Patrick F. Cummins

The power potential of tidal currents in channels

Interest in sources of renewable energy has led to increasing attention being paid to the potential of strong tidal currents. There is a limit to the available power, however, as too many turbines will merely block the flow, reducing the power generated. The maximum average power available from a tidal stream along a channel, such as that between an island and the mainland, is estimated and found to be typically considerably less than the average kinetic energy flux in the undisturbed state through the most constricted cross-section of the channel. A general formula gives the maximum average power as between 20 and 24% of the peak tidal pressure head, from one end of the channel to the other, times the peak of the undisturbed mass flux through the channel. This maximum average power is independent of the location of the turbine ‘fences’ along the channel. The results may also be used to evaluate the power potential of steady ocean currents.

The efficiency of a turbine in a tidal channel

There is an upper bound to the amount of power that can be generated by turbines in tidal channels as too many turbines merely block the flow. One condition for achievement of the upper bound is that the turbines are deployed uniformly across the channel, with all the flow through them, but this may interfere with other uses of the channel. An isolated turbine is more effective in a channel than in an unbounded flow, but the current downstream is non-uniform between the wake of the turbines and the free stream. Hence some energy is lost when these streams merge, as may occur in a long channel. We show here, for ideal turbine models, that the fractional power loss increases from 1/3 to 2/3 as the fraction of the channel cross-section spanned by the turbines increases from 0 to close to 1. In another scenario, possibly appropriate for a short channel, the speed of the free stream outside the turbine wake is controlled by separation at the channel exit. In this case, the maximum power obtainable is slightly less than proportional to the fraction of the channel cross-section occupied by turbines.

North Pacific internal tides from the Aleutian Ridge: Altimeter observations and modeling

Internal tides radiating into the North Pacific from the Aleutian Ridge near Amukta Pass are examined using 7 years of Topex/Poseidon altimeter data. The observations show coherent southward phase propagation at the M 2 frequency over a distance of at least 1100 km into the central Pacific. Barotropic and baroclinic models are applied to study this internal tidal signal. Results from the barotropic model show that the strongest cross-slope volume and energy fluxes occur in the vicinity of Amukta Pass, helping to establish this region as an important site for baroclinic energy conversion along the eastern half of the ridge. A two-dimensional version of the Princeton Ocean Model is used to simulate internal tide generation and propagation. A comparison between the altimeter data south of the ridge and the sea-surface signature of the internal tide signal of the model shows good agreement for the phase, both close to the source and well into the far field. Comparison of the phase between model and data also provides evidence for wave refraction. This occurs due to the slow modulation of wavelength associated with the variation in the Coriolis parameter encountered as the internal tide propagates southward. The model results suggest that the net rate of conversion of barotropic to baroclinic energy is about 1.8 GW in the vicinity of Amukta Pass. This represents about 6% of the local barotropic energy flux across the ridge and perhaps 1% of global baroclinic conversion.

Sensitivity of the GFDL Ocean General Circulation Model to a Parameterization of Vertical Diffusion

Abstract A coarse resolution, primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability. A case with constant diffusivity is compared with a case in which the diffusivity is inversely proportional to the local Brunt-V frequency. The stability-dependent parameterization of vertical diffusivity yields a poleward heat flux similar to that of a small, constant diffusivity. However, this parameterization increases the mean temperature in the deep ocean by about 0.8°C and the strength of the meridional circulation by over 40%. In addition, the stability-dependent diffusivity is found to increase the stratification in the deep ocean. The experiments suggest that it may be possible to calibrate the rate of deep-water formation of general circulation models, without affecting the poleward heat transport, by varying the magnitude of the vertical diffusivity below the thermocline. The explicit...

Low-Frequency Pycnocline Depth Variability at Ocean Weather Station P in the Northeast Pacific

Abstract Low-frequency variability of the depth of the main pycnocline at Ocean Weather Station P and over the northeast Pacific is examined in terms of the one-dimensional response to local Ekman pumping according to the Hasselmann stochastic climate model. The model is forced with monthly wind stress curl anomalies derived from the National Centers for Environmental Prediction reanalysis for the period 1948–2000. An empirical orthogonal function analysis shows that the leading mode of the response bears the signature of the Pacific (inter) Decadal Oscillation (PDO) and that the associated principal component captures the “regime shift” of 1976/77. The correlation is 0.77 between annually averaged pycnocline displacement anomalies hindcast from the model and anomalies in the depth of the main pycnocline at station P (50°N, 215°E) observed over a 43-yr period. The comparison indicates that variability in the depth of the upper layer on interannual to interdecadal timescales at station P occurs largely as ...

Fortnightly modulation of the estuarine circulation in Juan de Fuca Strait

Riverine discharge into the Strait of Georgia sets up a well-defined estuarine circulation within Juan de Fuca Strait, the main path for the freshwater outflow to the continental shelf. At the landward end of Juan de Fuca Strait, the water flows through narrow channels in which strong tidal currents are known to induce significant mixing of the water column, and a spring-neap modulation of the estuarine exchange. A three-dimensional prognostic numerical model has been developed to study the circulation around Vancouver Island, British Columbia. In a series of simulations, the estuarine circulation within Juan de Fuca Strait is established by the Fraser River freshwater discharge. A fortnightly modulation is imposed on the mixing over the various sills to simulate the spring-neap tidal mixing regime. The resulting variation in the estuarine circulation is found to be largely limited to the eastern section of Juan de Fuca Strait, in the vicinity of the sills. Data from current meter moorings and surface salinity data from lighthouse stations compare favorably with the model results. The effect of local wind forcing on the estuarine exchange is also examined. The model is capable of simulating those rare events during which a concurrence of river freshet, neap tide and northwest wind allows a stronger pulse of fresh surface water to escape relatively unmixed into the eastern end of Juan de Fuca Strait. The disturbance then propagates along the northern shore of the strait as a first mode internal Kelvin wave. Finally, the effect of the fortnightly modulation on the export of freshwater onto the continental shelf is examined. It is found that small amplitude coastal trapped waves are generated near the mouth of Juan de Fuca. However, this fortnightly signal is weak in comparison to the energetic wind-induced variations typically found over the shelf.

A regional index of northeast Pacific variability based on satellite altimeter data

[1]xa0An index of climate variability to monitor the state of the upper ocean is proposed for the northeast (NE) Pacific Ocean based on sea surface height (SSH) data from satellite altimetry. While sea surface temperature (SST) is often used characterize ocean variability, SSH reflects the integrated influence of temperature and salinity anomalies through the water column. A canonical correlation analysis shows that SSH and SST anomalies vary coherently at large spatial scales and low frequencies over the region. SSH anomalies are less subject to short period variability and the temporal components for SSH resemble smoothed, low-pass-filtered versions of the SST components. Also examined is the relationship between the SST-based Pacific Decadal Oscillation (PDO) index and the large-scale, upper ocean variability reflected in the leading principal component of SSH anomalies. The comparison demonstrates that the SSH principal component provides a robust index of regional climate variability that is less noisy than the PDO. The results are used to examine the 1998–2002 climate event over the NE Pacific.

Stratification and Mean Flow Effects on Diurnal Tidal Currents off Vancouver Island

Abstract A series of numerical experiments with a three-dimensional, baroclinic model were conducted to study the influences of density stratification and wind-driven currents on the K1 tide over the continental shelf off Vancouver Island. The region is one of anomalously large diurnal tidal currents due to the generation of coastally trapped waves at the entrance to Juan de Fuca Strait. Model results are compared with data obtained from a number of moorings deployed over the shelf, including a line extending for over 300 km in the alongshore direction. The results show that inclusion of stratification significantly improves the representation of K1 currents in the model, particularly with respect to the alongshore phase propagation of the clockwise and counterclockwise rotary components. With homogeneous water, the coastally trapped waves propagate relatively slowly and are dissipated before reaching Brooks Peninsula. In contrast, the ambient stratification permits coastally trapped motions to propagate ...

Numerical Simulations of a Buoyancy-Driven Coastal Countercurrent off Vancouver Island

Abstract A three-dimensional prognostic numerical model has been developed to study the ocean circulation around Vancouver Island, British Columbia. In a series of simulations, the model is applied to examine the role of buoyancy forcing in the dynamics of the summer coastal countercurrent found off the west coast of Vancouver Island. The forcing is provided by the Fraser River discharge into the Strait of Georgia. An estuarine circulation establishes itself in Juan de Fuca Strait, from which a distinctive right-bounded current is formed and advances along the coast. Sensitivity studies are conducted to determine the robustness of this current to initial conditions, opposing wind, enhanced vertical mixing, and grid resolution. Finally, various characteristics of the numerically modeled coastal flow are compared with observations.

Observations and modeling of seasonal variability in the Straits of Georgia and Juan de Fuca

The Strait of Georgia is a large semi-enclosed basin on the southern coast of British Columbia. Its main connection to the Pacific is to the south, through Juan de Fuca Strait. Abundant freshwater discharge, mainly from the Fraser River, forces an estuarine exchange with oceanic shelf water. The resulting circulation is modulated by tides and winds. Both the coastal wind stress and the flux of freshwater are subject to strong seasonal modulations, producing a marked seasonal cycle in the water properties and circulation of the region. The seasonal variability of the Strait of Georgia and Juan de Fuca Strait is described using data from a series of recent cruises conducted over a five-year period, in addition to longer term historical data sets. To complement the observations, the data are compared with numerical simulations based on the Princeton Ocean Model (POM). Forced with tides, freshwater discharge and seasonal wind stress, the model is integrated over a few years until the system approaches statistical equilibrium. Results show good agreement with observations from Juan de Fuca Strait, as well as over the upper part of the water column within the Strait of Georgia. However, simulation of the seasonal cycle of the deeper waters of the Strait of Georgia is more problematic. The deep water properties apparently are determined by a delicate balance between dense intrusions from the sill area and local vertical mixing.