资源环境科技发展态势分析平台

The mountainous subantarctic island of South Georgia lies within the belt of strong westerly winds that blow around the Southern Ocean. Interaction of the prevailing circumpolar westerly winds with the island's central mountain chain, which rises to nearly 3,000 m, generates warm, dry fohn winds on the downwind side of the island, which are a significant feature of the local climate. We make use of 10 years of automatic weather station observations from King Edward Point (KEP), on the northeastern (climatologically downwind) coast of South Georgia, to develop a climatology of the occurrence and properties of fohn winds in this region. Using objective criteria to identify the onset and cessation of fohn, we find that KEP experiences fohn conditions for around 30% of the time. Fohn events last for about 30 hr on average and are associated with significant increases in temperature and wind speed, and decreases in relative humidity. On average, a fohn event is observed every 4 days, with little seasonal variation seen in the frequency of occurrence. However, fohn events observed during the austral summer season are, on average, both longer and more intense (as measured by changes in temperature, relative humidity, and wind speed) than those occurring in other seasons. The fraction of the time for which fohn conditions are observed at KEP increases (decreases) during months when the prevailing westerlies are strengthened (weakened). Monthly mean temperatures at KEP are positively correlated with the fohn fraction but variations in the latter only explain 16% of the variance of monthly mean temperature. We conclude that, while fohn plays an important role in shaping the local climate of South Georgia, other processes may be of greater importance in controlling regional climate variability and change.