W5: Internal Wave Energy Dissipation and Wavenumber Spectra: Adaptive Sampling in the Ocean Interior

Principal investigators: Prof. Ralf Bachmayer (MARUM/University of Bremen), Dr. Maren Walter (MARUM/University of Bremen)

The objective of W5 is to observe the spatial distribution of the (spectral) properties of the internal wave field and the energy dissipation in the ocean interior, i.e. below the mixed layer and the surface ocean. 

The spatial distribution of internal wave energy and energy dissipation in the ocean is an important benchmark for numerical ocean and climate models. Yet, observations of these parameters are largely confined to ship-based point measurements and profiles, lacking good spatial coverage and long term observations of temporal variability. In recent years, autonomous vehicles and in particular autonomous underwater gliders, due to their buoyancy driven propulsion mode and therefore nonexistent propeller induced vibrations, have proven to be a valuable platform to extend the range of observations. Besides providing valuable and accurate dissipation measurements, their long-term and long range capabilities allow  unattended observations away from vessels for weeks over ranges of hundreds of kilometres freeing up valuable ship time for complementary observations. For truly autonomous observations with these platforms, algorithms are needed that make use of the observed bulk parameters to steer the vehicles towards regions of interest and meaningful trajectories, e.g. follow an eddy or moving weather system. In the ocean interior (depth larger than 200 m), additional navigational capabilities are needed, to enable sustained observation at certain depth levels or follow signals without the need to return to the surface and break the observational record.

During Phase II, we used existing platforms to develop and test algorithms for path planning and adaptive sampling. During the SONETT I expedition we adjusted horizontal glider pathways to effectively navigate in strong currents in eddies. During SONETT II we used a more complex guidance algorithm that uses quasi-real time sensor based data to adjust the glider dive parameter for increased sampling density. In doing so, we obtained two high quality data sets of glider data (including dissipation), which will be further analysed during Phase III.

In Phase III, W5 will build on the results from Phase II to focus on selected processes that will benefit from targeted observations. Specifically, we will address the following science questions:

  • How is the background internal wave field and the associated energy dissipation affected by the presence of mesoscale structures? 
  • What is the energy input into the interior internal wave field by severe storms?
  • How can we fuse multi-source information (from models, satellite observations, ...) with human input to best (cost/energy/time-efficiently) plan trajectories for autonomous underwater vehicles to sample these critical processes?
  • How can the autonomous underwater vehicle adapt the planned trajectory based on real-time measurements?

Upper-ocean energy spectrum, flux & dissipation

The use of a new technology combined with new sampling algorithms potentially offers unprecedented insights into deep ocean mixing and internal wave climate.

Ilmar Leimann, PhD L3 & W5

Hi! My name is Ilmar and I work as a PhD student at the MARUM/University of Bremen. I am supervised by Dr. Maren Walter (MARUM/University of Bremen) and Dr. Alexa Griesel (Universität Hamburg) and am part of the TRR subprojects L3
entitled “Meso- to Submesoscale Turbulence in the Ocean” and W5 „Internal Wave Energy Dissipation and Wavenumber Spectra: Adaptive Sampling in the Ocean Interior “.

Before I joined TRR, I lived in Kiel, where I got a bachelor degree in Physics of Earth and master degree in Climate Physics: Meteorology and Physical Oceanography at Christian Albrechts University Kiel & GEOMAR. I started my work as a part of TRR in September 2022.

In the first phase of L3, we assessed turbulence regimes with a focus on the Benguela upwelling region. Using a new scaling method and with adequate subsampling of the deployed surface drifters, we estimated a consistent energy transfer rate and identified an inverse cascade for scales 30-500 km close to the upwelling current. Now Our aim in the second phase is to extend the Lagrangian analyses and apply the structure-function diagnostic (in addition to the classical Lagrangian dispersion estimates) in an area offshore from the Benguela region that is characterized by high internal tide and eddy activity but without a deep baroclinic current. In a concerted effort (targeted measurements with gliders, ship ADCP, drifters) we will quantify horizontal wavenumber spectra for the upper ocean in the Walvis Ridge Region in close collaboration with W5 and W2. The subsampling methods developed from the analyses in the Benguela and Walvis Ridge regions, together with high-resolution modelling, will be used to extrapolate to the global ocean using the global drifter program.

The W5 Project is concerned with the shape of the internal wave energy spectrum, where our aim is to simultaneously observe the oceanic energy spectrum below the submesoscale range and the spatial distribution of energy dissipation, using adaptive/reactive sampling to guide the observations. For this purpose, we will deploy a new hybrid pelagic glider (developed by Prof. Ralf Bachmayer) using an innovative approach of combining advanced numerical model informed sampling techniques in real-time to observe internal wave spectra and turbulence in the ocean interior. As a key sensor, a pressure rated microstructure probe will be integrated into the pelagic glider system; this use of a new technology combined with new sampling algorithms potentially offers unprecedented insights into deep ocean mixing and internal wave climate. Obtained observational data will be contextualized by idealized and regional numerical modelling studies carried out in L3 and L2 and the results of this project will complement the observations, that will be jointly used to construct the upper- and pelagic oceanic energy spectrum within L3, and the observations towards obtaining a local energy budget.

  • Löb, J., Köhler, J., Walter, M., Mertens, C., & Rhein, M. (2021). Time Series of Near-Inertial Gravity Wave Energy Fluxes: The Effect of a Strong Wind Event. J. Geophys. Res. - Oceans 126, e2021JC01747, doi: https://doi.org/10.1029/2021JC017472.

  • Köhler J., Voelker, G.S. & Walter, M. (2018). Response of the internal wave field to remote wind forcing by tropical cyclones. J. Phys. Oceanogr. 48, 317-328, doi: https://doi.org/10.1175/JPO-D-17-0112.1