Time modulation of parametrised diapycnal mixing over Reykjanes ridge

Diapycnal mixing of water masses in the ocean drives the global overturning circulation and vertical flux of physical and chemical tracers. It is generated by the breaking of internal waves, particularly due to internal tide. The spatial variation of diapycnal mixing is understood quite well; however, there is less information about its temporal variability. Here we create spatiotemporal maps of mixing variability by modulating static diapycnal mixing parametrisations with mooring data from the Irminger Sea. For the first case, the barotropic tide and buoyancy frequency are extracted, and a bottom buoyancy frequency is defined. The barotropic tide and bottom buoyancy are applied as a time modulation to a parametrisation for dissipation over abyssal hills. This dissipative power is distributed using a topography- and buoyancy-dependent vertical structure to give turbulence production, diffusivity, and vertical heat flux. We find that buoyancy structure explains mixing variability, diffusivity varies through time by up to two orders of magnitude, and that mixing can follow a normal or skewed distribution.For the second case, we extract energies at near-inertial wave frequencies on a monthly basis and use these to modulate a parametrised dissipative power for near-inertial waves. We find that mixing due to near-inertial waves is more strongly intermittent. These results show that a static diffusivity parametrisation does not capture the time variability of diapycnal mixing. Periods of elevated or reduced diffusivity have a strong impact on tracer flux. Therefore, we recommend the inclusion of temporal variability in model parametrisations of diapycnal mixing.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)