JANUARY 1973 DAVID L. CUTCHIN AND ROBERT L . SMITH 73 Continental Shelf Waves: Low-Frequency Variations in Sea Level and Currents Over the Oregon Continental Shelf DAVID L. CUTCHIN1 AND ROBERT L. SMITH School of Oceanography, Oregon Stale University, Corvallis 97331 (Manuscript received 20 March 1972, in revised form 30 June 1972) ABSTRACT Sea level variations and currents on the Oregon continental shelf exhibit wavelike characteristics in a frequency band from approximately 0.15 to 0.45 cycle per day (cpd). Shelf wave dispersion curves and eigenfunctions for the Oregon continental shelf profile computed using a numerical technique are compared with a low-frequency (~0.03-0.75 cpd) spectral analysis of the current, sea level, and atmospheric pressure records. In a narrow band around 0.22 cpd the current, sea level relationship is consistent with the predicted values for free barotropic continental shelf waves. 1. Introduction diurnal tidal slot is occupied by an almost non-divergent oscillation similar to a continental shelf wave. Continental shelf waves, topographic Rossby waves In order to more closely examine this phenomena an trapped over the continental margin, have frequencies observational experiment was designed for the Oregon less than the local inertial frequency and wavelengths continental margin. The relatively uncomplicated ba- much greater than the depth. The waves progress thymetry and low mean flow off Oregon should ease parallel to the coast in one direction only, like classical the interpretation of data collected there. Since the Kelvin waves: in the Northern Hemisphere they travel theory for shelf waves predicts that prominent hori- with their right shoulder against the coast. A computer- zontal current fluctuations accompany rather modest assisted artist's concept of a shelf wave, with the sea sea level fluctuations, it was decided to place heavy surface displacement greatly exaggerated, is shown in emphasis on current measurements. Because of the Fig. 1. strong stratification and strength of the internal tide Continental shelf waves were first explicitly discussed (Mooers, 1970) in the region, we were prompted to by Robinson (1964) in a theory of quasi-geostrophic moor the three available current meters in a single waves resonant on a sloping shelf of finite width. vertical array to be able to distinguish any baroclinic Mysak (1967b) extended this to include the effects of shelf wave modes from the barotropic. a continental slope region, deep sea stratification, and The difficulties involved in continuously measuring longshore current. Buchwald and Adams (1968) ob- the distribution of wind stress and atmospheric pres- tained an analytical solution for the case of an expo- sure over the continental shelf, and the large uncertain- nentially varying shelf by neglecting horizontal diver- ties in parameterizing the momentum transfer through gence. Niiler and Mysak (1971) examined the effect the sea surface from the sparse standard meteorological of longshore lateral shear in western boundary currents data available, prevented us from investigating the on the propagation and stability of free barotropic forced wave problem. The analysis of the observations shelf waves. Caldwell et al. (1972) verified the existence presented in this paper is based on free shelf wave of continental shelf waves in a laboratory model. theory. Evidence for the existence of continental shelf waves in the ocean has been found through analysis of tide- gage records from the Australian coast (Hamon, 1966; 2. The experiment Mysak, 1967a), the Oregon coast (Mooers and Smith, a. Field measurements 1968), and the North Carolina coast (Mysak and Hamon, 1969). On the basis of current and sea level Fig. 2 shows the region of the observational experi- measurements made near the island of St. Kilda ment and the principal bathymetric features of the (Scotland), Cartwright (1969) noted that there the continental margin off Oregon. From mid-April to mid-September, 1968, a vertical 1 Present affiliation: Center for Great Lakes Studies, The Uni- array of three current meters was maintained in about versity of Wisconsin-Milwaukee. 100 m of water 7 n mi off Depoe Bay. In Fig. 2, this Unauthenticated | Downloaded 09/29/21 03:15 AM UTC 74 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 3 FIG. 1. A computer-assisted artist's conception of a first-mode continental shelf wave. The heavy arrow indicates direction of phase propagation and the lighter arrows indicate the water velocity under the crest. The vertical displacement of the sea surface is greatly exaggerated. station (DB-7) is indicated by a solid square. The current measuring devices used were Savonius rotor in- struments of the Braincon-type 316. During certain periods the data sampling interval was set at 10 min and during others at 20 min. One meter was set at each of the three depths: 25, 50 and 75 m. Due to instrument failures (particularly with the instrument at 75 m) and logistic errors, a complete set of data was not obtained. The instruments were serviced and replaced at about monthly intervals. The mooring system for the meters was the taut wire array reported by Pillsbury et at. (1969). Sea level elevation at the coast was provided by permanently maintained tidegages at Newport and Astoria, Ore. The tides at Newport are measured at a station 1 n mi into Yaquina Bay, a relatively small estuary without a large contributing river. The data are automatically recorded every 6 min to within 0.3 cm. Spectral tests showed very little energy in the Newport sea level record with periods <2 hr, so the data series were initially decimated to one point per hour. The tides at Astoria are measured by the National Ocean Survey (NOAA) at Tongue Point, about 12 n mi upriver from the mouth of the Columbia River estuary. The data are recorded in analog fashion and digitized every hour to within 3 cm. Atmospheric pressure data are collected at 3-hr in- tervals by National Weather Service observers at Newport and Astoria. b. The data The digitized current meter records, in N-S and E-W component form, were passed through symmetri- FIG. 2. The bathymetry of the experiment area. The dashed line indicates the base of the continental slope. The location of cal low-pass niters which suppressed the diurnal tide the current measurements (DB-7) is indicated by the square and and all higher frequencies. The filter, a "Cosine- the sites of the tidegages by triangles. The bathymetric profile along the line extending offshore from Depoe Bay is shown in Lanczos" taper, had a power response of 0.94, 0.5, and Fig. 7. 0.05 at 0.42, 0.71, and 0.82 cpd, respectively. The Unauthenticated | Downloaded 09/29/21 03:15 AM UTC JANUARY 1973 DAVID L. CUTCHIN AND ROBERT L. SMITH 75 AUGUST SEPTEMBER FIG. 3. Low-pass (half-power frequency is 0.71 cpd) current and sea level data. response was below 0.01 for frequencies >0.9 cpd and at DB-7, the current meter station, was located be- between 1.0 and 1.02 for the frequency range 0.0-0.34 tween 15 and 25 m depth. Current meters were not cpd. The records from the tidegages and the atmo- placed closer to the surface than 25 m in order to avoid spheric pressure data were put through symmetrical the confusing effects of surface wind waves on the filters with the same power response function as the data records. Had observations been made above the filters used on the currents. pycnocline they might have shown some oscillating During the first 45 days of the experiment, from shears. mid-April until the end of May, a distinct trend toward Because of the similarity of current fluctuations ob- northward flow and higher sea level was readily ap- served at the three depths, it was decided to examine parent from plots of the low-passed data. A severe in detail only the most complete of the current records. storm at the end of May caused substantial variations For the time frame selected for analysis, the 50 m in sea level and currents as well as unusually sharp record was the most complete with only one gap, from deviations from the typical late spring density struc- 1200 PST 17 June to 0000 PST 24 June, which was ture. The statistical analysis of a single time series interpolated with a straight line. containing either a slow oscillation with a time scale Most of the low-frequency oscillating flows observed on the order of the length of the series or an isolated at DB-7 were directed along 020° true, which closely burst of energy is difficult to interpret. Furthermore, coincides with the orientation of the local depth con- the available theory for continental shelf waves goes tours. It is therefore appropriate to display the current very little beyond the treatment of small-amplitude vector components in a coordinate system with + » barotropic disturbances. For these reasons we have toward 020° true and +u toward 110° true (Fig. 3). excluded from immediate consideration the data taken The tick marks along the bottom abscissa correspond during and prior to the storm. Plots of data taken to 0000 PST on every day. The first day of each month during the 100 days following the storm showed activity is identified by a larger mark. which was, in the statistical sense, better behaved. The data record chosen for analysis extended from 0000 Also shown in Fig. 3 are the low-passed sea level PST 5 June to 0000 PST 11 September, 1968. records from Newport and Astoria. These records have been adjusted for the effect of atmospheric pressure on The low-passed current records from the three depths, sea level according to the hydrostatic hypothesis, i.e., 25, 50 and 75 m, showed a steady shear between 25 a 1-mb increase in atmospheric pressure decreases sea and 75 m with the shallow water flowing more southerly level by 1 cm.