Synchronizing Antarctic sea ice models with measurements

Current speeds (in cm/s) and sea ice extent (15% concentration contour) are shown for a randomly chosen October in the high and low resolution runs for preindustrial ozone levels. Depleting ozone increases the westerly wind stress, and therefore strengthens the ocean currents. This research addresses whether the parameterized eddy response at low resolution (left panel) correctly reproduces the resolved eddy response at high resolution (right panel). Image courtesy of NICS.

Despite their similarities, the South Pole differs from its Northern counterpart in many ways. Some differences help us to characterize the Antarctic: penguins instead of polar bears, a different cast of stars in the night sky, and the notable presence of an entire continent. Other differences are more significant — and more mysterious. Unlike the Arctic, which is notoriously shedding more and more ice every year, the Antarctic sea ice is actually increasing in extent, in contradiction of existing models. And while scientists aren’t exactly sure why, they do have a few main suspects.

Investigating the mystery

The current culprit of choice is the hole in the ozone layer that hovers over the Antarctic continent. The ozone hole creates a number of natural phenomena, including the increased wind circulation which results in a lower surface temperature on the Antarctic continent and alters ocean heat transport.

The picture may be more complex than that, however, according to Cecilia Bitz, an atmospheric scientist at the University of Washington. Bitz, who was the principal investigator of the most detailed simulations of Antarctic sea ice to date, was a believer in the ozone hypothesis—until recently. 

Then her team, a branch of the National Science Foundation’s PetaApps program, ran 10-km simulations of the Antarctic sea ice using the Community Earth Systems Model to determine if, as expected, the depletion of ozone at the bottom of our planet is indeed causing the sea ice to expand.

Increasing the resolution

CESM is a fully-coupled, global climate model that provides state-of-the-art simulations of the Earth's past, present, and future climate states. It is among the most sophisticated climate models available, a global model that provides consistent simulation of high-resolution effects. However, even it has its problems, as previous models of the Antarctic sea ice showed an actual decrease in area annually in direct contrast with observations, which show an overall expansion.

Bitz’s hypothesis was that the models were too coarse, and by ramping up the resolution ten-fold her team could get to the truth. Ocean eddies, for example, are normally approximated to vary with wind strength. Bitz believed that, as a result, at coarse resolutions the model’s ocean response to increased wind strength at was incorrect, downplaying the contributions of environmental factors such as ocean currents and eddies to the overall expansion of the sea ice.

Surface temperature (in degrees Celsius) response to depleting ozone over the second half of the 20th century. The response at low resolution (left panel) is about twice as strong as at high resolution (right panel). Image courtesy of NICS.

After months of preparation, Bitz’s team began the two-month-long process of the actual simulations, which each consumed more than 11 million CPU hours on the University of Tennessee's Kraken supercomputer, and generate approximately 50 terabytes of data. Their intensive analysis of the simulation data didn't answer all of their questions, but it nonetheless yielded insight into the problem.

“A lot of the subtle behavior is different at fine resolution,” Bitz said. For example, the finer scale model shows some areas of Antarctic sea ice expanding, in keeping with real-world observations. But other areas that should be expanding in the model aren’t. Bitz believes that this may not be due to flaws in the model, but longer-term climate variability that isn’t influenced by human activity, such as the Little Ice Age.

However, one thing is clear: the model reveals that the expansion is probably not solely due to ozone. Next, Bitz said, her team will turn their attention north of the Equator, to the Arctic. For whatever reason, the North Pole has not been as well simulated as the Antarctic, Bitz said, and thus will require more effort to study.

A version of this story first appeared on the National Institute for Computational Sciences website.