OS24B-01 INVITED
The far-reaching back-effect of coastal tides upon open-ocean tides
As part of a study into various factors that impact the amplitudes of semidiurnal tides in the open and coastal ocean, we investigate the back-effect of coastal tides upon open-ocean tides. We use two tools for our study--a forward numerical model of the global ocean tides, and analytical solutions for a damped/driven non-rotating one-dimensional deep ocean basin of uniform depth coupled to a shelf of lesser uniform depth. The analytical model suggests that the presence of a resonant shelf can significantly alter the amplitudes of tides in the deep ocean, as shown by comparison of the coupled model to a simpler model in which the deep ocean basin stands by itself. Consistent with the analytical results, we find that if regions of known large coastal tides are blocked off, there is a very significant and far-reaching back-effect on the tides in a forward global numerical model. For instance, blocking off Hudson Strait leads to a global M2 tide with an rms amplitude 10 percent larger than is seen without the blocking. The disruptions in amplitude (and in phase) obtained by blocking Hudson Strait are largest near the Strait itself, but significant disruptions are seen as far away as the Indian Ocean. Blocking out the Northwest European Shelf, Gulf of Maine, Patagonian Shelf, East China Sea, or Northwest Australian Shelf also leads to significant and far-reaching changes to the global ocean tide. The conclusion is that shelf tides are not only forced by the deep ocean tide, but also have a substantial back effect on it. Implications for tides during the ice age, when sea levels were much lower and shelves were less extensive, will be briefly discussed.
OS24B-02 INVITED
Limits to Tidal Power
Ocean tides have been proposed as a source of renewable energy, though the maximum available power may be shown to be only a fraction of the present dissipation rate of 3.5 TW, which is small compared with global insolation (nearly 105 TW), wind dissipation (103 TW), and even human power usage of 15 TW. Nonetheless, tidal power could be a useful contributor in some locations. Traditional use of tidal power, involving the trapping of water behind a barrage at high tide, can produce an average power proportional to the area of the headpond and the square of the tidal range; the power density is approximately 6 W per square meter for a tidal range of 10 m. Capital costs and fears of environmental damage have put barrage schemes in disfavor, with interest turning to the exploitation of strong tidal currents, using turbines in a manner similar to wind turbines. There is a limit to the available power, however, as adding turbines reduces the flow, ultimately reducing the power. For sinusoidal forcing of flow in a channel connecting two large open basins, the maximum available power may be shown to be given approximately by 0.2ρ g a Q_max, where ρ is the water density, g gravity, a the amplitude of the tidal sea level difference along the channel, and Q_max is the maximum volume flux in the natural state. The same formula applies if the channel is the entrance to a semi-enclosed basin, with a now the amplitude of the external tide. A flow reduction of approximately 40% is typically associated with the maximum power extraction. The power would be reduced if only smaller environmental changes are acceptable, and reduced further by drag on supporting structures, dissipation in turbine wakes, and internal inefficiencies. It can be suggested that the best use of strong, cold, tidal currents is to provide cooling water for nuclear reactors.
OS24B-03
Dissecting the Pressure Field in Tidal Flow Past a Headland: When is Form Drag Real?
In previous measurements of form drag in the ocean, drag that is much larger than a typical bluff body drag estimate have been consistently found. In this work, theory combined with a numerical model of tidal flow around a headland in a channel gives insight into the mechanisms that create form drag in oscillating flow situations. The total form drag is divided into two parts: the inertial drag which is derived from the potential flow solution, and the separation drag which accounts for flow features such as eddies. The inertial drag can have a large magnitude, yet it cannot do work on the flow because its phase is in quadrature with the velocity. The separation drag has a magnitude equal to the bluff body drag and accounts for all of energy removed from the flow by the topography. This theory not only explains why form drag can be so large in the ocean, it also provides a method for separating the pressure field into the parts that can and cannot extract energy from the flow. In addition, the dependence of the form drag on the tidal excursion distance and the slope aspect ratio of the headlands was determined with a series of numerical experiments.
OS24B-04
Secular Changes in the Tide of the Gulf of Maine
For the past century the amplitude of the principal semidiurnal lunar tide in the Gulf of Maine has been dramatically increasing. For example, at Eastport, Maine, the trend in M2 amplitude is 13 cm/century, comparable to the rise in mean sea level. At the same time the solar tide S2 has been decreasing. These tidal changes, which occur throughout the entire Gulf, presumably reflect changes in basin configuration--- geometry, depth, or both. Tidal models that account for Holocene sea level rise do predict an amplification of M2, but much smaller than the currently observed trends. An increasing M2 and decreasing S2 could reflect a shift of the gulf's dominant resonant frequency toward M2 and away from S2. This seems unlikely, however, because Garrett and others have shown that the primary resonance of the Gulf of Maine lies near or below the frequency of N2, so any shift should affect M2 and S2 similarly. Preliminary analysis suggests a small perturbation in the Q of the gulf tide, thus pointing to dissipation rather the frequency-shifting as the cause. In addition, the changes in S2 appear to be induced by a far more widespread decrease in S2 amplitudes throughout much of the northeast Atlantic Ocean, extending at least as far as Bermuda. The cause of this is not understood, but radiational effects in S2 can be ruled out because similar changes occur in K2. We review the evidence---primarily from long tide-gauge time series---for these changes in tidal "constants" and the status of our still-incomplete theories of causative mechanisms.
OS24B-05 INVITED
Tidal Processes in the Columbia River Estuary and Plume
The Columbia River plume is strongly-influenced by the barotropic
tide, which (1) drives turbulence in the estuary and (2) sets
the strength and timing of each pulse of freshwater in the nearfield
plume. Together, these control turbulent entrainment and determine the
composition and structure of the resultant plume. Here we examine the
structure of turbulent mixing in the Columbia River estuary/plume
system during two contrasting periods of freshwater input Qf.
Within the estuary, intense turbulence observed on flood and ebb is
controlled by the bottom stress, which scales with tidally-dominated
near-bottom u3 and controls the mixing. In the nearfield plume,
the plume detaches from the bottom and mixing is driven by interfacial
stresses.
The salinity of the offshore Columbia River plume should be controlled
by the ratio of horizontal advection to turbulent mixing. This ratio
depends on the magnitude of freshwater river input as compared to the
integrated entraiment, which is dominated by tidal processes. Using
17 tidally-resolving offshore surveys during spring/neap tides and
low/high river flows, we show how the median salinity in the nearfield
oceanic plumes can be summarized in terms of the estuary Richardson
number RiE.
http://mixing.oregonstate.edu
OS24B-06
Laboratory Experiments Simulating the Effects of Variable Discharge on Buoyant Coastal Plumes
River plumes are of great importance to coastal ecosystems because they carry nutrients and contaminants from upstream, which can become trapped near the coast in a growing anticyclonic eddy, or bulge. The degree to which river water is trapped in this coastal eddy is associated strongly with the river discharge. In meso- to macro-tidal systems, ebb and flood tidal phases may result in increases and decreases, respectively, of the effective river discharge of a similar magnitude to the discharge itself. Thus, accumulation of fluid in the bulge may depend on the relative magnitude of the tidal forcing or other modulations of the river discharge. Field observations suggest that under some conditions, discharge variation can cause the anticyclonic eddy to become detached and swept downstream, rather than continuing to grow near the mouth. We carry out laboratory experiments to simulate the effects of periodically varying discharge on buoyant coastal plumes over a range of oscillation periods by injecting a sinusoidally pulsed freshwater inflow into a 2 meter diameter rotating tank of salt water. The depth of the plume in the vicinity of the river mouth is determined from an overhead camera using an optical thickness technique. Using this technique, the temporal evolution of the plume volume can be determined directly. Preliminary results confirm that approximately 65% of the discharge remains in the bulge region in the absence of tidal forcing. In the presence of tidal forcing plume growth appears to be slowed, thereby increasing the transport of buoyant water downstream in the coastal current. Finally, the plume is almost entirely arrested when the pulsing frequency is half of the rotation frequency.
OS24B-07
High efficiency of tide-induced mixing in estuaries
Estuaries convert tidal kinetic energy to potential energy by mixing fresh and salt water. Overall mixing efficiency can be expressed by the ratio of volume-integrated buoyancy flux to turbulent kinetic energy production, yielding an overall flux Richardson number Rfo. This quantity was estimated in two estuaries, the partially mixed Hudson River and the highly stratified Merrimack, based on control-volume estimates using velocity, elevation and salinity data, as well as direct integration of the buoyancy flux and kinetic-energy production from verified numerical simulations. The efficiency ranged from 0.025 for low-discharge, partially mixed conditions up to 0.12 for high-discharge, salt-wedge conditions. These values are well above the Simpson-Hunter value of ~ 0.003 for shelf seas, and they approach the Osborn (1980) value of 0.15 for the high-stratification regime. The increasing efficiency with higher stratification is a consequence of the increasing role of shear instability relative to boundary mixing. The efficiency in the partially mixed regime is limited by the restratification of the bottom boundary layer, in which longitudinal and lateral straining of the salinity are both important. Osborn, TR (1980). J. Phys. Oceanography, 10, 83-89.
OS24B-08
The subtidal momentum balance in a periodically stratified estuary: The dominance of tidal timescale processes
In the analysis of estuarine dynamics and transport, a tidally-averaged parameterization is often pursued. This approach frequently relies on defining a relationship between the tidally-averaged turbulent stresses and the tidally-averaged shear. With such a relationship, the tidally-averaged (subtidal) momentum balance is closed, and can be solved directly to predict the evolution of the estuary over long timescales. In this talk, we examine the role of tidal timescale processes in the subtidal momentum balance using a combination of observational and numerical analysis. Observations of turbulent stresses and mean velocities over an entire spring-neap cycle are used to evaluate the dynamics of tidally-averaged flows in a partially stratified estuarine channel. The subtidal stress divergence is seen to balance the tidally-averaged pressure gradient in the lower water column, but in the upper water column the tidal stresses are important contributors. The contribution of tidal stresses is seen to vary strongly on the spring-neap timescale, which much larger contribution during neaps than during springs. The use of an eddy viscosity in the subtidal momentum balance is then explored, but it is found that the subtidal momentum balance would require a negative eddy viscosity. This is due to the dominant contribution of tidally-varying turbulent momentum fluxes, which have no specific relation to the subtidal circulation, but through their asymmetries define the subtidal circulation. Using a numerical water column model, the validity of a subtidal eddy viscosity is explored; it is found to be an appropriate approach only for large values of the horizontal Richardson number.