William R. Crawford

The Vertical Structure of Turbulent Dissipation in Shelf Seas

Abstract The free-fall FLY profiler has been used to determine the variation in energy dissipation ϵ in the water column over a tidal cycle at mixed and stratified sites in the Irish Sea. It was found that ϵ exhibits a strong M4 variation with a pronounced phase lag that increases with height above the bed. In mixed conditions this M4 signal, which extends throughout the water column, is reasonably well reproduced by turbulent closure models of the vertical exchange. In the summer stratified situation, the M4, signal in ϵ is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed. The lowest levels of dissipation (∼10−5 W m−3), measured in the pycnocline, are significantly above the system noise level and much higher than predicted by a model using the Mellor-Yamada level 2 closure scheme (MY2.0). However, when allowance is made for the diffusion of TKE, the model (MY2.2) simulates the depth-time distribution of dissipation in the stratified case satisfactorily if...

Currents along the pacific coast of Canada

Abstract A series of oceanographic studies along the west coast of British Columbia in 1979–1982 produced several realizations of the seasonal cycle of currents. At one site, occupied for 3–1/4 years, the monthly mean currents are stable in speed and direction indicating that a single year of data reliably describes the pattern. Based upon three sets of observations at 15‐m depth (summer only) and 17 at 50‐m depth, the flow along Vancouver Island in winter is towards the northwest, parallel to shore, with some evidence that speeds are greater 15 km from shore (30–40 cm s−1) than farther offshore. In April the flow becomes more variable over the outer shelf a southeastward current develops near the shelf break in May, becoming strongest in August with speeds of 20 cm s−1. This southeastward flow appears to be a response to a shift to northerly winds in summer. Near shore during summer a flow tentatively labelled the Vancouver Island Coastal Current flows towards the northwest with speeds of 10–15 cms−1. By...

Bottom Stress Estimates from Vertical Dissipation Rate Profiles on the Continental Shelf

Abstract Measurements of the near-bottom distribution of the turbulent dissipation rate on the continental shelf west of Vancouver Island are used to calculate bottom stress. A free-failing vertical profiler with microstructure shear probes was used to measure the dissipation rate, from near the surface to within 0.15 m of the bottom. The shear probes measure velocity gradients at scales within the viscous subrange of the turbulence and therefore directly measure the rate at which kinetic energy is dissipated by viscosity. Friction velocities are computed from the formula uast; = (ϵκz/ρ)⅓, where the dissipation rate ϵ is measured in the constant stress layer. The technique is more reliable than estimates of the dissipation rate obtained by fitting spectral slopes to velocity spectra at scales in the inertial subrange. Near-bottom current measurements indicate that the bottom stress values obtained from the turbulent measurements are well correlated with the current magnitude. An estimate of the drag coeff...

Structure and variability of Langmuir circulation during the Surface Waves Processes Program

A cooperative, multiplatform field experiment was conducted in the eastern North Pacific during February and March of 1990 as part of the Surface Waves Processes Program (SWAPP). One of the experimental objectives was to investigate Langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood. The concurrent use of different observational techniques, ranging from simple surface drifters to complex Doppler sonar systems, resulted in new information about Langmuir circulation structure and variability. Estimates of Langmuir cell spacing indicated that a broad range of scales, from about 2 to 200 m, was excited during periods of strong surface forcing and that the energy containing scales evolved with time. Estimates of cell spacing based on Doppler velocities from a surface-scanning sonar directed crosswind showed this scale evolution, but estimates based on backscattered intensity did not. This was attributed to the fact that the intensity-based estimates were only indirectly related to circulation strength. The near-surface convergent velocities from the sonar were used to form an objective, quantitative measure of the temporal variations in Langmuir circulation strength. As expected, the circulation strength increased dramatically during strong wind events. However, circulation strength and wind stress did not decrease simultaneously, and Langmuir circulation was detectable for up to a day after abrupt reductions in wind stress. Energy from the surface wave field, which decayed more slowly than the wind, was apparently responsible for maintaining the circulation. The variation of circulation strength was found to be better related to (u*Us)½ than to u*, where u* = (τ/ρ)½ is the friction velocity, τ is the wind stress, and Us is the surface wave Stokes drift. This scaling is consistent with wave-current interaction theories of Langmuir cell generation.

Physical Characteristics of Haida Eddies

Haida Eddies are anti-cyclonic features that form in winter along the eastern continental margin of the Gulf of Alaska, west of the Canadian Queen Charlotte Islands. These eddies have been observed in many oceanographic programs, but their full life cycle has only been known for the past few years. Core waters of eddies are warmer below 100 m depth, and normally fresher at all depths than surrounding waters. Eddies formed in the ENSO winters of 1982/83 and 1997/98 were the highest observed, and were warmer than observed in other years. A study of historical variability of dynamic heights in regions where five eddies were observed shows that Haida Eddies are rare events in each region; and their impact cannot be simulated in any one year without directly modelling each individual eddy. The very large Haida Eddy formed in 1998 is the best sampled to date. Satellite-based measurements six months after its formation show the eddy to rise between 0.33 and 0.37 m above surrounding waters, with a Gaussian radius between 60 and 75 km. Dynamic height anomalies at this time, computed from ship-based observations along two lines through the eddy to 1500 metres depth, reveal a height of 0.32 and 0.35 m respectively, relative to surrounding waters, and Gaussian radius of 60 km. It was almost totally baroclinic, with one-half the baroclinic structure in the upper 375 m or so; however, its anti-cyclonic rotation depressed isotherms and isohalines at depths below 1000 m.

A high‐resolution assimilating tidal model for the northeast Pacific Ocean

A high-resolution, nonlinear, barotropic, finite element tidal model is developed for removing tidal elevations from satellite altimeter observations in the northeast Pacific Ocean. Surface elevations and currents for the constitutents M2, S2, N2, K2, K1, O1, P1, and Q1 are computed using boundary forcing from the Oregon State University global tidal model, TPXO.3, and the effects of the tidal potential, Earth tide, and ocean self-attraction and loading. Amplitudes and phases computed from the harmonic analyses of altimeter time series at crossover locations along the tracks of the TOPEX/Poseidon satellite are assimilated into the model, and the corrected harmonics are compared with pelagic and coastal tide and pressure gauge measurements, and with the TPXO.3 and FES95.2.1 global models. Diurnal tidal currents along the British Columbia and Alaska continental shelves are shown to be strongly influenced by the presence of continental shelf waves. The surface manifestation of these waves is clearly evident in K1 coamplitude plots. K1 and M2 energy fluxes are calculated along these shelves and the potential generation of internal tides is discussed. Barotropic tidal dissipation is estimated in several key subregions, and the M2 total for the Gulf of Alaska is found to be approximately half that of previous estimates.

Multi‐year meanders and eddies in the Alaskan Stream as observed by TOPEX/Poseidon altimeter

The birth, life and decay of long-lived anticyclonic eddies, or meanders, in the Alaskan Stream are observed from TOPEX/Poseidon (T/P) satellite. Their surface height anomalies approach 72 cm, average diameter is about 160km and they live for 1 to 3 years, propagating along the Stream with mean speed of 2.5 km d−1. Of the six observed, the last three were formed after the first T/P observations in September 1992. Of these, two evolved from eddies near Alaskan Panhandle in winter, drifting across the northern Gulf of Alaska before entering the Alaskan Stream, while another formed in the Stream west of Shelikof Strait.

A Comparison of Length Scales and Decay Times of Turbulence in Stably Stratified Flows

Abstract An examination of average values of a buoyancy length scale, LR = (ϵ/N3)½, and a Thorpe scale, LT, computed from vertical profiles of oceanic turbulence at 150°W in the tropical Pacific Ocean, shows reasonable agreement with the relation LR = 0.8LT found by Dillon. The present study uses direct measurements of velocity micro-structure to compute LR. It is also shown that if LR = 0.8LT, and the Brunt-Vaisala period 2π/N is constant, the decay-time constant of kinetic energy of turbulence in a stably stratified fluid is one-third to one-half of the Brunt-Vaisala period. Further investigation, using an exact formula for the energy, reveals that the assumption of constant N leads to an overestimate of the energy by a factor of 3. Correction for this factor reduces the decay time to between one-sixth and one-tenth of the Brunt-Vaisala period. These results are compared with previous observations. Only one previous study investigates the covariation of N and ϵ within a patch of turbulence, finding a de...

Turbulent Dissipation Over the Continental Slope Off Vancouver Island

Abstract Thirteen profiles of the rate of viscous dissipation of turbulent kinetic energy &epsi¯ were made over the continental slope off Vancouver Island between 12 and 14 May 1980 in conjunction with CTD and moored current-meter observations. Systematic variability was observed in the vertical but not in the horizontal direction. Above 200 m depth numerous salt-stabilized temperature inversions were seen and dissipation rates were significantly larger than below 200 m. Dissipation rates below 200 m are the lowest ever reported and coincide with a low level of energetics revealed by the current meter moorings. Comparison with the Garrett-Munk internal wave spectrum indicates an e-folding decay time of internal wave energy of ∼50 days at depths below 200 m.

Turbulence and mixing: Sources of nutrients on the Vancouver island continental shelf

Abstract Estimates of the rate of dissipation of turbulent energy,?v, were made on the continental shelf off the southwest coast of Vancouver Island throughout the diurnal tidal cycle at six stations. If turbulent mixing takes place away from boundaries, and a constant fraction of the energy supplied to turbulence is converted into potential energy, this dissipation rate per unit volume, ?v, is shown to be proportional to the rate of turbulent mass and nutrient flux across the isopycnals. We compute these fluxes to determine the source of nutrients in the upper mixed layer on the shelf. It is found that tidal mixing on the shelf throughout the water column contributes less than 10% of the flux of nutrients supplied by the estuarine outflow out of Juan de Fuca Strait. Strong winds during upwelling events supply nutrients at rates greater than those due to tidal mixing, but at rates that are likely smaller than the Juan de Fuca source. Therefore, the nutrient‐rich waters observed in the euphotic zone in spr...