Scientists discover Westerly winds have bigger role in climate change

Their information is based on pollen and wood extracted from peat deposits on Campbell Island that show there have been major changes in the cover of tundra plants, grassland, shrub and low forest over the last 17,000 years.

“Using statistical techniques, these changes in vegetation (as indicated by pollen) can be translated into a record of summer temperatures. Two 16,500 year-long records of summer temperature have been produced for the island, one at sea level and the other near the limit to woody vegetation on the island” co-author Dr Janet Wilmshurst says.

Previous work on temperature records from the surrounding southern ocean, glacier advances in the Southern Alps, and vegetation change on the mainland of New Zealand and southern South America has been consistent in showing that warming of the Southern Ocean began at around 18,000 years ago, peaked at warmer than present day (up to 3oC warmer) temperatures between 12,000 and 8000 years ago and thereafter temperatures cooled.

In contrast, the Campbell Island results show that summer temperatures remained cooler than now until around 9000 years ago, after which they warmed to approximately present day levels. However, marine cores from the same water mass show opposite temperature trends. “It has long been thought that sea surface temperatures and land temperatures are linked. However, these results show that even on a small island, they do not necessarily track together,” Dr McGlone says.

“The explanation for the ocean-land temperature divergence of the past 16,000 years relies on changing position and intensity of the westerly winds. At present, the strongest westerly winds in the Southern Ocean lie directly over the island. The argument is that the vast north-south interchange of heat represented by these westerly winds tends to warm the atmosphere over the island in summer (relative to the ocean) and cool it in winter. We know from the absence of wind-blown silt and stones in the peat between 12,000 and 9000 years ago, that westerlies were weaker over the island at this time. Less warm air was brought south to the island in summer, but also less cold air in winter. It was therefore a time of low seasonality. When the westerlies strengthened over the island after 9000 years ago, the reverse happened, with summer warming and winter cooling. The ocean was relatively unaffected by these seasonal changes.”

Therefore, the Campbell Island past climate record suggests strongly that very large changes in the position and intensity of the westerly winds have occurred over the past 12,000 years. These in turn have major effects on rainfall, temperature and windiness over New Zealand.

Today, similar changes happen to the westerly winds in response to changes in the atmospheric pressure pattern called the Southern Annular Mode (SAM). When the SAM is in a positive state, with lower than normal atmospheric pressures over Antarctica, westerlies strengthen over Campbell Island but weaken over New Zealand; when it is in a negative state, the reverse occurs. These fluctuations have a strong effect on New Zealand weather on a weekly to annual basis.

The trend over the past few decades has been towards a more positive SAM, associated with settled, warmer summers over New Zealand. The trend is thought to be related to stratospheric ozone depletion (the ozone hole) and to greenhouse gas increase. Climate model projections suggest that the trend will continue into the future.
“On the other hand, if the past is a guide to the future, a warmer climate may lead to more negative SAM, with stronger westerlies over the New Zealand mainland, and a reduced rate of warming during summers,” Dr James Renwick from NIWA says.

“The records we have developed give us a very real insight into what the future might bring," says co-author Prof. Chris Turney of the University of Exeter.
 
Authors: Matt McGlone, Janet M.Wilmshurst (Landcare Research), Chris Turney (University of Exeter) James Renwick (NIWA) and Katharina Pahnke (University of Hawaii)