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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


Air–Sea Momentum Flux in Tropical Cyclones

The intensity of a tropical cyclone is influenced by two competing physical processes at the air–sea interface. It strengthens by drawing thermal energy from the underlying warm ocean but weakens due to the drag of rough ocean surface. These processes change dramatically as the wind speed increases above 30 m/s.

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30 Mar 2018

The project is driven by the following science questions: (1) How important are equilibrium-range waves in controlling the air-sea momentum flux in tropical cyclones? We hypothesize that for wind speeds higher than 30 m/s the stress on the ocean surface is larger than the equilibrium-range wave breaking stress. (2) How does the wave breaking rate vary with wind speed and the complex surface wave field? At moderate wind speeds the wave breaking rate increases with increasing speed. Does this continue at extreme high winds? (3) Can we detect acoustic signatures of sea spray at high winds? Measurements of sea spray in tropical cyclones are very rare. We will seek for the acoustic signatures of spray droplets impacting the ocean surface. (4) What are the processes controlling the air-sea momentum flux?

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

Decomposition of the multimodel multidirectional M2 internal tide field

Zhao, Z., J. Wang, D. Menemenlis, L. Fu, S. Chen, and B. Qiu, "Decomposition of the multimodel multidirectional M2 internal tide field," J. Atmos. Ocean. Technol., EOR, doi:10.1175/JTECH-D-19-0022.1, 2019.

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22 Apr 2019

The M2 internal tide field contains waves of various baroclinic modes and various horizontal propagation directions. This paper presents a technique for decomposing the sea surface height (SSH) field of the multimodal multidirectional internal tide. The technique consists of two steps: First, different baroclinic modes are decomposed by two-dimensional (2D) spatial filtering, utilizing their different horizontal wavelengths; second, multidirectional waves in each mode are decomposed by 2D plane wave analysis. The decomposition technique is demonstrated using the M2 internal tide field simulated by the MITgcm. This paper focuses on a region lying off the US West Coast ranging 20° – 50°N, 220° – 245°E. The lowest three baroclinic modes are separately resolved from the internal tide field; each mode is further decomposed into five waves of arbitrary propagation directions in horizontal. The decomposed fields yield unprecedented details on the internal tide’s generation and propagation, which cannot be observed in the harmonically fitted field. The results reveal that the mode-1 M2 internal tide in the study region is dominantly from the Hawaiian Ridge to the west, but also generated locally at the Mendocino Ridge and continental slope. The mode-2 and mode-3 M2 internal tides are generated at isolated seamounts, as well as the Mendocino Ridge and continental slope. The Mendocino Ridge radiates both southbound and northbound M2 internal tides for all three modes. Their propagation distances decrease with increasing mode number: Mode-1 waves can travel over 2000 km; while mode-3 waves can only be tracked for 300 km. The decomposition technique may be extended to other tidal constituents and to the global ocean.

The global mode-2 M2 internal tide

Zhao, Z., "The global mode-2 M2 internal tide," J. Geophys. Res., 123, 7725-7746, doi:10.1029/2018JC014475, 2018.

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1 Nov 2018

The surface tide flowing over bottom topography converts part of its energy into the internal tide. The internal tide propagates away from the generation site and eventually dissipates in the ocean. The propagation distance may be up to 3,500 km. The global internal tide is very complicated because (1) the internal tide can be generated over numerous generation sites and (2) the internal tide is a superposition of orthogonal baroclinic modes. Previously, I have addressed the first issue by developing a plane wave analysis method, which is a variant of harmonic analysis. My previous studies focus on the dominant mode‐1 internal tide. In this study, I address the second issue. This is an important and challenging scientific question, because different modes have different vertical structures and dissipation rates. To fully understand the internal tide field, we should investigate the internal tide's generation and propagation for each mode. In this study, I construct the first global map of the mode‐2 M2 internal tide. I find that mode 2 makes significant contributions to internal tidal energetics and sea surface height variance. My satellite results contain rich information on the global internal tide and reveal some unprecedented fundamental features.

Deep sea currents driven by breaking internal tides on the continental slope

Xie, X., Q. Liu, Z. Zhao, X. Shang, S. Cai, D. Wang, and D. Chen, "Deep sea currents driven by breaking internal tides on the continental slope," Geophys. Res. Lett., 45, 6160-6166, doi:10.1029/2018GL078372, 2018.

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28 Jun 2018

Mooring data collected on the continental slope of the South China Sea show that along‐slope deep sea bottom currents are generated when large spring internal tides (internal waves with tidal frequency) are observed, with the maximum velocity amplitude exceeding 0.15 m/s. The observations are consistent with predictions that near‐bottom breaking of internal waves can result in generation of along‐slope flows when these waves obliquely approach the slope. A linear internal tide model in one horizontal dimension with realistic topography and stratification is used to show that the breaking of internal tides is likely due to near‐critical reflection on the slope. Combining the mooring observations and the model simulation, an along‐slope near‐bottom transport of ~0.5 Sv is estimated. Along‐slope bottom flows caused by breaking internal waves potentially provide a significant way to deform continental slopes and affect deep water exchange between the marginal sea and open ocean.

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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