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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

Global assessment of semidiurnal internal tide aliasing in Argo profiles

Hennon, T.D., M.H. Alford, and Z. Zhao, "Global assessment of semidiurnal internal tide aliasing in Argo profiles," J. Phys. Oceanogr., 49, 2523-2533, doi:10.1175/JPO-D-19-0121.1, 2019.

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1 Oct 2019

Though unresolved by Argo floats, internal waves still impart an aliased signal onto their profile measurements. Recent studies have yielded nearly global characterization of several constituents of the stationary internal tides. Using this new information in conjunction with thousands of floats, we quantify the influence of the stationary, mode-1 M2 and S2 internal tides on Argo-observed temperature. We calculate the in situ temperature anomaly observed by Argo floats (usually on the order of 0.1°C) and compare it to the anomaly expected from the stationary internal tides derived from altimetry. Globally, there is a small, positive correlation between the expected and in situ signals. There is a stronger relationship in regions with more intense internal waves, as well as at depths near the nominal mode-1 maximum. However, we are unable to use this relationship to remove significant variance from the in situ observations. This is somewhat surprising, given that the magnitude of the altimetry-derived signal is often on a similar scale to the in situ signal, and points toward a greater importance of the nonstationary internal tides than previously assumed.

Measuring global ocean heat content to estimate the Earth energy imbalance

Meyssignac, B., and 36 others including Z. Zhao, "Measuring global ocean heat content to estimate the Earth energy imbalance," Front. Mar. Sci., 6, doi:10.3389/fmars.2019.00432, 2019.

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20 Aug 2019

The energy radiated by the Earth toward space does not compensate the incoming radiation from the Sun leading to a small positive energy imbalance at the top of the atmosphere (0.4–1 Wm-2). This imbalance is coined Earth’s Energy Imbalance (EEI). It is mostly caused by anthropogenic greenhouse gas emissions and is driving the current warming of the planet. Precise monitoring of EEI is critical to assess the current status of climate change and the future evolution of climate. But the monitoring of EEI is challenging as EEI is two orders of magnitude smaller than the radiation fluxes in and out of the Earth system. Over 93% of the excess energy that is gained by the Earth in response to the positive EEI accumulates into the ocean in the form of heat. This accumulation of heat can be tracked with the ocean observing system such that today, the monitoring of Ocean Heat Content (OHC) and its long-term change provide the most efficient approach to estimate EEI. In this community paper we review the current four state-of-the-art methods to estimate global OHC changes and evaluate their relevance to derive EEI estimates on different time scales. These four methods make use of: (1) direct observations of in situ temperature; (2) satellite-based measurements of the ocean surface net heat fluxes; (3) satellite-based estimates of the thermal expansion of the ocean and (4) ocean reanalyses that assimilate observations from both satellite and in situ instruments. For each method we review the potential and the uncertainty of the method to estimate global OHC changes. We also analyze gaps in the current capability of each method and identify ways of progress for the future to fulfill the requirements of EEI monitoring. Achieving the observation of EEI with sufficient accuracy will depend on merging the remote sensing techniques with in situ measurements of key variables as an integral part of the Ocean Observing System.

Energy flux observations in an internal tide beam in the eastern North Atlantic

Köhler, J., and 7 others including Z. Zhao, "Energy flux observations in an internal tide beam in the eastern North Atlantic," J. Geophys. Res., 124, 5747-5764, doi:10.1029/2019JC015156, 2019.

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1 Aug 2019

Low‐mode internal waves propagate over large distances and provide energy for turbulent mixing when they break far from their generation sites. A realistic representation of the oceanic energy cycle in ocean and climate models requires a consistent implementation of their generation, propagation, and dissipation. Here we combine the long‐term mean energy flux from satellite altimetry with results from a 1/10° global ocean general circulation model that resolves the low modes of internal waves and in situ observations of stratification and horizontal currents to study energy flux and dissipation along a 1000 km internal tide beam in the eastern North Atlantic. Internal wave fluxes were estimated from twelve 36‐ to 48‐hr stations in along‐ and across‐beam direction to resolve both the inertial period and tidal cycle. The observed internal tide energy fluxes range from 5.9 kW m-1 near the generation sites to 0.5 kW m-1 at distant stations. Estimates of energy dissipation come from both finestructure and upper ocean microstructure profiles and range, vertically integrated, from 0.5 to 3.3 mW m-2 along the beam. Overall, the in situ observations confirm the internal tide pattern derived from satellite altimetry, but the in situ energy fluxes are more variable and decrease less monotonically along the beam. Internal tides in the model propagate over shorter distances compared to results from altimetry and in situ measurements, but more spatial details close the main generation sites are resolved.

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