Steig on Antarctic warming: "Rossby wave trains"

From the University of Washington, comes this upcoming paper by corresponding author Dr. Eric Steig (and co-authors)that the mechanism for warming of the Antarctic Peninsula is mostly weather pattern driven. The mechanism cited? “Rossby Wave Train” aka the circumpolar vortex aka the southern polar jet steam transferring warmth from the tropical Pacific.

I’ll have more on this after the UW press release below – Anthony

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West Antarctic warming triggered by warmer sea surface in tropical Pacific

By Vince Stricherz

News and Information

The Antarctic Peninsula has warmed rapidly for the last half-century or more, and recent studies have shown that an adjacent area, continental West Antarctica, has steadily warmed for at least 30 years, but scientists haven’t been sure why.

New University of Washington research shows that rising sea surface temperatures in the area of the Pacific Ocean along the equator and near the International Date Line drive atmospheric circulation that has caused some of the largest shifts in Antarctic climate in recent decades.

The warmer water generates rising air that creates a large wave structure in the atmosphere called a Rossby wave train, which brings warmer temperatures to West Antarctica during winter and spring.

Antarctica is somewhat isolated by the vast Southern Ocean, but the new results “show that it is still affected by climate changes elsewhere on the planet,” said Eric Steig, a UW professor of Earth and space sciences and director of the UW Quaternary Research Center.

Steig is the corresponding author of a paper documenting the findings that is being published April 10 in the journal Nature Geoscience. The lead author is Qinghua Ding, a postdoctoral researcher in the UW Quaternary Research Center. Co-authors are David Battisti, a UW atmospheric sciences professor, and Marcel Küttel, a former UW postdoctoral researcher now working in Switzerland.

The scientists used surface and satellite temperature observations to show a strong statistical connection between warmer temperatures in Antarctica, largely brought by westerly winds associated with high pressure over the Amundsen Sea adjacent to West Antarctica, and sea surface temperatures in the central tropical Pacific Ocean.

They found a strong relationship between central Pacific sea-surface readings and Antarctic temperatures during winter months, June through August. Though not as pronounced, the effect also appeared in the spring months of September through November.

The observed circulation changes are in the form of a series of high- and low-pressure cells that follow an arcing path from the tropical Pacific to West Antarctica. That is characteristic of a textbook Rossby wave train pattern, Ding said, and the same pattern is consistently produced in climate models, at least during winter.

Using observed changes in tropical sea surface temperatures, the researchers found they could account for half to all of the observed winter temperature changes in West Antarctica, depending on which observations are used for comparison.

“This is distinct from El Niño,” Steig said. That climate phenomenon, which affects weather patterns worldwide, primarily influences sea-surface temperatures farther east in the Pacific, nearer to South America. It can be, but isn’t always, associated with strong warming in the central Pacific.

Steig noted that the influence of Rossby waves on West Antarctic climate is not a new idea, but this is the first time such waves have been shown to be associated with long-term changes in Antarctic temperature.

The findings also could have implications for understanding the causes behind the thinning of the West Antarctic Ice Sheet, which contains about 10 percent of all the ice in Antarctica.

Steig noted that the westerly winds created by the high pressure over the Amundsen Sea pushes cold water away from the edge of the ice sheet and out into the open ocean. It is then replaced by warmer water from deeper in the ocean, which is melting the seaward edge of the ice sheet from below.

The work was funded by the National Science Foundation.

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In the northern hemisphere, Rossby waves look like this:

Source: http://www2.pvc.maricopa.edu/ssd/geog/outlines/GPH212/chap12.html

I’d show a Southern Hemisphere example of artwork like above, but there don’t seem to be any good ones handy. It isn’t studied that much in the SH because, well, so few people are affected by the weather the polar jet brings in the SH.

Here’s what they look like from satellite imagery over the south pole:

Note that while the Antarctic peninsula is traversed by Rossby wave systems, the Antarctic main continent gets little such weather due to the strong, cold, Antarctic polar high.

 

I argued in April 2009:

What happens to Steig et al’s warming when you divide Antarctica into two distinct climate zones?

As seen in the map above, currently Antarctica is classified per the Köppen climate classification system entirely as EF, or “Ice Cap”. But here is what it might look like if the peninsula was classified differently.

Jeff Condon did an analysis at my behest, removing the Antarctic Peninsula from the trend analysis of surface data and came up with this:

Now removing the peninsula does have basis in science because the ultra thin strip of land is primarily dependent on ocean temperatures and currents. It will be seen as cherry picking because I’ve clipped the part of the Antarctic warming the most. Before TCO or someone points out that I wouldn’t clip it if it didn’t have warming, keep in mind that I show it both with and without the peninsula and I make no claim that clipping the peninsula is the preferred method. It does make some sense though.

First the full trend.