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Richard E. Thomson

Abstract

This paper describes the circulation, water properties and energetics of an observed cyclonic eddy that formed over the continental margin of Vancouver Island between late July and early September, 1980. The eddy was characterized by a depth scale of 1 km, a radius of 50 km and a maximum near-surface geostrophic flow of 50 cm s−1. Within the middepth core of the eddy, isopycnal surfaces were domed upward by 50 m and were comprised of relatively warm, saline and low dissolved oxygen water that appeared to originate with the California Undercurrent.

The eddy is shown to have been generated through dynamic instability of the seasonal mean flow along Vancouver Island. The appearance in late July of the undercurrent over the slope may have been an important factor in the amplification of the mesoscale meander that eventually deformed into the eddy. Calculation of each of the terms in the integrated energy balance reveals that both barotropic and baroclinic instability contributed to the amplification and that 87% of the energy flux from the mean to the perturbed flow occurred within the upper 150 m. The baroclinic source term alone accounted for 82% of the total energy flux within the upper 500 m of the water column. The measured change in the potential energy distribution and, to a lesser degree, the tilt of the perturbation streamlines with depth are consistent with generation of the eddy through the instability process. An estimate of 25 ± 8 days is obtained for the e-folding growth time of the instability.

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Richard E. Thomson

Abstract

We consider the effect of a randomly varying horizontal advective flow on internal gravity waves propagating vertically in a nonrotating, stratified and unbounded fluid. Attention is focused on the alteration in the vertical growth rate for the waves when the background flow U is of the form U=Uo+μ, where μ is a centered stationary random function of height (z) and Uo=constant. We show that for both long and short correlation lengths the randomness in the wind (or current) leads to a dissipation of wave energy such that the normal upward amplification of the waves can be significantly reduced. Similarly, the normal downward attenuation can be significantly enhanced. The alteration in phase speed produced by the random flow is also considered although it is not discussed in detail.

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Richard E. Thomson

Abstract

The Alaskan Stream boundary current south of the uniformly curving coastline formed by the Alaskan peninsula-Aleutian Island chain is examined analytically via steady, barotropic frictional theory. It is shown that, as a result of the changing zonal orientation of this boundary, there is an alteration in the characteristic vorticity balance in the current as it progresses westward from the Gulf of Alaska. Where the curving coastline becomes approximately zonal, this vorticity distribution is such that, unless the clockwise vorticity generated at the coast by the no-slip condition is balanced by a vorticity source external to the current, instabilities and separation of the Alaskan Stream from the coast will occur.

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Richard E. Thomson and Susumu Tabata

Abstract

Thirty-year time series of hydrographic observations from Ocean Station PAPA and Line ‘P’ are used to estimate secular trends in monthly mean steric sea level heights relative to depths of 100 and 1000 decibars in the northeast Pacific Ocean. Linear trends at station ‘P’ (50°N, 145°W) indicate that steric heights relative to the 1000 db (approx. 1000 m) level are rising at a rate of 1.1 mm yr−1, comparable with the Order 1 mm yr−1 global trends suggested by analysis of selected long-term coastal tide gauge records. Approximately 67% of the increase in steric levels is due to thermosteric change at depths below 100 m, the smaller 33% contribution from the halosteric component apeasrs to be confined to the upper 100 m. Steric height trends at fine ‘P’ locations are also of order 1 mm yr−1 but, in contrast to station ‘P’ trends, arise mainly through the halosteric component.

Confidence levels for the linear trends an calculated in two ways. (i) using the Student-t test assuming that cub monthly observation is a statistically independent sample; and (ii) using the Student-t test in conjunction with the effective number of degrees of freedom derived from integral time scales. For station ‘P’, trends based on (i) are reliable to the 99% confidence level while for line ‘P’ only stations on the eastern portion of the fine have significant trends relative to the 1000 db level. Confidence levels obtained from (i) fail to take into consideration the long-term fluctuations in steric level records. To obtain more reliable estimates of the confidence intervals, we use integral time scales to determine the effective number of degrees of freedom for each monthly time series. Subsequent recalculation of trend-line confidence intervals indicates that the total steric height trends at Station ‘P’ remain significant at the 90% confidence level. The halosteric trend relative to 100 db is significant at 90% while the thermosteric trend relative to 1000 db is marginally significant at 70 to 80%. With the exception of stations 5 and 6, trends for line ‘P’ stations are no longer significant above the 70% level. The lower statistical reliability in the line ‘P’ trends is due, in part, to the sparse sampling rate relative to station ‘P’. We conclude that steric sea levels in the northeast Pacific are rising, at approximately 1 mm yr−1 and that this increase may be associated with a combined regional warming of the deeper waters and dilution of the surface waters. Although the observed trends appear to be linked to climate-induced eustatic changes in global sea level, the records are not of adequate length or spatial coverage to rule out effects of shifting regional circulation patterns.

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Richard E. Thomson and Robert E. Wilson

Abstract

Cape St. James is an extensive triangular-shaped promontory located in a tidally energetic region at the southern tip of the Queen Charlotte Islands approximately 150 km off the mainland coast or British Columbia. Several years of oceanographic data collected in vicinity of the cape reveal a regional circulation characterized by a strong (0.50 m s−1) coastal current along the western continental margin and respective clockwise and counterclockwise rotating mesoscale baroclinic eddies to the west and south of the cape. The coastal current flows counter to the prevailing winds while the anticyclonic eddy to the west of the cape is a particularly intense feature that appears consistently in AVHRR imagery of the region. The structure of the mean flow, combined with the marked O(0.1 0 m s−1) low-frequency current variability at fortnightly and monthly tidal periods plus significant coherence at fortnightly periods between low-frequency currents and demodulated tidal flow, suggests that rectification of the strong diurnal and semidiurnal tidal currents is the principal cause of the residual circulation in the vicinity of the cape.

Results from an analytical model indicate that generation of the mean residual circulation is due primarily to the M2 tidal current constituent and that maximum countercurrent velocities occur over the inner portion of the continental shelf. The fortnightly modulation of the mean flow is effected by both diurnal and semidiurnal currents but with a tendency for semidiurnal contributions to dominate in regions of greatest counterflow. Generic depth-dependent numerical simulations for nondimensional frictional parameters typical of the region verify that the asymmetry in the observed location and intensity of the eddy field, together with the presence of the strong coastal countercurrent on the west side of the cape and a narrow jet to the south of the cape, are associated with tidal rectification. These models also suggest that residual vertical motion due to topographic lifting and Ekman suction are responsible for the observed tilting of the isopycnals and thereby the development of baroclinicity in the residual horizontal motion.

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Susan E. Allen and Richard E. Thomson

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Linear analytical solutions for bottom-trapped subinertial oscillatory flow over simple ridge topographies in a stratified (two-layer) rotating fluid are presented. Results are compared to moored current meter observations of bottom-intensified motions over the Endeavour Segment of Juan de Fuca Ridge in the northeast Pacific. The solutions reproduce many of the observed features including preferential amplification of the clockwise rotary component of velocity over the ridge and increased velocity amplification with proximity to the ridge crest. For a given internal deformation radius, the degree of current amplification increases with increased bottom slope, ridge height, and oscillation frequency. Amplification decreases with increased width of the ridge relative to the deformation radius.

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Richard E. Thomson and Jason H. Middleton

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We derive expressions that predict the variations of Cartesian, rotary and elliptical properties of free and forced barotropic continental shelf waves as functions of alongshore and cross-shore location. Bottom friction is shown to significantly complicate these expressions. Particular attention is given to the spatial variability in the phases of forced waves as functions of the wavenumber of the forcing and the corresponding free wave mode. Consideration of the alongshore and across-shelf structure predicted by the theory indicates that, for a given frequency, the relative merits of Cartesian or rotary Fourier analysis of data depends on the location of the observation stations in the across shelf direction and on the geometry of the continental shelf and slope. The specific case of observed, diurnal period (K1) continental shelf waves off Vancouver Island is used to illustrate how the free and forced shelf wave models lead to different interpretations for the wavelengths of the free wave component. The results demonstrate the nontrivial nature of the forced problem and emphasize the need for accurate resolution of the wavenumber of the driving mechanism.

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Andrew J. Willmott and Richard E. Thomson

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The authors examine barotropic nondivergent shelf waves generated on an exponential continental shelf that has an abrupt change in width. Three types of forcing are considered: 1) a tidal period volume flux through a gap in the coastline located along the discontinuity, 2) an alongshore propagating wind stress over the continental shelf, and 3) an alongshore propagating perturbation in the streamfunction at the edge of the continental slope. Dimensional results for the linearized models are derived for the northwest coast of Vancouver Island, British Columbia, where the shelf abruptly widens into Queen Charlotte Sound. Because of the change in wave scales and numbers of shelf modes possible on either side of the coastline discontinuity, the response for the discontinuities width shelf differs markedly from that for a uniform width shelf. Results show that shelf wave energy generated by fortnightly tidal flow through the gap (or coastal strait) is radiated in a narrow “beam” across the broader portion of the shelf. Diurnal period motions are trapped near the mouth of the strait and do not contribute significantly to the shelf current. The use of realistic periods and wavelengths (5–10 days and 500–1000 km) for the alongshore forcing terms yields propagating eddylike circulation patterns that closely resemble the flow patterns commonly seen in satellite thermal imagery over the narrow portion of the Vancouver Island shelf. At low forcing frequencies a distinct “shadow zone” with relatively weak barotropic response is found over the wide portion of the shelf in the vicinity of the coastal discontinuity.

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Richard E. Thomson and William R. Crawford

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Continental shelf waves of diurnal period are shown to be generated by tidally induced Reynolds stresses within a bottom Stokes-scale boundary layer. The theory is applicable to a uniformly rotating, homogeneous ocean of two-dimensional depth variability in which alongshore variations occur over scales large compared to the shelf width. Explicit solutions are derived for the shelf wave velocity components and for the cross-shelf sea surface slope in the case of a traveling Kelvin wave forcing. Numerical values are presented for an exponential depth profile H = H0 exp(−2αx), where x is the coordinate normal to the coast. Results indicate that the amplitude of the shelf wave current can exceed that of the astronomical tidal current and that the alongshore component of the shelf wave current will consistently lead the alongshore component of the tidal current by 180° to 0° over one, wavelength in the direction of phase propagation.

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Richard E. Thomson and Isaac V. Fine

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Estimates of mixed layer depth are important to a wide variety of oceanic investigations including upper-ocean productivity, air–sea exchange processes, and long-term climate change. In the absence of direct turbulent dissipation measurements, mixed layer depth is commonly derived from oceanic profile data using threshold, integral, least squares regression, or other proxy variables. The different methodologies often yield different values for mixed layer depth. In this paper, a new method—the split-and-merge (SM) method—is introduced for determining the depth of the surface mixed layer and associated upper-ocean structure from digital conductivity–temperature–depth (CTD) profiles. Two decades of CTD observations for the continental margin of British Columbia are used to validate the SM method and to examine differences in mixed layer depth estimates for the various computational techniques. On a profile-by-profile basis, close agreement is found between the SM and de facto standard threshold methods. However, depth estimates from these two methods can differ significantly from those obtained using the integral and least squares regression methods. The SM and threshold methods are found to approximate the “true” mixed layer depth whereas the integral and regression methods typically compute the depth of the underlying pycnocline. On a statistical basis, the marginally smaller standard errors for spatially averaged mixed layer depths for the SM method suggest a slight improvement in depth determination over threshold methods. This improvement, combined with the added ability of the SM method to delineate simultaneously ancillary features of the upper ocean (such as the depth and gradient of the permanent pycnocline), make it a valuable computational tool for characterizing the structure of the upper ocean.

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