By Jonathan Amos
Science reporter, BBC News, San Francisco

US scientists have reconstructed a 40,000-year record of wind conditions at the South Pole.
They assembled the climate data by measuring the distribution of dust layers seen in two ice boreholes.

A comparison of the layers allowed the University of California-Berkeley team to gauge how rough snow surfaces would have been in ancient times.

The researchers then used this "proxy" to assess the probable strength of wind needed to produce those features.

The technique needs refinement and works best in the deeper parts of the ice. Nonetheless, scientists are confident it gives at least a broad record of conditions at the pole some 30,000 to 70,000 years ago.

And if combined with a number of palaeo-wind records gathered from around the continent, it could provide a useful tool to test climate computer models, they believe.

"At this point we can't make too many firm conclusions, but we see changes at millennial to multi-millennial timescales, where the roughness increases by a factor of five," explained Ryan Bay, a physicist at UC-Berkeley.

"There are periods where the ancient roughness is fairly low, and those are associated we think with fairly low winds. And then there are periods which are very distorted and presumably the sastrugi were much higher," he told BBC News.

On a string

Sastrugi are the meringue-like waves sculpted in the Antarctic snowscape by strong winds.

Over time, they are compacted down, along with the features of snow drifts and dunes. Dust that settles out of the atmosphere is also incorporated into the rising ice column and traces the different layers.

By mapping the vertical separations of common dust horizons in two holes, it then becomes possible to infer past surface topographies.
High winds will produce large bumps on a surface and correspondingly large dust displacements in the hole pair; low winds will produce less roughness and smaller displacements.

Dr Bay and his colleagues used optical probes to log the dust patterns by looking out into the ice from two 2,500m-deep holes sunk some 400m apart with a hot-water drill.

The holes were made to house light sensors for a neutrino observatory being built at the pole called IceCube.

Strings of these sensors must be lowered into the holes rapidly before their openings freeze over. The Bay team's probes were attached to the ends of the strings as they went down.

"We were along for the ride - but it meant sacrificing our dust loggers," said Dr Bay.
"Although there are some interesting features in our record, we need additional logs so we can constrain our systematics.

"And this coming season we're moving to re-useable dust loggers. We have a winch and we will drive up to a hole quickly, as soon as it becomes available, and, within four hours, will log down and up and get out of the way so the next string can be deployed."

The big storm

This is not the first palaeo-wind record to be produced in the Antarctic.

The Epica coring project has also produced one in the region known as Dome C. They were forced to drill two holes side- by-side when their initial cut into the ice jammed.

A leader on that study, Dr Eric Wolff, said the technique had great promise but was still in development.

"One big assumption is that wind is the only thing controlling the bumpiness of the surface, which probably isn't true and certainly hasn't been calibrated over a long time period to check," the British Antarctic Survey scientist told BBC News.

"The other big uncertainty is what wind you are actually looking at - is it average wind or the maximum wind you had for a particular year; in other words, is the surface roughness you see all shaped by a particular storm in that year?"
But with the necessary refinement and with sampling from across the continent, it should be possible to gather the data needed to test whether models of past atmospheric circulation over Antarctica are working properly, he said.

"And Ryan has shown that you could go out and get lots of these records quite rapidly and obtain the kind of spatial survey that's needed to make this important," Dr Wolff added.

The White Continent today is characterised by strong and persistent air flows which drain off the elevated ice domes of the interior. At lower coastal regions, this can produce gale-force winds that blow for weeks.

At the South Pole, average wind speeds are 1-2km/h in summer and 7-9 km/h in winter. About 50 km/h in winter would be a high.

Dr Bay has been describing his group's work here at the American Geophysical Union's Fall Meeting.