Uncovering Ocean Mixing near Rough Bathymetry : Using Broadband Acoustics

Sammanfattning: Ocean mixing related to rough bathymetry is highly dynamic and exhibits large spatial and temporal variability. Therefore, established in-situ methods as well as numerical models often lack the resolution to capture this type of mixing. In this thesis, acoustics are used to observe, map and quantify stratified mixing at unprecedented resolution. Acoustic broadband data from a Simrad EK80 and co-located microstructure data from a Sea & Sun Technology MicroStructure profiler (MSS) were collected during two cruises on R/V Electra in Feb-March 2019 and 2020 in the Southern Quark region in the northern Åland Sea, Baltic Sea. In the first manuscript (M1), an existing acoustic model to quantify turbulent mixing from acoustic backscatter is revisited and applied to the co-located data sets. Possibilities and limitations of applying the acoustic model are investigated in detail. In a following case study (M2), the acoustic model is applied to a subset of the data where rough bathymetry reaches into stratified flow and highly increases mixing across the halocline. The dominant mixing mechanism is revealed to be wake vortices and their impact is estimated using a diffusion model. The final case study (M3) emphasizes the impact of mixing in the Southern Quark on exchange processes between the distinctly different Northern Baltic Proper and the Bothnian Sea. The acoustic observations uncover the detailed structure (M1-M3) and temporal development (M3) of turbulent diapycnal mixing in heterogeneous flow over rough bathymetry. This thesis is a step towards increased applicability and automatized analysis of acoustic broadband data for identifying and quantifying turbulent diapycnal mixing.