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Alexander B. Rabinovich and Richard E. Thomson

Abstract

Satellite-tracked surface drifters deployed in September 1993 in the vicinity of the Kuril–Kamchatka Trench were advected onto the Pacific continental shelf of the Kuril Islands where they encountered strong (40–50 cm s−1) diurnal tidal currents. One of the drifters subsequently passed through Friz Strait into the Sea of Okhotsk, experiencing intense (>100 cm s−1) diurnal currents in the strait and strong (35–40 cm s−1) diurnal currents over the Okhotsk shelf of the Kuril Islands. The across-shelf structure of the diurnal tidal currents is shown to be consistent with that of free, topographically trapped subinertial waves propagating along the continental margin of the islands. Of the three continental shelf wave models considered (a barotropic model with zero mean flow, a barotropic model with nonzero alongshore mean flow, and a baroclinic model based on the observed density structure), only the baroclinic model accurately explains the main features of the diurnal currents for the Pacific and Okhotsk shelves. Both first and second mode waves contribute to the diurnal currents.

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Alexander B. Rabinovich, Georgy V. Shevchenko, and Richard E. Thomson

Abstract

The authors describe a two-dimensional (vector) regressional model for examining the anisotropic response of ice drift and ocean current velocity (“drift velocity”) to surface wind forcing. Illustration of the method is limited to sea ice response. The principal mathematical and physical properties of the model are outlined, together with estimates of the “response matrices” and the corresponding “response ellipses” (drift velocity response to a unity wind velocity forcing). For each direction, ϕ, of the wind vector the method describes a corresponding “wind factor” α(ϕ) (relative drift speed) and “turning angle” θ(ϕ) (the angle between the drift velocity and wind vector). The major ellipse axis corresponds to the direction of the “effective wind” (ϕ = ϕ max) and the minor axis to the direction of the “noneffective” wind. The eigenvectors of the response matrix define wind directions that are the same as the wind-induced drift velocity directions. Depending on the water depth and offshore distance, six analytical cases are possible, ranging from rectilinear response ellipses near the coast to purely circular response ellipses in the open ocean. The model is used to examine ice drift along the western shelf of Sakhalin Island (Sea of Okhotsk). Responses derived from the vector regression (four parameter) method are less constrained and therefore more representative of wind-induced surface motions than those derived using the traditional complex transfer function (two parameter) approach.

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Georgy V. Shevchenko, Alexander B. Rabinovich, and Richard E. Thomson

Abstract

Ice-drift velocity records from coastal radar stations, combined with data from moored current meters and coastal wind stations, are used to examine sea-ice motion off the northeastern coast of Sakhalin Island in the Sea of Okhotsk. Ice motion is shown to be governed primarily by diurnal tidal currents and wind-induced drift, which explain 92%–95% of the total ice-drift variance. Diurnal tidal motions predominate off the northern Sakhalin coast, accounting for 65%–80% of the variance, while low-frequency wind-induced motions prevail off the south-central coast, accounting for over 91% of the ice-drift variance. Maximum diurnal tidal ice-drift velocities range from 90–110 cm s−1 on the north coast to 10–15 cm s−1 on the south coast, in good agreement with the barotropic model of Kowalik and Polyakov. The presence of diurnal shelf waves accounts for the strong diurnal currents on the steeply sloping northern Sakhalin shelf, while the absence of such waves explains the weak diurnal currents on the more gently sloping south-central shelf. Using a vector regression model, the authors show that wind-induced ice-drift “response ellipses” (the current velocity response to a unity wind-velocity forcing) are consistent with a predominantly alongshore response to the wind, with wind-induced currents most pronounced off the south-central coast where water depths are relatively shallow. Time–frequency analysis of wind and ice-drift series reveals that, in winter, when sea ice is most extensive and internally cohesive, the ice response is almost entirely aligned with the alongshore component of the wind; in spring, when sea ice is broken and patchy, the ice responds to both the cross- and alongshore components of the wind.

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Steven J. Bograd, Alexander B. Rabinovich, Richard E. Thomson, and A. Jane Eert

Abstract

The effects of reduced sampling schedules (duty cycles) on velocity statistics derived from satellite-tracked drifters in the northeast Pacific Ocean are investigated. Continuous segments of the drifter records (in which all available satellite positions fixes are recorded and processed by Service ARGOS) are degraded to match the standard duty cycle used in the World Ocean Circulation Experiment–Surface Velocity Program, in which there are 48 h of no data transmission followed by 24 h of received transmission (48–24 h). Also examined are duty cycles of 32–16 h and 16–8 h. It is found that the strong inertial motions prevalent in the drifter records result in significantly biased statistics derived from the degraded series. Reproduction of the original prime (mean and standard deviation) and rotary spectral statistics requires an interpolation that takes into account the oscillatory component of the drifter motions. Duty cycles having shorter but more frequent gaps (e.g., 16–8 h) are not sufficient to resolve the main features of the flow. The authors recommend that interpolations over duty cycle segments of drifter records be customized to account for the dominant modes of variability observed in available continuous segments.

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