T3: Energy transfers in gravity plumes

Principal investigators: Dr. Kerstin Jochumsen (Universität Hamburg), Prof. Torsten Kanzow (MARUM-AWI), Dr. Martin Losch (Afred-Wegener-Institute for Polar and Marine Research)

Dense gravity plumes are frequent phenomena in the deep ocean. Often they are associated with rapid water mass modification due to vigorous mixing and entrainment of ambient water. The plume immediately downstream of Denmark Strait is a prototype example for these processes (see Figure below); 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 this plume. We will identify the regions of enhanced entrainment of ambient waters into the plume because here the different dynamical regimes exhibit a particularly low degree of scale-separation between the mesoscale, turbulence and dissipative scales.

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 Denmark Strait and descends southwestwards into the Irminger Basin. Red crosses mark the locations where we will make in-situ measurements.

Our main questions are:

  • Where are the hot spots of entrainment along the descent of the overflow plume and which physical processes are active at these locations?
  • How do both plume-eddy and plume-topography interactions drive the energy transfer across the scales?
  • How much of the transfer from potential energy to kinetic energy and mixing can be attributed to gravity wave dissipation and how does this transfer compare to the effects of vertical current shear in overflow eddies?

We address these questions by analyses of new and historical observational data, accompanied by model simulations. Specific model configurations will be established in order to represent the topography and impact of mixing hot spots associated with enhanced turbulence in the Denmark
Strait Overflow plume (see general scheme below). These simulations will also serve to study the performance of different parameterizations of eddy dissipation in both high resolution grids and coarse grids used in climate modeling. The observations and model studies will contribute to understanding the energetics of mixing and overflow dynamics. The specific model simulations will be used as a test bed for coarse climate ocean models and hence contribute to the overall goal of the TRR to develop, test and implement new and consistent parameterizations for the effect of unresolved processes.

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

Reports from the scientific front

Investigating the Denmark Strait Overflow plume

By Ryan North

In October 2016 I joined the TRR 181 as a postdoc at the Universität Hamburg in the T3 subproject: Energy transfers in gravity plumes. Our subproject aims to improve our ability to parameterize the energetics and mixing within gravity plumes by investigating the Denmark Strait Overflow plume. This plume was chosen as an ideal study case because of its relevance to the global ocean circulation, and the long history of observational data in the Strait. My role within the subproject mainly involves working with this historical data and the collection of new data. The data will be used both on its own and for collaborative modelling work. Using the results of the observational and modeling components, we will investigate the role entrainment plays in the evolution of the plume. In particular, we are interested in investigating the hypothesis that enhanced entrainment occurs where the plume interacts with mesoscale eddies or topography. The modeling component will help to put the results in perspective across a range of scales, from the turbulent scale up to the mesoscale.

Prior to joining the Institute of Oceanography I followed a winding career path. Beginning at Canada’s Queen’s University, my career has taken me through structural and coastal engineering, lake, river and coastal hydrodynamic modeling, climate related hypoxia in lakes, and submesoscale eddies in the coastal ocean (at HZG in nearby Geesthacht). With this new position I have finally managed to move beyond inland waters and the coastal shelf break to reach properly deep water!

I am looking forwarding to meeting more members of TRR 181, and to opportunities to work together in the near future. Currently, I am onboard the FS Meteor helping out fellow TRR project members investigate filaments forming within the Benguela upwelling system off the coast of Namibia.