Aspects of Wave Radiation in the World Oceans
- (Woods Hole Oceanographic Institution)
The long-term goal of this project is to observe and understand radiating barotropic and baroclinic Rossby waves in the world oceans. Our emphasis will be on the properties and consequences of waves that radiate away from regions of vigorous instability, e.g., mid-latitude western boundary currents and equatorial currents. The data analysis will draw primarily on along-track and gridded altimetry products and will be complemented by theoretical and numerical analysis.
Our hypothesis is that regions of especially vigorous instability (the major western boundary currents, the Southern Ocean, and the equatorial current systems) radiate energy into the more quiescent ocean interior (Pedlosky, 1977; Harrison and Robinson, 1979), and that this nonlocal effect of instabilities contributes to the eddy field and other variability in less energetic, interior regions of the oceans. Some evidence for nonlocal influence of Gulf Stream instabilities of this sort was discussed by Bower and Hogg (1992). That study was based on isolated, moored velocity records. While the statistical evidence of remotely forced radiation at the mooring sites is in some ways quite detailed, it is nevertheless not particularly compelling or lucid because the propagation path of the waves could not be directly traced back to the generation site with field data existing in the 1980s. The modern altimetry record can enable new progress in understanding the extent to which radiation from the major unstable current systems contributes to variability elsewhere.
A recent example of the promise of satellite altimetry for shedding light on radiating instabilities comes from an analysis by Farrar (2011), which showed that formation of Tropical Instability Waves (TIWs) in the equatorial Pacific leads to northward radiation of barotropic Rossby waves. That analysis showed that sea level variability near Hawaii is coherent with sea level some 1600 km closer to the equator in the zonal-wavenumber/frequency bands associated with TIWs. The sea level signal is transmitted northward via barotropic Rossby waves. These waves are present to at least 20*N, and new preliminary evidence discussed here suggests the waves may be an important signal all the way north to the Gulf of Alaska.
The proposed work would use the altimetry data in a novel way to allow insight into the existence, properties, and dynamics of radiating, wave-like variability in the world oceans. The basic idea of the analysis approach is to perform cross-spectral calculations between locations of vigorous instability and all other locations to isolate distant signals associated with radiation from the instabilities. We plan to start with a detailed observational and theoretical process study of the northward radiation from the unstable equatorial currents. Our preliminary results are consistent with the hypothesis that the instability of the equatorial current system and its subsequent northward radiation of 30-day barotropic Rossby waves fills the North Pacific with barotropic, 30-day variability, but the amplitude and phase of this variability is influenced in a complicated way by refraction, reflection, and wave interference. Later in the project, we will use the insights obtained from the process study to examine radiation from other major unstable currents of the world oceans.