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T3: Energy Transfers in Gravity Currents

Principal investigators: Prof. Torsten Kanzow (University of Bremen/Alfred Wegener Institute for Polar and Marine Research), Dr. Martin Losch (Alfred Wegener Institute for Polar and Marine Research), Dr. Friederike Pollmann (Universität Hamburg)

The potential energy contained in gravity currents drives intense localized mixing, entrainment, and the circulation in the deep ocean. For the long-term goal of parameterizing gravity currents (see video below) in climate models, we need to understand different plume systems. In the first phase of the CRC, subproject T3 investigated energy transfers related to the Denmark Strait Overflow (DSO) plume, which is a key element of the global overturning’s northern limb.  In this second phase, we investigate energy transfers associated with the Weddell Sea Bottom Water (WSBW) gravity current, which is part of the overturning’s southern limb and supplies the densest waters to the global ocean.

Physical processes acting in overflows, mostly not resolved by climate models (Legg, et al., 2009, http://dx.doi.org/10.1175/2008BAMS2667.1).

The energy transfers in the WSBW plume include processes that are missing in the DSO plume system: (1) they are more strongly influenced by the interaction with tides and tidally generated internal gravity waves (IGWs) and (2) they give rise to a feedback loop, in which energy fluxes from eddies created within the plume set the conditions for the dense water formation feeding the plume. Characterising energy transfers in such different types of plume systems will allow us to later combine the results from the first and second phase into a comprehensive parameterization for plume-induced mixing.

 


Video "Overflows easily explained":

Objectives of Phase 1

Fig. 1: Topography of the study region in the northeastern Atlantic. The Denmark Strait Overflow Plume is highlighted in purple as it leaves the Nordic Seas via Den-mark Strait and descends southwestwards into the Irminger Basin. Red crosses mark the locations where we will make in-situ measurements.

The gravity current immediately downstream of Denmark Strait is a known for rapid water mass transformation and vigorous mixing (see Figure 1); its current variability is dominated by eddies on timescales of 2–10 days. Using observational and numerical modeling efforts we aim to understand the pathways and processes by which kinetic energy is transferred from the mesoscale eddy field to dissipative turbulent scales within the Denmark Strain Overflow (DSO) plume.

 

 

 

Main results of Phase 1

Fig. 2: Top: Entrainment rates derived from field observations (North et al., 2018). Region of strong entrainment and mixing is highlighted with light blue shading. Bottom: Energy density transfer between mean and eddy kinetic energy in the Denmark Strait overflow plume, derived from a regional numerical model (PhD student Deniz Aydin).

Entrainment rates in the interfacial layer (separating plume and warmer ambient waters) were determined through the analysis of over 30 years of vertical profiles of temperature, salinity and horizontal velocity taken within 200 km downstream of the Denmark Strait sill. Flow type (high vs low transport flow) explains most of the variability in the entrainment rates but the area approximately 75 to 175 km downstream (light blue shading in the upper plot of Fig. 2) was identified as an entrainment hotspot related to high vertical shear in the interfacial layer.  These results were corroborated with model simulations of the overflow, which showed that the largest energy transfers occurred in nearly the same downstream region (light blue shading in the lower plot of Fig. 2). Comparing model simulations with grid spacing (horizontal resolution) of 18, 9, 4, 2, and 1 km also implies that there is a convergence of energy transfers with higher resolution, as the computed transfers for the 1 and 2 km simulations display rather similar amplitudes with significantly larger values than those resulting from the 4 km simulation.

Fig. 3: The Hovmöller diagrams show potential temperature (top) and along-stream velocity (bottom), from a mooring located in the DSO plume 120 km downstream of the Denmark Strait sill (PhD student Stylianos Kritsotalakis ). White dashed lines indicate the upper limits of the bottom (BL) and interfacial (IL) layers of the DSO pl ume, respectively.

 

Mooring data collected in the DSO plume 120 km downstream of the Denmark Strait sill revealed that high (>1 m/s) plume velocities coincide with cyclonic eddying activity and low Richardson numbers (<0.25). Low Richardson numbers are a necessary condition for Kelvin-Helmholtz instabilities to occur, suggesting that the interplay of high plume velocities and cyclonic eddy activity provides favorable conditions for a downscale energy transfer toward turbulent mixing (Fig. 3). 

 

Fig. 4: Time series from consecutive vertical profiles collected 120 km downstream of the Denmark Strait sill. Hovmöller diagrams show conservative temperature (top) and absolute horizontal velocity (middle) with the boundaries of the well-mixed bottom (BL) and shear-stratified interfacial (IL) layers shown as solid and dashed lines, respectively. Bottom: Estimated internal wave energy in the ambient flow above the plume (green) (Postdoctoral researcher Rebecca Adam McPherson) and turbulent kinetic energy dissipation rates in the interfacial layer (grey) (North et al., 2018). A total of 31 profiles comprise this time series.

The role of internal waves in the transfer of energy to turbulence in the DSO plume can be observed in consecutive conductivity-temperature-depth (CTD) casts (Fig. 4, top panels), which show the temporal evolution of overflow structure and dynamics. Internal wave energy (EIW) above the overflow (Fig. 4, bottom) was generally enhanced when the plume was thick and propagating slowly (Fig. 4, middle panels), and decreased when the plume thinned and flowed faster. Conversely, turbulent kinetic energy dissipation rates in the interfacial layer (IL) were highest when the overflow was thinner and velocities were high, most likely caused by shear-driven turbulent mixing. This suggests that when the overflow was thicker and less vertically sheared, relatively more large-scale mean kinetic energy was converted to internal wave energy than to shear driven dissipation. The source of the internal waves, and their role in transferring energy away from the overflow, cannot be determined by observations alone. Thus, modelling efforts will be used in the future to examine the generation and impact of internal waves on the overall energy of the DSO.

Objectives of Phase 2

In this second phase, we investigate energy transfers associated with the Weddell Sea Bottom Water (WSBW) gravity current, which is part of the overturning’s southern limb and supplies the densest waters to the global ocean.


Our main hypotheses are:

  • Tides and topographic waves in the Weddell Sea contribute substantially to driving dense shelf water over the continental shelf break where it feeds the WSBW plume
  • Both eddies and IGWs have first-order effects on energy conversions in the WSBW plume. Numerical models need to include these effects to realistically simulate the characteristics of the WSBW plume
  • IGW-driven energy conversions in the plume substantially modify the feedback of eddy energy from the WSBW plume to the supply of Circumpolar Deep Water (CDW) to the shelf region, where dense water is formed
  • The nonlinear IGWs generated near their critical latitude play an important role in energy transfers and mixing in the Weddell Sea
     
Fig. 1: Eddy-rich snapshot of potential temperature (in degC) at z=-240 m from a global MITgcm LLC4320 simulation. The planned regional model domain is adapted to available observations (the location of a mooring array is indicated in black). The white boxes indicate representative areas for which the idealized models may be set up. The white dots mark available CTD casts.

We address these hypotheses by analyses of new and historical observational data, accompanied by idealized and realistic model simulations that also take into account tides (see figures 1 & 2). Specific model configurations will be established in order to represent the topography and impact of mixing hot spots associated with enhanced turbulence in the  WSBW plume. These simulations will also serve to study the performance of different parameterizations of eddy dissipation in both

Fig. 2: Potential temperature distribution and kinetic energy analysis from the mooring array shown in Figure 1.

 

 

high resolution grids and coarse grids used in climate modeling. The observations and model studies will contribute to understanding the energetics of mixing and plume dynamics.

Research Stay in Hokkaido by Nicolas Dettling (Nov 23)

My name is Nicolas Dettling, I am a PhD student in subproject T3 „Energy Transfers in Gravity Currents“. So far I have worked on applying and improving eddy parameterizations in the presence of gravity currents, such as the ones exporting dense water from the Antarctic continental shelves. Now, it is time to apply what we have found in idealised models to regional models of the Antarctic marginal seas.

For this purpose, I visited Prof. Yoshihiro Nakayama and his research group at the Institute for Low Temperature Science at Hokkaido University, Japan in November. Over the last years, Prof. Nakayama and his group have set up a number of regional ocean model simulations targeting key questions concerning the dynamics of the ocean around the Antarctic continental shelf and slope. Over the course of three weeks, I was introduced to a model of the Cape Darnley region, where dense water flows down the continental slope into the abyssal ocean, providing a nice test case for my previous parameterization work. I learned how to set up, run and interpret the model at different resolutions and we discussed the steps towards applying an eddy parameterization in the model. I am very grateful for the support and the fruitful discussions during my time at the institute.

The Hokkaido University campus is centred around a park and every morning I would walk along the Ginkgo Avenue enjoying the autumn colours on my way to work. Fortunately, there was also time to explore the City of Sapporo and the beautiful nature of Hokkaido, where the first snow of the season had already arrived. Luckily, the next Hokkaido Soup Curry or a bunch of Gyozas (fried dumplings) were never far away to recover after working or travelling.

I genuinely enjoyed to work in Japan and to engage in this cultural exchange. I would like to thank Prof. Nakayama and his group for their hospitality and I am very much looking forward to working together again in the future. Finally I want to thank the TRR 181 for funding the research visit.

To resolve or not to resolve?

I’m investigating effects of grid resolution on the modification of overflow and ocean energetics.

Deniz Aydin, PhD T3

Bathymetry of the study region with different resolutions.

I am Deniz and I work on the T3 ‘Energy transfers in gravity plumes’ project as a PhD candidate at AWI. In particularly we are interested in the Denmark Strait Overflow (DSO) which is between Greenland and Iceland. This location is special because the DSO carries most of the dense and cold Arctic water entering the North Atlantic. Thus contributing to the deep southward flowing part of the Atlantic meridional overturning circulation.

As soon as the dense water on the sill starts descending, it undergoes a significant amount of mixing and entrainment of ambient water. By 200km downstream of the sill, volume and tracer properties of the overflow water are substantially modified due to combination of different processes. In our subproject we try to understand the interactions of all these different processes at different scales using observational and numerical modeling analysis.

It’s difficult to properly represent overflows in a global ocean model with the coarse resolution climate models generally have. For my part in this subproject, I use a general circulation model (MITgcm) in a regional setup with a 1year of simulation period. I’m investigating effects of grid resolution on the modification of overflow and ocean energetics. For this purpose I use 6 different horizontal resolutions ranging from eddy resolving (1km) to coarse resolution (36km). At the moment, I am analyzing the results from higher resolution simulations. Soon coarser resolutions will come into the picture and analysis of eddy parameterization schemes along with them. My research will contribute to a better understanding of consequences of lacking smaller scale processes and better representation of them in coarser models.