High‐frequency internal wave observations in the marginal ice zone

Abstract

Three thermistor chains extending from 15 to 65 m were deployed from ice floes in the Marginal Ice Zone north of Svalbard in a triangle with sides of 500-800 m for a period of 7 days. Vertical displacement spectra indicate that the internal wave energy level is lower than the Garrett and Munk spectrum by a factor of 2-3 but higher than other observations in the Arctic. A group of internal waves (a wave train), with frequency of 2-3 cph was observed to propagate at 0.05 m/s through the triangle in a west-southwesterly direction during a 4-to 6-hour period. A case study of this wave train, where the horizontal phase differences between the three thermistor chains were analyzed, showed wavelengths of 100-200 m and phase velocities of 0.10-0.15 m/s. Observation of the density field and the velocity field indicated that stratification dominated over vertical shear. Consequently, the dispersion relation was calculated by solving numerically the vertical mode equation for internal waves in a shearless ocean. The estimated wavelengths and phase velocities agree with the dispersion relation for first-mode internal waves. The generating mechanisms may be tidal oscillations associated with a seamount located east of the triangle, or ice keels moving in relation to the water creating lee waves in the pycnocline. 1. INTRODUCTION Internal wave investigations in ice covered regions of the world oceans are few. Experiments from drifting pack ice in the Arctic Ocean indicate a lower energy level of internal waves (hereafter denoted IW) than in other parts of the ocean [-Morison, 1986]. During the Norwegian Remote Sensing Experiment (NORSEX) in 1979 [-Johannessen et al., 1983-[ a synthetic aperture radar (SAR) image exhibited IW with wavelengths of 400-1000 m just off the ice edge (Figure 1) at approximately 81ø35'N, 7øE. This is in the same area where the 1983 experiment took place. Under calm to moderate wind conditions, IW have also manifested themselves in radar images as convergence lines of ice belts [-Johannessen et al., 1985-[. However, no in situ IW experiments in the marginal ice zone (MIZ) have so far been reported.One component of the 1983 Marginal Ice Zone Experiment (MIZEX '83), carried out on the Yermak Plateau north of Svalbard, was a 7-day IW experiment staged from R/V Polarbjorn, which was moored to a drifting ice floe 20 km in from the ice edge. Three thermistor chains were suspended in a triangle from drifting ice floes. A number of other experiments were run in parallel, describing the ocean, the ice, and the atmosphere [-MIZEX Group, 1986-[. For high-frequency IW studies, the most important data sources were the thermistor chains with sampling frequency well above the Brunt-Viiisiilii frequency, whereas useful background information came from conductivity-temperature-depth (CTD) casts, Aanderaa current meters, profiling current meters, ice drift, and wind measurements.In the first part of this paper the experiment and selected IW observations are described. Then energy spectra, horizon-691 tal coherence, and ...

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