stratie cation greatly increases the probability of a bloom, semidiurnal periodic stratie cation does not increase the likelihood of a phytoplankton bloom over that of a constantly unstratie ed water column. Thus, for phytoplankton blooms, the physical regime of periodic stratie cation is closer to complete mixing than to persistent stratie cation. Furthermore, the details of persistent stratie cation are important: surface layer depth, thickness of the pycnocline, vertical density difference, and tidal current speed all weigh heavily in producing conditions which promote the onset of phytoplankton blooms. Our model results for shallow tidal systems do not conform to the classical concepts of stratie cation and blooms in deep pelagic systems. First, earlier studies (Riley, 1942, for example) suggest a monotonic increase in surface layer production as the surface layer shallows. Our model results suggest, however, a nonmonotonic relationship between phytoplankton population growth and surface layer depth, which results from a balance between several ‘ ‘ competing’ ’ processes, including the interaction of sinking with turbulent mixing and average net growth occurring within the surface layer. Second, we show that the traditional SCDM must be ree ned for application to energetic shallow systems or for systems in which surface layer mixing is not strong enough to counteract the sinking loss of phytoplankton. This need for ree nement arises because of the leakage of phytoplankton from the surface layer by turbulent diffusion and sinking, processes not considered in the classical SCDM. Our model shows that, even for low sinking rates and small turbulent

A short-term in situ CO 2 enrichment experiment on Heron Island (GBR)

Ocean acidification poses multiple challenges for coral reefs on molecular to ecological scales, yet previous experimental studies of the impact of projected CO2 concentrations have mostly been done in aquarium systems with corals removed from their natural ecosystem and placed under artificial light and seawater conditions. The Coral–Proto Free Ocean Carbon Enrichment System (CP-FOCE) uses a network of sensors to monitor conditions within each flume and maintain experimental pH as an offset from environmental pH using feedback control on the injection of low pH seawater. Carbonate chemistry conditions maintained in the −0.06 and −0.22 pH offset treatments were significantly different than environmental conditions. The results from this short-term experiment suggest that the CP-FOCE is an important new experimental system to study in situ impacts of ocean acidification on coral reef ecosystems.

Laboratory experiments simulating a coastal river inflow

The dynamics of buoyant water entering a rotating basin are studied using a series of laboratory experiments designed to elucidate the alongshore transport mechanisms in river plumes. Inflowing water, which is discharged perpendicular to the tank wall, is observed to form a growing anticyclonic bulge and a coastal current downstream of the bulge. Detailed simultaneous measurements of the velocity and buoyancy fields in the plume confirm that the bulge momentum is in a gradient–wind balance and the coastal current is geostrophic. The growth of the bulge and accumulation of fluid within it coincides with a reduction in coastal current transport to approximately 50 % of the inflow discharge. The bulge is characterized by a depth scale, h ,w hich is proportional to the geostrophic depth, hg, and two time-dependent horizontal length scales, yc, the displacement of the bulge centre from the wall, and rb, the effective radius of the bulge. These two length scales are proportional to the inertial radius, � —‐

Thermally Driven Exchanges between a Coral Reef and the Adjoining Ocean

Abstract In this paper hydrographic observations made over a fringing coral reef at the northern end of the Gulf of Aqaba near Eilat, Israel, are discussed. These data show exchange flows driven by the onshore–offshore temperature gradients that develop because shallow regions near shore experience larger temperature changes than do deeper regions offshore when subjected to the same rate of heating or cooling. Under heating conditions, the resulting vertically sheared exchange flow is offshore at the surface and onshore at depth, whereas when cooling dominates, the pattern is reversed. For summer conditions, heating and cooling are both important and a diurnally reversing exchange flow is observed. During winter conditions, heating occupies a relatively small fraction of the day, and only the cooling flow is observed. When scaled by ΔV, the observed profiles of the cross-shore during cooling velocity collapse onto a single curve. The value of ΔV depends on the convective velocity scale uf and the bottom s...