The characteristics and temporal variability of föhn winds at King Edward Point, South Georgia
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 3000 m, generates warm, dry föhn winds on the downwind side of the island, which are a significant feature of the local climate. We make use of ten 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 föhn winds in this region. Using objective criteria to identify the onset and cessation of föhn, we find that KEP experiences föhn conditions for around 30% of the time. Föhn events last for about 30 hours on average and are associated with significant increases in temperature and wind speed, and decreases in relative humidity. On average, a föhn event is observed every four days, with little seasonal variation seen in the frequency of occurrence. However, föhn 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 föhn 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 föhn fraction but variations in the latter only explain 16% of the variance of monthly mean temperature. We conclude that, while föhn 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.
Authors: Bannister, Daniel ORCID record for Daniel Bannister, King, John C.