Liu leads new era of model-data comparison in simulating Earth's climate
December 15, 2015
By Rachael Lallensack
Because Professor Zhengyu Liu’s background is concentrated in oceanography, some people wonder how he ended up in Madison.
"They say, 'That's really weird. There are no oceans in the Midwest,'" Liu joked.
That doesn’t prevent Liu from applying an oceanic perspective to his climate research. He studies the interaction between the atmosphere, the ocean system and the climate.
This comprehensive approach comes through in his expansive knowledge of El Nino — the weather pattern now gathering steam in the Pacific Ocean, portending a range of global impacts, including the warm, dry winter Madison will likely experience this year.
With that in mind, the Nelson Institute Center for Climatic Research has been a perfect fit for Liu for the past 22 years. As a past director of the center, he has helped maintain CCR's record as a world leader in historical climate modeling and improving the models’ predictive power. Liu recently chatted about how his work began and where it’s headed.
How did you originally become interested in studying the ocean?
As a graduate student, I was initially intrigued by El Nino. I was studying meteorology before, but realized when we talk about El Nino, it really depends on both the ocean as well as the atmosphere, not just one of them at a time. You have to understand how they interact.
I thought this was so interesting, so I knew I had to study some oceanography if I wanted to come back to explore the complete ocean-atmosphere system. That’s how I ended up with a Ph.D. in oceanography.
What are you currently researching?
I’m working along several lines. One is how the climate has evolved since the Last Glacial Maximum, about 21,000 years ago. Global climate has changed dramatically in the last 21,000 years, including El Nino. I studied the first climate model simulation of El Nino evolution in the last 21,000 years and found that the change of El Nino can be traced to the South Pacific Ocean, where the water temperature is changed by the changing solar radiation.
In short, we are studying how El Nino is excited locally in the equatorial Pacific, or remotely, from outside the tropical Pacific.
I'm also working with some international collaborators in China to study Bjerknes compensation, or how the ocean and atmosphere transfer heat from tropical latitudes to our high latitude. The current hypothesis [named for meteorologist Jacob Bjerknes, the first to make this suggestion] is that when one transfer increases, the other transfer will decrease, as a compensation.
That, however, is only a hypothesis, and there has never been a theory to explain why that happens, and under what conditions it happens, so we’re developing a theory that might explain it. We’re also verifying it with complex models. It’s a fundamental issue in understanding the climate. This has been a classical problem for many years; the first time it was raised was in 1964.
Are there new projects on the horizon that you’re especially excited about?
I’ve recently started working on the first set of isotope-enabled Earth System Model simulations of the transient climate and isotope evolution. In collaboration with other scientists, I am building a new generation of this state-of-the-art climate model that incorporates key isotopic geotracers -- notably, water isotopes and carbon isotopes.
This is important because an isotope-enabled climate simulation will allow for a direct comparison of proxy data [from natural recorders of past climate conditions, such as ice cores and fossil pollen] with the model, and therefore reduce the great uncertainty of proxy interpretation. This marks a new era of model-data comparison.