Dongxiao Zhang

Physical Oceanographer

Eyes on the ocean in a changing climate

DongxaioDr. Dongxiao Zhang is a physical oceanographer studying large-scale ocean circulation, its roles in climate variation, and impacts on marine ecosystem and chemistry.

He earned his Ph.D. at the Rosenstiel School of Marine and Atmospheric Sciences, University of Miami. As a student, Dongxiao worked on the project of PCM1 under Professors Tom Lee and Bill Johns, as a part of World Ocean Circulation Experiment (WOCE). The PCM1 was the first current meter array to measure the full cross-section of Kuroshio on seasonal to interannual time scales. The Kuroshio is a strong Western Boundary Current in the western Pacific, which transports an enormous amount of heat from the tropics to higher latitudes, to keep the North Pacific warm and our Northwest climate mild in winter. His study shows surprisingly large amplitude of fluctuation in the current strength and its northward heat transport. The revealed close interaction of the strong current and mesoscale ocean eddies suggests the difficulty for large scale climate models, which often do not have enough resolution, to faithfully simulate the Kuroshio current system.

Trained as an observational oceanographer, he fully understands the difficulties in obtaining data from the seas and always tries to make full use of this valuable data. Attracted by the Tropical Atmosphere and Ocean (TAO) array maintained by JISAO and PMEL scientists.

Dongxiao came to JISAO to work on the tropical oceans, which can perturb weathers around the globe through tropical phenomena such as ENSO. The TAO array revolutionized the way oceanographers monitor the ocean from the depth. It is now operational and generously shares its data with weather forecast centers around the world in real time. Dongxiao combined the modern TAO dataset with historical hydrographic surveys by research vessels, to document the changes of upper ocean circulation that brings cold subtropical waters to the tropical Pacific during the last 50 years. He found a significant slowdown trend of the current, which is closely related to the warming of the eastern tropical Pacific. This might have a strong impact on the decadal ENSO modulation, biological activities and out-gassing of the anthropogenic CO2in the tropical Pacific. The work was reported in an article in Nature [pdf], and Dongxiao shares the NOAA outstanding research paper award of 2004 with Dr. Michael McPhaden, the TAO project office director.

Dongxiao clearly understands the strength of numerical models in climate change study. Although state-of-the-art coupled climate models successfully simulated the global scale climate change over the last centuries, they are less satisfactory in simulating regional climate change—especially in the tropical oceans. As part of the Climate Model Evaluation Project (CMEP), he compared the observed changes in the tropical Pacific to model simulations in 14 of the different climate models submitted for the fourth IPCC report for climate simulation and projections. Dongxiao found that most of the models correctly simulated the relation of the ocean currents and sea surface temperature (SST) on decadal variability, the observed ties between trends in currents and SST are however less clear in the models. The de-coupling between ocean circulation and SST in the modeled trends might be related to the too-weak trend of hydrological cycle change simulated by these models. He strongly believe that such exercises of model-data comparison will eventually improve the models' performance in predicting regional climate variability. He is currently funded to work with scientists at NOAA/National Weather Service and NASA Goodard Institute for Space Studies to correct biases of coupled models in tropical oceans and to improve the ENSO forecast.

Working in JISAO and PMEL—both interdisciplinary institutes, Dongxiao has opportunities to work on other aspects of the ocean during climate change. He and his colleagues are working on the dissolved oxygen change in the ocean. They find that oxygen in the North Pacific thermocline have undergone large decadal variations with little trend in the last several decades, a more complicated view than a simple depleting trend of oxygen in a warming ocean. He is also collaborating with marine biogeochemical modelers to investigate the biological activity changes in the tropical Pacific associated with changes of ocean circulation. He has also worked on upper thermocline circulation and the deep Atlantic Thermohaline Circulation in the tropical Atlantic. He is interested in the circulation change on decadal-to-multidecadal time scales and their impacts on the hydrological cycle on lands.

While not working, Dongxiao enjoys the time with his family. At home, he tries his best not to talk about work with his wife, Dr. Wei Cheng, who is also a JISAO scientist, a numerical modeler, working on both large-scale global coupled climate model and coastal ocean physical-biological model. His most relaxing and enjoyable time is when his 4-year old daughter, Claire, reads books to him.