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Natural Oscillation

There’s new research that casts doubt on recent claims ozone depletion and global warming have combined to alter atmospheric circulation patterns around the South Pole, patterns that tend to decrease Antarctica’s temperature. It now appears there were similar fluctuations before the widespread release of greenhouse gases and ozone-destroying chemicals.
To set the stage, in October 2004, Drew Shindell and Gavin Schmidt of NASA’s Goddard Institute for Space Studies (GISS) published a report in Geophysical Research Letters in which they claim Antarctica’s cooling trend of the last thirty years was caused by a combination of global warming and a hole in the ozone layer. In the paper and its accompanying NASA press materials, Shindell and Schmidt warn that this cooling trend is likely to “rapidly reverse” and result in enhanced warming over the South Pole during the next fifty years. The accompanying press release promises “ice sheets melting and sliding into the ocean” leading to “greatly increasing sea levels.”
The basis for their assertion is the Southern Hemisphere’s dominant mode of atmospheric circulation variability — the Antarctic Oscillation or Southern Annular Mode (SAM). The name describes a ring-like pattern of flow around the South Pole. Because SAM has been trending toward its positive phase for the last two or three decades, its ‘positive phase’ has become associated with general cooling over polar regions in the Southern Hemisphere.
General circulation model (GCM) simulations suggest that both ozone depletion (what commonly is referred to as the ‘ozone hole’) and greenhouse gas increases lead to such positive (cooling) trends in the SAM. Using NASA’s GISS climate model, Shindell and Schmidt test future changes in the SAM and Southern Hemisphere climate based on projected changes in ozone, greenhouse gases, and their combined effects. They claim the combination of ozone and greenhouse gas forcing in their GCM reproduces the observed cooling trend over the last two decades “quite well.”
Take a close look at Figure 1. It compares the model trends with observed trends in temperature and tells a different story. The model produces widespread mid-latitude (as opposed to polar) warming, whereas only isolated and sporadic areas of warming actually have been observed.
In another comparison (not shown), their model captures the general ring-like upper-atmospheric circulation pattern of the SAM, the magnitude of changes over the past twenty years, and the location of anomaly regions. The model produces only weak negative trends over Antarctica; and the area of strongest negative trends is completely absent in observations. Similarly, areas of the largest negative (warming) trends in observations are severely underestimated by the model which goes on to strongly overestimate positive trends in the Southern Hemisphere’s mid-latitudes and completely miss the largest area of observed positive trends.

So which is responsible for past trends in the Antarctic: ozone depletion or greenhouse gases? The answer is uncertain, at best. Both an increased concentration of greenhouse gases and depleted ozone seem to produce positive trends in the SAM. Where and when you look — and which climate model you use to do the looking — appear to dictate their relative importance.
Changes in atmospheric pressure in the mid-troposphere imply ozone accounts for most of the change. If one looks at pressure changes at the surface, however, greenhouse gases might be equally as important. Then there is the factor of seasonality in each of the signals. In other words, as in any modeling exercise, the results depend on which model is used and which atmospheric level is investigated in which season. Variations cause different results to emerge.
According to the GISS climate model results the ozone hole can be expected to recover. Shindell and Schmidt reason that such ozone ‘recovery’ will produce a negative trend in the SAM and cancel the positive effects of greenhouse gases. Although no trend in the SAM logically would suggest no pronounced changes over the next decades, Shindell and Schmidt argue that the mid-latitude warming in the Southern Hemisphere will continue. Somehow SAM, the dominant mode of atmospheric variability in the Southern Hemisphere, will cease to influence climate, according to their interpretation.
How much faith can be vested in projections of future climate using a model that only poorly reproduces past climate and produces inconclusive results? Don’t ask us; ask Julie Jones and Martin Widmann of Germany’s GKSS Research Center.
Jones and Widmann produced a 100-year reconstruction of the SAM and published their results in Nature. Their index of SAM variability shows it to have exhibited a large positive (cooling) anomaly around 1960, followed by fifteen to twenty years of strong negative (warming) trends (see Figure 2). Furthermore, SAM’s trend in recent decades is comparable to those of the 1940s to the 1960s. Between 1940–1960 it would have been impossible for chlorofluorocarbons to have played a role in eroding the ozone layer. That time period only was at the very outset of any possible human-caused warming from use of fossil fuels. In other words, the observed trend over recent decades is not unprecedented. Any suggestion that ozone and/or greenhouse gases have altered the SAM is therefore questionable. Natural variability has produced similar trends in the absence of anthropogenic forcing.
Jones’ and Widmann’s research also seriously brings into question Shindell’s and Schmidt’s gloom-and-doom scenario for Antarctic ice and rising sea levels. If a shift from a positive to a neutral SAM phase — as predicted by the GISS model — will “rapidly reverse” the temperature trends over Antarctica, why didn’t the completely reversed, negative SAM phase from 1960 to the mid-1980s have the same effect when combined with anthropogenic warming and ozone depletion from the 1970s onward?

References:
Shindell D. T. and G. A. Schmidt, 2004: Southern Hemisphere climate response to ozone changes and greenhouse gas increases. Geophys. Res. Lett., 31, L18209, doi:10.1029/2004GL020724.

Jones, J. M. and M. Widmann, 2004: Early peak in Antarctic oscillation index. Nature, 432, 290–291, doi:10.1038/432290b..