Reports Area W

A Memory of Pre-Pandemic Times and a Glimpse at the hopefully soon-to-be Future: My Visit at MIT and the AGU Fall Meeting 2021

With two successful talks, one on my research at the CRC181 and one on my science policy activities, I am more than happy with the received exposure and appreciation of our work.

Georg Sebastian Voelker, Postdoc in W1, S2

Having been in the home office for a long time during the last two years I am sure everyone wonders: Remember how things were before the virus hit? And how things will be afterwards? I was asking myself the very same questions while having a travel grant available I had won mid 2020 from the DFG research unit MS-GWaves which was still sitting in the accounts waiting to be used. My visit had been planned for a long time but had also been delayed by the pandemic. So when the US started opening up to foreign visitors in late summer 2021 I decided to try to move forward with the plan we had been setting aside for so long. And despite the restrictions and insecurities linked to long distance travel I should very soon be rewarded. On November 8 I boarded an airplane to Cambridge, Massachusetts to visit the long research partner of out group, Prof. Triantaphyllos Akylas at the Massachusetts Institute of Technology.

Our former and ongoing research project with T. R. Akylas is concerned with the background-modulated wave-wave interaction of internal gravity waves. In a previous manuscript we had been able to show that wave modulation by a sheared mean flow can significantly inhibit the energy exchange through a near-resonant triadic interaction. However, the assumptions of Boussinesq dynamics and a constant stratification limited the applicability of the findings to the atmospheric context. We thus took on the task to extend the theory to semi-incompressible dynamics with both a variable stratification and sheared mean winds. Having derived the theory beforehand we used the 5 weeks together at MIT to explore the combined effects of the modulation by the wind and the stratification on the wave interaction. Interestingly the two modulation mechanisms can counteract each other opening up the possibility of strong interactions in regions with both changing stratification and strong shear. As the tropopause region typically exhibits these features it is of particular interest to be studied. A manuscript is now in preparation and planned to be submitted later this year.

Having already traveled to the US another possibility opened for me: the in-person attendance of the fall meeting of the American Geophysical Union in New Orleans. Traveling to conferences has always been one of my favorite parts of being a scientist. I am particular fond of getting to know places and people, exchanging ideas about our research, networking among peers and like-minded people and making friends throughout the world. The idea of attending a conference on site for the first time in two years was therefore especially tempting for me. Even though it came with the huge insecurity of sharing the venue with another 10,000 people during a pandemic the stringent health policies helped keeping the participants safe and the number of infections low.

With two successful talks, one on my research at the CRC181 and one on my science policy activities, I am more than happy with the received exposure and appreciation of our work. Fostering existing connections and forging new ones additionally rendered the conference experience as a very positive one. But maybe most importantly, I also realized what I had been missing out in the past months. Even though video conferences can account for the majority of the scientific collaboration it will not be able to replace the experience of and the human relationships associated to a person to person contact. Partnerships are build on these relationships and I am hoping that there will be a time soon where we can find a way to get back together. Personally I feel motivated to move forward and make progress in ways that I had not expected when I boarded that airplane on November 8. I would therefore like to particularly thank the CRC181, the research group MS-GWaves, the WilhelmHeraeus Visiting Professorship program and not at last Prof. Ulrich Achatz and Prof. Triantaphyllos Akylas for enabling this collaboration and the conference participation for me.

Internal Gravity Wave Interaction, Propagation, and Breaking in the Atmosphere

It has becomes clear that, even though the vertical propagation of gravity waves may be dominant for many atmospheric phenomena, the horizontal propagation cannot be neglected.

Georg Sebastian Voelker, Postdoc W1

My name is Georg Sebastian and I am currently a postdoc in the TRRENERGYTRANSFERS, working in the project W1: “Gravity Wave Parameterization for the Atmosphere” under the supervision of Prof. Ulrich Achatz at the Goethe University Frankfurt am Main.

I had started out by studying the generation of internal gravity waves below the ocean surface by wind during my PhD under the supervision of Maren Walter. Recently, I moved to working on internal gravity waves in the atmosphere. Thus having studied both physical oceanography and atmospheric dynamics I am keen to investigate internal gravity waves in both environments considering various aspects of their dynamics.

The W1 Project is concerned with the lateral propagation of internal gravity waves in the atmosphere. In particular we employ the ray-tracing algorithm MS-GWaM to model sub-grid scale gravity waves including their transient propagation characteristics. That is, we model the lifetime of gravity waves including their interaction with the background – such as Doppler shifting or wave modulation by the mean flow.

In contrast, current state of the art parameterizations of gravity waves in atmospheric models assume that gravity waves propagate in the vertical instantaneously neglecting their finite vertical group velocity. Moreover these parameterizations do not allow for a horizontal propagation at all. However, it has becomes clear that, even though the vertical propagation of gravity waves may be dominant for many atmospheric phenomena, the horizontal propagation cannot be neglected. With the implementation of these effects in the numerical weather forecasting code ICON-NWP we hope to support that finding and gain new insight into the net effects and importance of the horizontal propagation.

Propagation is, however, only a part of a gravity wave’s life story. On its journey it can undergo a myriad of processes. Of special interest in various contexts are for instance the triad resonant interaction (TRI) and finally its breaking mechanisms.

Recently, we successfully described resonantly interacting gravity wave packets using a ray tracing algorithm utilizing weakly non-linear WBKJ theory in a Boussinesq environment including a slowly varying background flow. While interacting, the triad members are simultaneously modulated by a horizontal jet, leading to a reduction in energy exchange as the waves spectrally pass through the exact resonance conditions.

Moreover we are working on identifying instability mechanisms for strongly non-linear gravity waves in the vicinity of a mean-flow jet. Our theoretical study shows that in contrast to the upper jet edge the lower jet edge can sustain a novel type of modulational instability. New numerical evidence also shows that breaking mechanisms at the jet edges have distinct structures and might be associated to modulational instabilities or TRI (see Fig. 1 a and b).

This ongoing work is-among others–conducted in collaboration with Gergely Bölöni (DWD), Triantaphyllos Akylas (MIT, MA, USA), Mark Schlutow (FU Berlin, GER), and Ulrich Achatz (Goethe Universität Frankfurt).

Figure 1: Simulation of a strongly nonlinear, stationary internal gravity waves breaking in the vicinity of the lower (a) and upper (b) jet edge. Depicted are the vorticity and contours at a Richardson number Ri=0.25.

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.

The fate of low mode internal tides

The major goal of our subproject is to study the fate of low mode internal tides and the processes that operate along their pathways.

Janna Köhler, Postdoc in W2

I am an observational physical oceanographer working as a Postdoc at the University Bremen. Prior to joining the TRR I mainly studied temporal variability of internal waves using time series of moored instruments, and related the observed changes in the internal wave energy to generation processes such as topography-current interaction or energy input into the internal wave field by the wind.

In the TRR I am part of the subproject W2 “Energy transfer through low mode internal waves”. Low mode internal waves possess a major part of the entire energy of the internal wave field, which makes them an important component of the oceanic energy pathways.

The major goal of our subproject is to study the fate of low mode internal tides and the processes that operate along their pathways. A seamount south of the Azores provides a good study case for these processes, as it is one of the main generation sites of internal tides in the Atlantic. Here we will conduct shipboard measurements and deploy a mooring for about one year to study spatial and temporal variability of internal tide energy. We will then compare the observed spatial and temporal variability along the tidal beam with output of dedicated runs of the STORMTIDE model carried out by our project partners at the MPI in Hamburg.

 

M2 internal tide energy flux from the STORMTIDE model (Müller 2013) with the planned cruise track (white line) and mooring position (black triangle). References: Müller, M. "On the space-and time-dependence of barotropic-to-baroclinic tidal energy conversion." Ocean Modelling 72 (2013): 242-252.

Implementation of Lee Waves in IDEMIX

I’m investigating what and how big of a role lee waves play in transferring energy between large scale geostrophic motions and scmall scale turbulent mixing.

Thomas Eriksen, PhD W4

The purpose of my project is to investigate what and how big of a role lee waves play in transferring energy between large scale geostrophic motions and small scale turbulent mixing. Lee waves are formed when geostrophic motions interact with bottom topography. They radiate away from the topography and eventually break. When they break, the kinetic energy that they contain is used for dissipation, which, ultimately, raises potential energy. The issue of their role in the general circulation has been raised due to observed increased mixing rates near the ocean bottom in the Drake Passage and the Scotia Sea.

Previous estimates of the energy transfer from geostrophic motions into lee waves are around 1/3 of the energy input into gravity waves from winds. However, they are very few and differ roughly by a factor of 4. Furthermore, this energy transfer estimate has so far only been diagnosed and not used as in integral part of an ocean model. The contribution of lee waves in driving the large scale motions themselves – the overturning circulation, for example – are therefore largely unknown. The proper way of including lee waves in an energetically consistent ocean model would thus be to diagnose the energy c o n t a i n e d in lee waves e v e r y w h e r e in the ocean, let this energy travel and eventually be used for dissipation – in my case using an internal wave model – and then subtract it from its source.

This is exactly what is done in my model. The objective of my study is therefore to extend the IDEMIX model with an inclusion of lee wave energetics. This means that the energy being transferred into lee waves will be able to affect the rest of the ocean through diapycnal diffusivity – similarly to other types of gravity waves.

So far in my study, I have diagnosed the global energy transfer into lee waves to around 0.3TW. This is in accordance with previous estimates. The implementation of lee wave energetics into IDEMIX is underway. The lee wave energy flux is split into four directional compartments (N, S, E, W) will enter the gravity wave field as a bottom boundary flux, and the wave energy will thus be able to travel in the same manner as energy from other gravity waves. This is a fundamentally different way of treating lee waves compared to previous studies.

The next step is to study the differences in diapycnal diffusivity in model runs with and without lee waves. To what degree lee waves are able to account for the observed increased mixing rates in the deep Southern Ocean is till an open question, which I would like to answer. After this, I would like to address the question of what role lee waves play in setting the overturning circulation.

How the background mean flow effects internal gravity waves

From my work, hopefully general rules may be seen that can be included in parameterisations for internal gravity waves.

Rachael Ewins, PhD in W4

I am investigating the effect background mean flow has on the propagation of internal gravity waves. From this hopefully general rules may be seen that can be included in parameterisations for internal gravity waves. For this ray tracing is used to follow the positions and properties of wave packets that interact with an idealised current.

The test wave packets are populated randomly over a range of physical positions and also phase space, which allows exploration of the importance to various properties to how the test wave packets interact with the background current. The key property that is being tracked is the energy of the packets and from this the transfer of energy to and from the current can be seen.

Ray tracing simply propagates the position and wave numbers of the wave packets over a series of time steps given that background properties of background flow velocity, the local buoyancy frequency. The energy of the wave packets can be followed due to the conservation of Action. The results means that individual wave packets can be followed to different end conditions namely critical layer absorption, wave capture or refraction away from the current flow. The net energy transfer from the waves to the background flow (or from) can be seen by the end energy of the waves that enter critical layers or are captured by the current.

By varying the properties of the background current the effects of various shears in the current can be seen which will lead to more information about the key properties of both internal wave and background flow that lead to wave captures and critical layer absorption. In addition the background flow can be changed into configuration to simulate eddies, using the same processes.

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No reports available.