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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

Internal tides can provide thermal refugia that will buffer some coral reefs from future global warming

Storlazzi, C.D., O.M. Sheraton, R. van Hooidonk, Z. Zhao, R. Brainard, "Internal tides can provide thermal refugia that will buffer some coral reefs from future global warming," Sci. Rep., 10, 13435, doi:10.1038/s41598-020-70372-9, 2020.

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10 Aug 2020

Observations show ocean temperatures are rising due to climate change, resulting in a fivefold increase in the incidence of regional-scale coral bleaching events since the 1980s; analyses based on global climate models forecast bleaching will become an annual event for most of the world's coral reefs within 30–50 yr. Internal waves at tidal frequencies can regularly flush reefs with cooler waters, buffering the thermal stress from rising sea-surface temperatures. Here we present the first global maps of the effects these processes have on bleaching projections for three IPCC-AR5 emissions scenarios. Incorporating semidiurnal temperature fluctuations into the projected water temperatures at depth creates a delay in the timing of annual severe bleaching ≥ 10 yr (≥ 20 yr) for 38% (9%), 15% (1%), and 1% (0%) of coral reef sites for the low, moderate, and high emission scenarios, respectively; regional averages can reach twice as high. These cooling effects are greatest later in twenty-first century for the moderate emission scenarios, and around the middle twenty-first century for the highest emission scenario. Our results demonstrate how these effects could delay bleaching for corals, providing thermal refugia. Identification of such areas could be a factor for the selection of coral reef marine protected areas.

Flow-topography interactions in the Samoan Passage

Girton, J.B., J.B. Mickey, Z.X. Zhao, M.H. Alford, G. Voet, J.M. Cusack, G.S. Carter, K.A. Pearson-Potts, L.J. Pratt, S. Tan, and J.M. Klymak, "Flow-topography interactions in the Samoan Passage," Oceanography, 32, 184-193, doi:10.5670/oceanog.2019.424, 2019.

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1 Dec 2019

Mixing in the Samoan Passage has implications for the abyssal water properties of the entire North Pacific — nearly 20% of the global ocean's volume. Dense bottom water formed near Antarctica encounters the passage — a gap in a ridge extending from north of Samoa eastward across the Pacific at around 10°S — and forms an energetic cascade much like a river flowing through a canyon. The 2011–2014 Samoan Passage Abyssal Mixing Experiment explored the importance of topography to the dense water flow on a wide range of scales, including (1) constraints on transport due to the overall passage shape and the heights of its multiple sills, (2) rapid changes in water properties along particular pathways at localized mixing hotspots where there is extreme topographic roughness and/or downslope flow acceleration, and (3) diversion and disturbance of flow pathways and density surfaces by small-scale seamounts and ridges. The net result is a complex but fairly steady picture of interconnected pathways with a limited number of intense mixing locations that determine the net water mass transformation. The implication of this set of circumstances is that the dominant features of Samoan Passage flow and mixing (and their responses to variations in incoming or background properties) can be described by the dynamics of a single layer of dense water flowing beneath a less-dense one, combined with mixing and transformation that is determined by the small-scale topography encountered along flow pathways.

Mapping internal tides from satellite altimetry without blind directions

Zhao, Z., "Mapping internal tides from satellite altimetry without blind directions," J. Geophys. Res., 124, 8605-8625, doi:10.1029/2019JC015507, 2019.

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1 Dec 2019

Satellite altimetry is one practical technique for observing internal tides on the global scale. However, it is a great challenge to extract weak internal tide signals. This paper presents a new technique for mapping internal tides from satellite altimeter data. Along‐track high‐pass filtering is needed to remove long‐wavelength nontidal noise and the barotropic tidal residual; however, the filter also removes internal tides having large angles with respect to satellite ground tracks. It thus causes blind directions in mapping internal tides from satellite altimetry: Generally west‐east propagating internal tides are missed. The new technique addresses the blind‐direction issue by replacing the problematic one‐dimensional (1‐D) high‐pass filter with a two‐dimensional (2‐D) band‐pass filter. This mapping technique is able to retrieve ubiquitous westbound and eastbound internal tides not captured in previous estimates. Long‐range westbound and eastbound waves travel over thousands of km from numerous generation sites such as the Emperor seamount chain, the Hawaiian Ridge, and the Kermadec trench. Evaluation using independent Cryosat‐2 data reveals that the new internal tide model may reduce more SSH variance than a model built in 2016 does in regions of strong internal tides. However, this mapping technique makes no improvement in strong boundary current regions, due to the dominance of mesoscale motions. Moreover, the new internal tide model contains leaked noise from westward propagating tropical instability waves (TIWs), which can be suppressed by prior along‐track high‐pass filtering. This paper suggests that better internal tide models may be constructed using both 1‐D and 2‐D filters with optimized parameters.

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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