Upper Ocean Turbulence in the Icelandic Basin Under Strong Forcing

Harper L Simmons, University of Alaska Fairbanks, Fairbanks, AK, United States, Louis C St. Laurent, Applied Physics Laboratory University of Washington, Seattle, United States, Luca Raffaele Centurioni, Scripps Institution of Oceanography, La Jolla, CA, United States, Sophia Merrifield, Massachusetts Institute of Technology, Cambridge, MA, United States and Justin Shapiro, Applied Physics Laboratory University of Washington, Seattle, WA, United States
Concurrent near surface measurements of turbulence and air-sea interactions can be difficult to obtain in the open ocean, and the challenges are exacerbated in high sea states. In May 2018 a field program in the Icelandic Basin made measurements of air-sea interactions, significant wave height and upper ocean turbulence.

Winds were strong through most of the mission with a series of storms with significant wave heights from 5m to over 8m. The combination of nearby ship sampling, drifters and autonomous turbulence measurements from a Slocum Glider equipped with a Rockland Microrider turbulence package allowed comparisons of known shear and convective turbulence scalings to direct shear-probe estimates of turbulence in the upper 200m of the ocean. Mixed layers depths varied in space and time from 150 to 200m so that the measurements were predominantly in the upper ocean boundary layer. As part of the same project, measurements in 2019 captured similar surface conditions with a shallower MLD (<50m) and are presented for reference (S. Merrifield abstract).

Conventional near surface dissipation scalings are based on competition between buoyancy and shear-driven production. Buoyancy forcing transitioned from cooling in the first week to warming, however the turbulence was in a shear-dominated regime throughout the mission. Turbulence exceeded scaling predictions suggesting that the observations are in strongly a forced wind-wave range and highlight the need for improvements to boundary layer turbulence in extreme sea states.