Examining the Mixing Mechanisms Forced by Non-Linear Internal Waves

The internal wave field is a dominant driving force for turbulent diapycnal mixing in the ocean interior, with particularly intense mixing associated with the energetic flows generated by non-linear internal waves (NLIW). Despite its importance, field measurements of diapycnal mixing are scarce, difficult to interpret and plagued by uncertainty introduced by mixing model assumptions. Furthermore, difficulty in resolving the smallest turbulent scales have impeded mixing measurements in highly energetic flows. These limitations have led to a lack of understanding of the mechanisms that generate diapycnal mixing and control its spatio-temporal variability. Recent developments have improved the ability of moored platforms to obtain estimates of mixing from both fine- and micro-structure measurements, thus providing the opportunity to study the mechanisms through which NLIW generate mixing.

Here we examine NLIW-forced diapycnal mixing at a continental shelf ridge approximately 45km SSE of the Imperieuse Reef on the Australian North West Shelf (NWS) over a 30 day record in March 2019. Three moorings on a cross-slope transect at the 150, 200 and 330 m depth contours captured the NLIW dynamics. Fine- and micro-structure turbulence measurements at the 150 and 200 m site provide estimates of diapycnal mixing. The field observations revealed an energetic environment in which NLIWs with amplitudes as great as 80 m shoal up the ridge, with some waves becoming unstable and breaking, generating highly turbulent flows and intense mixing (e.g. see attached figure). Furthermore, observations show both mode-1 and mode-2 NLIWs shoaling up the ridge, allowing us to comment on the modal-variability of diapycnal mixing at our site. These results help us to better understand how the mechanisms that generate turbulent diapycnal mixing influence its intensity and spatio-temporal variability.