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Stronger Polar Winds May Be Cause of Antarctica's Mixed Climate Message

Antarctica is proving to be somewhat of a climate conundrum for scientists. Although the polar regions are thought to be especially sensitive to global climate change, the continent has provided many complex, and at times confounding, signals over the past few years. Most noticeably, some Antarctic areas are cooling, whereas others are warming much faster than the rest of the world. Now a report appearing in the current issue of the journal Science suggests that a change in the pattern of the westerly winds that encircle Antarctica may be behind this mixed message.

David W. J. Thompson of Colorado State University and Susan Solomon of the National Oceanic and Atmospheric Administration examined 30 years of climate data collected from surface stations and from weather balloons launched from sites around Antarctica. The readings revealed a shift in the Antarctic Oscillation (AAO), the seesawing pattern of pressure that controls the winds around the South Pole: in recent years, it seems, the AAO has been spending more time in its positive, strong-wind phase. According to the report, these shifts in circulation could account for almost 90 percent of the summertime cooling over parts of Antarctica. They can also explain more than half of the warming recorded over the Antarctic Peninsula, because "anomalously strong westerlies should act to decrease the incidence of cold air outbreaks from the south and lead to increased warm advection from the Southern Ocean."

The researchers also suggest that the hole in the ozone layer could be partially to blame for the stronger gales. Previous research had shown that the loss of ozone molecules (which absorb solar energy) cools the lower stratosphere by six degrees Celsius each spring. Moreover, this cooling strengthens westerly winds in the stratosphere in much the same way that the AAO affects winds in the troposphere, the lowest six miles of the atmosphere. Thompson and Solomon compared the timing of the changing patterns in the two layers of the atmosphere and found a strong correlation, suggesting that Antarctic ozone depletion has played an important role in driving the continent's climatic variability. Further research is required to elucidate this link, however, because ozone is most likely only one of many factors behind the gustier winds.

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