Internal wave observations from the Arctic environmental drifting buoy

Abstract

A free‐drifting buoy serving as an ice anchor and flotation sphere for a 125‐m instrumented mooring line was deployed in the Arctic pack ice by the R/V Polarstern in August 1987 at the northernmost point achieved by the Arktis IV expedition. Current measurements were made from this Arctic environmental drifting buoy (AEDB) using a 150‐kHz acoustic Doppler current profiler (ADCP) attached to the mooring line at a depth of 16 m below the flotation sphere with the transducers facing downward. Velocity profiles spanning depths between 35 and 327 m with 15‐m resolution were collected at half‐hour intervals over a period of 216 days while the buoy drifted from its starting point near 86°N, 22°E to about 70°N, 16°W. The full experimental record was separated into three sections corresponding to periods of the drift spent in the Nansen Basin, over the Yermak Plateau, and in the Greenland Sea. Velocity time series and spectra were computed, and results for the three analysis sections were compared. Spectra in the Nansen Basin were found to have energy levels significantly less than those typical for mid‐latitudes, roughly one third of the canonical Garrett‐Munk (GM) level, and a spectral slope between ƒ and 2.5ƒ that was significantly flatter than that expected from the GM model. Between 83°N and 81°N, as the buoy passed over the Yermak Plateau, internal wave energy increased to levels comparable to GM. Isolated, energetic near‐inertial wave packets with upward group velocity were observed in this portion of the record, presumably generated by interaction of the barotropic tide with the bottom topography of the plateau. Estimates of the upward energy flux from these near‐inertial packets were comparable to the downward fluxes from typical surface sources. Interpretation of the Greenland Sea section was complicated by ADCP sampling problems and a lack of Argos fixes, but the available data showed internal wave spectral levels that were intermediate between those of the Nansen Basin and Yermak Plateau and no evidence of upward‐propagating near‐inertial energy.