This is an NSF-funded collaborative project with Dr. Harper Simmons (U. Alaska), aiming at constructing a global map of low-mode internal tide energy flux and dissipation by the application of state-of-the-art techniques and a combination of satellite altimetry, mooring data, and a numerical model output.
About 1 TW (10^12 W) of barotropic tidal energy is converted to baroclinic motions in the deep ocean. Some of this energy is lost to dissipation in the nearfield, but the vast majority escapes the conversion region as low-mode internal tides, and propagates thousands of kilometers prior to dissipating. The fate of low-mode internal tides and the geography of the internal-tide induced ocean mixing are still largely unknown. It is believed that diapycnal mixing in the ocean interior is the driving force of the large-scale oceanic meridional overturning circulation (MOC), wherein about 30 Sv of deepwater forms at high latitudes and returns back to the surface at low latitudes, playing a critical role in the global climate system.
The global coverage of satellite altimeters makes them the only practical observational tool available for the task. Altimeters observe internal tides by detecting the internal tide-induced sea surface height variations. The internal tidal motions (e.g., isopycnal displacement, current) in the ocean interior are then obtained through the dispersion relation. However the poor spatial resolution of any single satellite, and the inability of altimetry to detect temporally incoherent signals, have hampered the interpretation of past altimetric estimates of low-mode internal tide energy and energy flux. The proposed work addresses these shortcomings in order to produce the needed global maps:
(1) To address the low-resolution problem, we will expand on our previous work (in which we used the T/P-Jason tandem mission) by combining multiple satellite altimetric data from T/P-Jason, T/P-Jason tandem, GFO, and ERS. We recently demonstrated the multisatellite technique in the North Pacific, demonstrating that spatial resolution is improved to the point where the altimetric estimates agree with high-resolution numerical models.
(2) To understand the loss of coherence of internal tide propagating in an ever-changing ocean, we will analyze a new high-resolution global simulation that includes a realistic internal tide field as well as realistic meso- and large-scale ocean circulations. The model estimate of how the non-uniform moving ocean makes internal tide incoherent will be validated by the analysis of several long moored time series collected around the globe.
We therefore believe that, with these improvements, our techniques are now up to the task of mapping the low-mode internal tide’s energy flux and dissipation on the globe. In doing so, this project will lead to a better understand the processes that affect the propagation and dissipation of internal tide on a global scale.
Multi-satellite altimeter data. (#alias to infinity period. &alias to annual period.)
PRELIMINARY REGIONAL RESULTS
Coherent mode-1 M2 internal tidal beams from the Hawaiian and Aleutian ridges, estimated from multi-satellite altimetry by our improved plane-wave fit technique. The colors denote amplitude and black contours are phase lines (at an interval of one wavelength) for the largest north-bound (left panel) or southbound (right panel) wave. [from Zhao, Alford, and Girton, Oceanography, 2012]. See also the movie.
Phase structure (color) and energy flux (arrows) of mode-1 M2 and O1 internal tides radiating from Luzon Strait (LS) westward into the South China Sea and eastward into the Pacific Ocean, illustrating multiple wave regimes and interference patterns. Both components are derived from multi-satellite altimetry by plane-wave fitting, with the fluxes and phase lines corresponding to the largest out-bound wave (i.e., directed away from Luzon Strait) found in each 120 km window.
Mode-1 M2 internal tide radiating northward (left panel) and southward (right panel) from Mendocino Escarpment. The southward component propagates more than 2000 km across the open ocean with flat bottom. On the contrary, the northward component dissipates within less than 500 km, likely due to the relatively rough bottom topography.