Propagation of Low-Mode Internal Waves through the Ocean

Luc Rainville Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, California

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Robert Pinkel Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, California

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Abstract

The baroclinic tides play a significant role in the energy budget of the abyssal ocean. Although the basic principles of generation and propagation are known, a clear understanding of these phenomena in the complex ocean environment is only now emerging. To advance this effort, a ray model is developed that quantifies the effects of spatially variable topography, stratification, and planetary vorticity on the horizontal propagation of internal gravity modes. The objective is to identify “baroclinic shoals” where wave energy is spatially concentrated and enhanced dissipation might be expected. The model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field. The refraction of tidally generated internal waves at the Hawaiian Ridge is examined using an ensemble of mesoscale background realizations derived from weekly Ocean Topography Experiment (TOPEX)/Poseidon altimetric measurements. The path of mode 1 is only slightly affected by typical currents, although its phase becomes increasingly random as the propagation distance from the source increases. The effect of the currents becomes more dramatic as mode number increases. For modes 3 and higher, wave phase can vary between realizations by ±π only a few wavelengths from the source. This phase variability reduces the magnitude of the baroclinic signal seen in altimetric data, creating a fictitious energy loss along the propagation path. In the TOPEX/Poseidon observations, the mode-1 M2 internal tide does appear to lose significant energy as it propagates southwestward from the Hawaiian Ridge. The simulations suggest that phase modulation by mesoscale flows could be responsible for a large fraction of this apparent loss. In contrast, northeast-propagating internal tides encounter a less energetic mesoscale and should experience limited refraction. The apparent energy loss seen in the altimetric data on the north side of the ridge might indeed be real.

Corresponding author address: Luc Rainville, Woods Hole Oceanographic Institution, MS #21, Woods Hole, MA 02543. Email: lrainville@whoi.edu

Abstract

The baroclinic tides play a significant role in the energy budget of the abyssal ocean. Although the basic principles of generation and propagation are known, a clear understanding of these phenomena in the complex ocean environment is only now emerging. To advance this effort, a ray model is developed that quantifies the effects of spatially variable topography, stratification, and planetary vorticity on the horizontal propagation of internal gravity modes. The objective is to identify “baroclinic shoals” where wave energy is spatially concentrated and enhanced dissipation might be expected. The model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field. The refraction of tidally generated internal waves at the Hawaiian Ridge is examined using an ensemble of mesoscale background realizations derived from weekly Ocean Topography Experiment (TOPEX)/Poseidon altimetric measurements. The path of mode 1 is only slightly affected by typical currents, although its phase becomes increasingly random as the propagation distance from the source increases. The effect of the currents becomes more dramatic as mode number increases. For modes 3 and higher, wave phase can vary between realizations by ±π only a few wavelengths from the source. This phase variability reduces the magnitude of the baroclinic signal seen in altimetric data, creating a fictitious energy loss along the propagation path. In the TOPEX/Poseidon observations, the mode-1 M2 internal tide does appear to lose significant energy as it propagates southwestward from the Hawaiian Ridge. The simulations suggest that phase modulation by mesoscale flows could be responsible for a large fraction of this apparent loss. In contrast, northeast-propagating internal tides encounter a less energetic mesoscale and should experience limited refraction. The apparent energy loss seen in the altimetric data on the north side of the ridge might indeed be real.

Corresponding author address: Luc Rainville, Woods Hole Oceanographic Institution, MS #21, Woods Hole, MA 02543. Email: lrainville@whoi.edu

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