Zhongxiang Zhao (University of Washington)
"Global M2 Internal Tides and Internal Tide Oceanic Tomography
A global map of mode-1 M2 internal tides is constructed using sea surface height measurements from multiple satellite altimeters during 1992–2012, representing a 20-year coherent internal tide field. A two-dimensional plane wave fit method is employed to 1) suppress mesoscale contamination by extracting internal tides with both spatial and temporal coherence and 2) separately resolve multiple internal tidal waves. Global maps of amplitude, phase, energy, and flux of mode-1 M2 internal tides are presented. Multiwave interference of some degree is widespread because of the M2 internal tide’s numerous generation sites and long-range propagation. Global integration of the satellite observations yields a lower bound energy of 36 PJ for the coherent mode-1 M2 internal tides. Satellite observed M2 internal tides compare favorably with mooring measurements and a global eddy-resolving numerical model.
Inspired by our advances in observing internal tides by satellite altimetry, a concept of internal tide oceanic tomography (ITOT) is proposed to monitor ocean warming. ITOT is similar to acoustic tomography bought up by W. Munk about 40 years ago, but that work waves are internal tides. ITOT detects ocean temperature changes by precisely measuring travel time changes of long-range propagating internal tides. The underlying principle is that upper-ocean warming strengthens ocean stratification and thus increases the propagation speed of internal tides. Two examples are presented to demonstrate the feasibility and usefulness of ITOT. In the eastern tropical Pacific, the yearly time series of travel time changes of the M2 internal tide is closely correlated with the ENSO index. In the North Atlantic, significant interannual variations and bidecadal trends are observed and consistent with the changes in ocean heat content measured by Argo floats. ITOT offers a long-term, cost-effective, environmentally friendly technique for monitoring global ocean warming.
(1) Zhao, Z., M. H. Alford, J. B. Girton, L. Rainville, and H. Simmons, Global observations of open-ocean mode-1 M2 internal tides, J. Phys. Oceanogr., 46, 1657–1684, doi: 10.1175/JPO-D-15-105, 2016.
(2) Zhao, Z., Internal tide oceanic tomography, Geophys. Res. Lett., 43, 9157–9164, doi:10.1002/2016GL070567, 2016."