Abstract
AbstractOcean microstructure, current, and hydrography observations from June 2016 are used to characterize the turbulence structure of the Lofoten Basin eddy (LBE), a long-lived anticyclone in the Norwegian Sea. The LBE had an azimuthal peak velocity of 0.8 m s−1 at 950-m depth and 22-km radial distance from its center and a core relative vorticity reaching −0.7f (f is the local Coriolis parameter). When contrasted to a reference station in a relatively quiescent part of the basin, the LBE was significantly turbulent between 750 and 2000 m, exceeding the dissipation rates ε in the reference station by up to two orders of magnitude. Dissipation rates were elevated particularly in the core and at the rim below the swirl velocity maximum, reaching 10−8 W kg−1. The sources of energy for the observed turbulence are the background shear (gradient Richardson number less than unity) and the subinertial energy trapped by the negative vorticity of the eddy. Idealized ray-tracing calculations show that the vertical and lateral changes in stratification, shear, and vorticity allow subinertial waves to be trapped within the LBE. Spectral analysis shows increased high-wavenumber clockwise-polarized shear variance in the core and rim regions, consistent with downward-propagating near-inertial waves (vertical wavelengths of order 100 m and energy levels 3 to 10 times the canonical open-ocean level). The energetic packets with a distinct downward energy propagation are typically accompanied with an increase in dissipation levels. Based on these summer observations, the time scale to drain the volume-integrated total energy of the LBE is 14 years.
Highlights
The Lofoten Basin of the Norwegian Sea is surrounded by the main branches of the Norwegian Atlantic Current (NwAC) carrying warm and saline Atlantic Water (AW) along the shelf break with the slope current and along the Mohn Ridge with the front current (Fig. 1) (Orvik and Niiler 2002)
More recent observations include trapped near-inertial wave packets in a warm-core ring (Joyce et al 2013), dissipation rates elevated in a deep Southern Ocean eddy (Sheen et al 2015), increased shear variance caused by waves trapped in submesoscale cyclonic vortex filaments in the north wall of the Gulf Stream (Whitt et al 2018), and high dissipation rates away from the seafloor inside the midocean fracture zones, caused by transfer of nearinertial wave energy to turbulence in a critical layer (Clément and Thurnherr 2018)
We presented the first observations of dissipation rates in the long-lived anticyclonic vortex located in the deepest part of the Lofoten Basin, often referred to as the Lofoten Basin eddy (LBE)
Summary
The Lofoten Basin of the Norwegian Sea is surrounded by the main branches of the Norwegian Atlantic Current (NwAC) carrying warm and saline Atlantic Water (AW) along the shelf break with the slope current and along the Mohn Ridge with the front current (Fig. 1) (Orvik and Niiler 2002). Under the ‘‘Water-mass transformation processes and vortex dynamics in the Lofoten Basin of the Norwegian Sea (ProVoLo)’’ project, full-depth ocean microstructure profiles were collected in June 2016, to study the turbulent structure of the LBE in unprecedented detail. Observations include trapped near-inertial wave packets in a warm-core ring (Joyce et al 2013), dissipation rates elevated in a deep Southern Ocean eddy (Sheen et al 2015), increased shear variance caused by waves trapped in submesoscale cyclonic vortex filaments in the north wall of the Gulf Stream (Whitt et al 2018), and high dissipation rates away from the seafloor inside the midocean fracture zones, caused by transfer of nearinertial wave energy to turbulence in a critical layer (Clément and Thurnherr 2018). The VMP was deployed 10–15 min before a CTD/LADCP profile, giving collocated, approximately simultaneous measurements of stratification, currents, and microstructure. Good-quality measurements were averaged over the two estimates, except when the two measurements differed by more than a factor of 10, the minimum dissipation value was used
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