W2: Energy transfer through low mode internal waves

Principal investigators: Prof. Monika Rhein (MARUM/University of Bremen), Prof. Jin-Song von Storch (Max Planck Institute for Meteorology)

Processes and observational techniques associated with the generation, propagation, and dissipation of near inertial waves and internal tides. Internal tides are induced by barotropic tides over topographic features (depicted by the bathymetry of a seamount). Wind generates inertial oscillations in the surface mixed layer that generate near inertial waves below. Both high and low modes are excited by wind and tides. Low mode waves propagate long distances, while higher modes have stronger shear that results in local dissipation and mixing. The pattern of vertical displacement of an internal M2 tide as inferred from satellite altimetry is shown at the bottom (data provided by B. Dushaw). Measurements of internal wave energy fluxes will be carried out by moored instruments or by repeatedly lowering the instruments from a ship over the duration of one or two tidal cycles.

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. Observations indicate that the local ratio of generation and dissipation of internal waves is often below unity and thus the energy available for mixing must be redistributed by internal tides and near-inertial waves at low vertical wavenumber that can propagate thousands of kilometers from their source regions. Eddy-permitting global ocean circulation models are able to quantify the different sources of energy input and can also simulate the propagation of the lowest internal wave modes. However, the variation of the internal wave energy flux along its paths by wave-wave interaction or refraction by mesoscale features as well as its ultimate fate by dissipation remains to by parameterized.

This project aims to quantify the generation and propagation of internal waves in the global ocean, study the pathways of radiated low mode internal waves including processes operating along the pathways, identify regions of sources and sinks, estimate the contribution to local dissipation and identify the involved processes.

For these purposes we will use

  1. dedicated global high resolution (1/10° or higher) model runs, with idealised forcing mechanisms
  2. observations of internal wave energy fluxes along paths where satellite altimetry shows beams of converging low mode internal waves
  3. and a combination of the model simulations with the available observations

to produce the best estimate of the global distributions of sources and sinks needed for an energetically consistent model of the diapycnal diffusivity induced by internal waves breaking.

Barotropic to baroclinic tidal energy conversion in W m^−2 (color scale is logarithmic) for the semidiurnal M2 tide from the high-resolution ocean circulation model STORMTIDE (Müller, 2013). Arrows denote energy flux (taken from Alford, 2003) for low mode internal tides from historical mooring records.
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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.

Propagation of long internal waves

The proper protocol and experiment setup for numerical experiments is crucial.

Zhuhua Li, Postdoc in W2

I am a postdoc at Max Planck Institute for Meteorology (MPI-M), working with Dr. Jin-Song von Storch. We care about the long internal waves in the ocean’s interior, which carry most of the internal wave energy. During their long-range propagation, their energy can be transferred to smaller scales, leading eventually to diapycnal mixing needed for supporting the global overturning circulation. Due to a lack of knowledge about the ultimate location and strength of their dissipation, their effect on mixing is generally parameterized by a spatially uniform diffusion in most ocean general circulation models (OGCMs). It is still a challenge to properly include their effects in OGCMs.

The high-resolution OGCMs, which directly resolve long internal waves (in addition to the general circulation), serve as an indispensable tool for investigating the fate of these waves. They allow us to study long internal waves in a realistic wave environment with realistic stratification and circulation. One such model is the 1/10° STORMTIDE model. This model simulates the long internal waves at tidal frequencies (internal tides) well (Fig. 1), in particular in regions where nonlinear interactions between internal tides and circulation are weak and linear waves predominate. This is reflected by the good agreement of the wavelengths of the most energetic mode simulated by the STORMTIDE model with those derived from linear theory (Fig. 2).

For the subproject W2, we will investigate the long internal waves generated by surface winds and tides within the same modeling framework. As a starting point, we analyze the global propagation of the long internal tides by using the energy flux simulated by the STORMTIDE model. This is a first step towards a better understanding of the sources and sinks of internal tides.

Vertically integrated kinetic energy (J/m2) of the M2 internal tides in logarithmic scales simulated by the 1/10° STORMTIDE model (Li et al. 2016). The M2 internal tides have a period of about 12.42 hours and they are the most energetic component among internal waves at all tidal frequencies.
Wavelengths (km) of the most energetic mode (mode 1) of the M2 internal tides (a) from the STORMTIDE simulation and (b) from linear internal wave theory. In brief: The STORMTIDE model simulates the M2 internal tides well (Li et al. 2016).
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