W2: Scattering and Refraction of Low-Mode Internal Tides by Interaction With Mesoscale Eddies

Principal investigators: Dr. Manita Chouksey (MARUM/University of Bremen), Prof. Dirk Olbers (MARUM/University of Bremen), Prof. Monika Rhein (MARUM/University of Bremen), Prof. Jin-Song von Storch (Max Planck Institute for Meteorology/Universität Hamburg)

Internal gravity waves in the ocean are generated by tides, wind, and interaction of currents with rough seafloor topography. Models predict a global energy supply for the internal wave field of about 0.7–1.3 TW by the conversion of barotropic tides at mid-ocean ridges and abrupt topographic features. Winds acting on the oceanic mixed layer contribute 0.3–1.5 TW and mesoscale flow over rough topography adds an additional amount of 0.2 TW. Globally, 1–2 TW are needed to maintain the observed stratification of the deep ocean by diapycnal mixing that results from the breaking of internal waves. Ocean circulation models show significant impact of the spatial distribution of internal wave dissipation and mixing on the ocean state, e.g. thermal structure, stratification, and meridional overturning circulation.

The scattering and refraction of low-mode internal tides by interactions with mesoscale eddies provides a direct and important energetic link between mesoscale processes and the internal wavefield. Despite their relevance for the geographical distribution and intensity of mixing in the ocean, these processes have not been well studied or understood. In the second phase of the TRR181 W2 aims to improve our understanding of the involved processes and to quantify the eddy-induced changes in the magnitude, direction, and vertical structure of internal tides in the mid-latitude ocean.

For this we will combine
(a) In-situ observations to infer eddy-induced refraction and changes in the vertical structure and magnitude of the internal tides, dissipation rates from finestructure, and spectral energy transfers.
(b) An ICON-o telescope simulation with a resolution of up to O(500m) that will include tides, resolve higher vertical modes and simulate interactions between eddies and internal tides.
(c) Kinetic equations in a modal framework, evaluated for different parameters of the eddy field to estimate the energy transfer between tide and eddies.

The Walvis Ridge region in the southeast Atlantic (Fig. 1) is chosen as a representative for the mid- latitude open ocean as here energetic low-mode \(M_{2}\) internal tides propagate away from the ridge and cross the path of eddies in the form of both Agulhas rings and other mesoscale features.

Fig. 1: M2 internal tide energy flux from STORMTIDE in the background showing the energetic low-mode internal tides generated at Walvis Ridge. Green stars denote positions of the planned moorings, white dots those of the PIES. The main pathway of the eddies is shown as blue shading, the black box denotes the focus area of the CRC SONETT cruise.

Research Stay in San Diego by Zoi Kourkouraidou (Feb 24)

Last February I had my first research visit. After participating at the "Ocean Sciences Meeting 2024" in New Orleans, I crossed the continent and landed in beautiful San Diego, California. I visited the MOD Lab at SCRIPPS Institute of Oceanography, a team of oceanographers, engineers and PhD students who work on multi scale ocean dynamics. 

Most specifically, I visited Dr. Amy Waterhouse who arranged for a rich and very fruitful schedule of meetings with both senior and early career researchers both from SCRIPPS and also from the University of San Diego (UCSD). I had the chance to learn about their work, was given a nice tour through their lab and also attended some of their seminars and one PhD defence. At the end of the week I was given the opportunity to present my own work in the CASPO seminar, where I got many interesting questions and inputs for my research. 

The week went by very fast unfortunately, but I'm still very grateful for having the chance to network with so many researchers, learn about their science and visit this legendary institute! On top of these, I will certainly not easily forget the beautiful walks along the La Jolla beach, the stunning sunsets and the unique experience of going surfing during lunch break! 

I am thankful to Amy for hosting me and taking care of my schedule and of course to the CRC181 for the funding.

Investigating internal wave energy fluxes

In my current work, I also look into the impact of mesoscale motion on the energy flux in this dataset.

Jonas Löb, PhD W2

My name is Jonas and I am a PhD Student in the subproject W2 “Low mode waves” in the working group Oceanography at the University Bremen. In this project I calculate low mode internal wave energy fluxes from mooring measurements and compare the results with measurements from satellite altimetry and a 1/10° ocean model (STORMTIDE2). Energy flux is an important quantity for these models because its divergence identifies sources and sinks.

Internal gravity waves occur all over the stratified ocean and can be grouped in different categories varying on their generation mechanism. I focus mainly on internal tides in the semidiurnal frequency M2 generated by the barotropic tides over rough topography. Internal tides are a response of the astronomical gravitational forces of the ocean via oscillations in the sea surface elevation with horizontal tidal currents through the entire water column. These waves in the stratified ocean take the form of standing vertical oscillations of horizontal currents, called modes. The “zeroth” (barotropic) mode of horizontal velocity corresponds to horizontal ocean currents that are uniform from top to bottom. The first depth dependent (baroclinic) mode is characterized by flow in one direction at the top and in the opposite direct at the bottom. Higher modes have a more complicated vertical structure and their phase speed decreases with increasing mode number. The vertical structure of a mode can be calculated by the stratification, and velocity profiles can be fitted onto a linear combination of these modes. Low mode motions contain appreciable energy but quickly propagate away laterally. To study these low mode internal waves, we deployed a mooring inside a tidal beam in the eastern North Atlantic, south of the Azores, where a seamount chain stands out as a generation site for internal tides. In our study region the energy flux correlates reasonably well in direction, coherent – uncoherent portioning and mode ratio between mooring and model time series and satellite data. With regard to the total energy flux, the model and satellite observations underestimate the flux compared to the in situ data.

In my current work, I also look into the impact of mesoscale motion on the energy flux in this dataset. A surface eddy was crossing the mooring, and in the process dampening the energy flux in the first two modes by about one third, while a passing subsurface eddy dampened the energy mainly in the second mode. These observations support the idea that eddy interactions transfer energy from low modes into higher modes that can lead to increased dissipation. An open question is how much of the energy converted from lower to higher modes result in local dissipation, which is a crucial information in creating energy consistent ocean-climate models.