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Tidally-forced lee waves drive turbulent mixing

Observations north of Svalbard reveal a pathway of energy transfer from tidal currents to turbulent mixing.

Ilker_Heilge
Preparation of the vertical microstructure profiler. Ilker Fer, Eivind Kolås, and Helge Bryhni (from left to right)
Photo:
Algot K Peterson

During the first physical oceanography process cruise of the Nansen LEGACY project, we worked a section from the shelf to deep ocean along 18E, north of Svalbard. Immediately after completing the section, we produced distributions of density surfaces and turbulence dissipation rates (a measure of how turbulent the water is) and observed an abrupt dip of the density surfaces and very turbulent waters in the whole water column near the shelf break.

Excited by the observation, which I hypothesized was due to breaking of tidally forced internal waves, I proposed we occupy a process station at this location. This meant nonstop profiling using our microstructure profiler, repeatedly and continuously for about 24 hour period, covering the complete daily tidal cycle. This labor-intensive work suggestion was not popular at all and almost led to a mutiny! After some discussion and attempts to convince the other cruise participants, we collected a unique data set showing the evolution of a nonlinear wave and how it leads to turbulent mixing.

In the paper recently published in Geophysical Research Letters, we describe the observations from the shelf north of Svalbard, showing a turbulent event driven by tidal currents. The energy is trapped and accumulated at the time of maximum flow, and is released at the turn of the tide when the entire water column becomes highly turbulent. Our observations imply that this process is an important source of mixing in the Arctic Ocean. The pathway for the energy from tides to turbulence in the Arctic Ocean, the magnitude and distribution of the ocean mixing rates, and the role of feedbacks between mixing rates, stratification, sea ice and the tide are key to predicting the fate of the Atlantic water in the Arctic, and the evolution of the Arctic Ocean in a warming world.

The paper, “Tidally-forced lee waves drive turbulent mixing along the Arctic Ocean margins”, is freely accessible from here: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL088083.

Cruise data used in this analysis are available at https://doi.org/10.21335/NMDC-2047975397 through the Norwegian Marine Data Centre with a Creative Commons Attribution 4.0 International License.