Observations of current and mixing around the shelf break in Pribilof Canyon in the Bering Sea
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Tide-induced vertical mixing along the shelf break in the eastern Bering Sea is considered one of the main physical processes that sustain local summertime high biological production. However, observations based on microstructure measurements that show enhanced tidal mixing are scarce. In this study, repeated casts of current and turbulence in the vicinity of the shelf break within Pribilof Canyon were conducted over a day in June 2012, enabling us to evaluate the representativeness of the vertical mixing intensity during one day and to detect the relationship between turbulence and tidal current. The cross-sectional distributions of the one-day averaged vertical diffusivity and the turbulent energy dissipation rate showed that strong vertical mixing occurred at the subsurface within about 15 km of the shelf break and near the seabed of the outer shelf. This result agrees with prior observations that were made by us and based on a single profile at each station, which indicated that the observed spatial pattern of turbulence is robust. Diurnal and semidiurnal tidal currents dominated the flow variations off the shelf break, and a statistically significant positive correlation was detected between the vertical shear of the horizontal tidal currents and the turbulent energy dissipation rate. This result suggests that the high turbulent energy dissipation and the enhanced vertical mixing off the shelf break were induced by the strong vertical shear of tidal currents.
KeywordsVertical turbulent mixing Microstructure observation Tidal current Pribilof Canyon
This paper forms a part of Ph.D. thesis (Tanaka 2014; Graduate School of Science, The University of Tokyo). We appreciate helpful comments from Profs. T. Hibiya, Y. Masumoto, K. Iga, M. Uematsu, and Y. Tanaka, and two anonymous reviewers. We are grateful to Prof. K. Kuma for kindly providing data on iron concentration. We are also grateful to the officers and crew of the T/S Oshoro-maru and to all of the scientists onboard it for their assistance in conducting observations and their thoughtful arrangement of the field observations. This work was supported by a JSPS Research Fellowship and Grant-in-Aid for Scientific Research (KAKENHI, 20221002/25257206/25121503). We thank Dr. R. Saito for his help obtaining AVISO data.
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