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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