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Zhongxiang Zhao

Principal Oceanographer





Department Affiliation

Ocean Physics


B.S. Physics, Shandong University, 1994

Ph.D. Oceanography, University of Delaware, 2004


Monitoring Global Ocean Heat Content Changes by Internal Tide Oceanic Tomography

This study will obtain a 20-year-long record of global ocean heat content changes from 1995–2014 with a method called Internal tide oceanic tomography (ITOT), in which the satellite altimetry data are used to precisely measure travel times for long-range internal tides.

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29 Jul 2016

Ocean Heat Content (OHC) is a key indicator of global climate variability and change. However, it is a great challenge to observe OHC on a global scale. Observations with good coverage in space and time are only available in the last 10 years with the maturing of the Argo profiling float array. This study will obtain a 20-year-long record of global OHC changes from 1995–2014 with a method called Internal tide oceanic tomography (ITOT), in which the satellite altimetry data are used to precisely measure travel times for long-range internal tides. Just like in acoustic tomography, these travel times are analyzed to infer changes in OHC. This analysis will double the 10 years of time series available from Argo floats. More importantly, ITOT will provide an independent long-term, low-cost, environmentally-friendly observing system for global OHC changes. Since ocean warming contributes significantly to sea level rise, which has significant consequences to low-lying coastal regions, these observations have the potential for direct societal benefits. The project will communicate some of its results directly to the public. The team will make an educational animation showing how ocean warming is measured and how the novel ITOT technique works from the vantage point of space. This animation will be played for students visiting the lab, and in science talks and festivals in local K-12 schools. In addition, three summer undergraduate students will be trained in data analysis and interpretation, and poster presentation.

The analysis technique to be applied over the global ocean in this project is based on the preliminary regional analysis already conducted by this team. About 60 satellite-years of altimeter data from 1995-2014 will be analyzed. Specifically, it will (1) quantify annual variability, interannual variability, and bidecadal trend in global M2 and K1 internal tides, (2) construct the conversion function from the internal tide's travel time changes to OHC changes, and (3) construct a record of 20-year-long global OHC changes, and assess uncertainties using Argo measurements. The ultimate goal for this project is to develop the ITOT technique for future global OHC monitoring. This will improve our understanding of the temporal and spatial variability of global OHC, particularly in combination with in situ measurements from Argo floats, XBTs, and WOCE full-depth hydrography. The ITOT observations will provide useful constraints to ECCO2. The internal tide models may be used to correct internal tide noise in the Argo and XBT measurements. In addition, the monthly and yearly internal tide fields produced will provide constraints to global high-resolution, eddy-permitting numerical models of internal tides.


2000-present and while at APL-UW

Satellite investigation of the M2 internal tide in the Tasman Sea

Zhao, Z., M.H. Alford, H.L. Simmons, D. Brazhnikov, and R. Pinkel, "Satellite investigation of the M2 internal tide in the Tasman Sea," J. Phys. Oceanogr., 48, 687-703, doi:10.1175/JPO-D-17-0047.1, 2018.

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1 Mar 2018

The M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 15° toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5°S, 155.5°E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.

The life cycle of semidiurnal internal tides over the northern Mid-Atlantic Ridge

Vic, C., A.C. Naveira Garabato, J.A. Mattias Green, C. Springys, A. Forryan, Z. Zhao, and J. Sharples, "The life cycle of semidiurnal internal tides over the northern Mid-Atlantic Ridge," J. Phys. Oceanogr., 48, 61-80, doi:10.1175/JPO-D-17-0121.1, 2018.

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1 Jan 2018

The life cycle of semidiurnal internal tides over the Mid-Atlantic Ridge (MAR) sector south of the Azores is investigated using in situ, a high-resolution mooring and microstructure profiler, and satellite data, in combination with a theoretical model of barotropic-to-baroclinic tidal energy conversion. The mooring analysis reveals that the internal tide horizontal energy flux is dominated by mode 1 and that energy density is more distributed among modes 1–10. Most modes are compatible with an interpretation in terms of standing internal tides, suggesting that they result from interactions between waves generated over the MAR. Internal tide energy is thus concentrated above the ridge and is eventually available for local diapycnal mixing, as endorsed by the elevated rates of turbulent energy dissipation ε estimated from microstructure measurements. A spring–neap modulation of energy density on the MAR is found to originate from the remote generation and radiation of strong mode-1 internal tides from the Atlantis-Meteor Seamount Complex. Similar fortnightly variability of a factor of 2 is observed in ε, but this signal’s origin cannot be determined unambiguously. A regional tidal energy budget highlights the significance of high-mode generation, with 81% of the energy lost by the barotropic tide being converted into modes >1 and only 9% into mode 1. This has important implications for the fraction (q) of local dissipation to the total energy conversion, which is regionally estimated to be ~0.5. This result is in stark contrast with the Hawaiian Ridge system, where the radiation of mode-1 internal tides accounts for 30% of the regional energy conversion, and q < 0.25.

Propagation of the semidiurnal internal tide: Phase velocity versus group velocity

Zhao, Z., "Propagation of the semidiurnal internal tide: Phase velocity versus group velocity," Geophys. Res. Lett., 44, 11,942-11,950, doi:10.1002/2017GL076008, 2017.

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16 Dec 2017

The superposition of two waves of slightly different wavelengths has long been used to illustrate the distinction between phase velocity and group velocity. The first-mode M2 and S2 internal tides exemplify such a two-wave model in the natural ocean. The M2 and S2 tidal frequencies are 1.932 and 2 cycles per day, respectively, and their superposition forms a spring-neap cycle in the semidiurnal band. The spring-neap cycle acts like a wave, with its frequency, wave number, and phase being the differences of the M2 and S2 internal tides. The spring-neap cycle and energy of the semidiurnal internal tide propagate at the group velocity. Long-range propagation of M2 and S2 internal tides in the North Pacific is observed by satellite altimetry. Along a 3,400 km beam spanning 24° – 54°N, the M2 and S2 travel times are 10.9 and 11.2 days, respectively. For comparison, it takes the spring-neap cycle 21.1 days to travel over this distance. Spatial maps of the M2 phase velocity, the S2 phase velocity, and the group velocity are determined from phase gradients of the corresponding satellite observed internal tide fields. The observed phase and group velocities agree with theoretical values estimated using the World Ocean Atlas 2013 annual-mean ocean stratification.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center