1. 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 de-
pends 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 oceanogra-
phy if I wanted to come back to ex-
plore 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 sim-
ulation 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 chang-
ing 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 interna-
tional collaborators in China to study
Bjerknes compensation, or how the
ocean and atmosphere transfer heat
from tropical latitudes to our high lat-
itude. 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 compensa-
tion.
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 ex-
plain 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 Sys-
tem Model simulations of the tran-
sient 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 iso-
tope-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.
Lallensack is a senior at UW-Madison
majoring in environmental studies and
journalism.
B
ecause 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.
Liu leads new era of model-data
comparison in simulating
Earth’s climate
BY RACHAEL LALLENSACK