Global Internal Tide Energy Flux and Dissipation from Satellite Altimetry

Zhongxiang Zhao1, Matthew H Alford2, James B Girton3, Luc Rainville1, Harper L Simmons4, Amy Frances Waterhouse2 and Caitlin Beth Whalen2, (1)Applied Physics Laboratory University of Washington, Seattle, WA, United States, (2)Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, United States, (3)University of Washington, Applied Physics Laboratory, Seattle, WA, United States, (4)University of Alaska Fairbanks, Fairbanks, AK, United States
Abstract:
We aim to quantify the internal tide’s contribution to deep-ocean mixing, as opposed to the wind’s contribution via near-inertial internal waves. We construct a global internal tide model consisting of M2, S2, O1 and K1, using 50 satellite-years of altimeter sea-surface height (SSH) measurements. Two-dimensional plane wave fits are employed to (1) suppress mesoscale contamination by extracting internal tides with both spatial and temporal coherence, and (2) separately resolve internal tides in multiple propagation directions. Energy and flux are computed from the internal tide’s SSH amplitude, using transfer functions derived from the climatological hydrographic profiles in the WOA 2013. The satellite products represent multiyear coherent internal tide fields, neglecting the incoherent component. M2 is the dominant constituent; however the contribution of S2, O1 and K1 is greater in a few regions (e.g., the western Pacific). M2 and S2 have similar, but different, geographic patterns. O1 and K1 internal tides mainly occur in the Pacific and Indian oceans. Global integration of the mode-1 M2 internal tide gives a lower bound energy of 36 PJ. Its residence time is estimated to be 1~1.4 days, equivalent to an average propagation distance of 400 km from its generation sources (compared to the longest propagation distance >3500 km). M2 internal tidal beams propagate across critical latitudes for PSI (28.8 S/N) with little energy loss. In the eastern Pacific Ocean, M2 internal tides lose significant energy in propagating across the Equator, likely due to the loss of coherence in the varying equatorial jets. In contrast, little energy loss is observed in the equatorial zones in the Atlantic, Indian, and western Pacific oceans. The dissipation rate (which includes the likely loss of coherence) is estimated from the divergence of energy flux and compared with recent observations using historical microstructure profiles and Argo float profiles, and a global numerical model GOLD.
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