Föhn winds on South Georgia and their impact on regional climate
South Georgia is a small and mountainous island, located in the remote Southern Ocean. The island’s subantarctic climate is controlled by its location and steep orography; with 19 peaks over 2000m and situated within a belt of strong westerly winds South Georgia acts as an effective barrier to the winds that impinge upon it. Since the 1920s, average summer temperatures have risen by ~1oC on South Georgia. Coupled with this has been an increase in the rate of glacial retreat throughout the last century, with glaciers on the northeast leeside of the island retreating at a faster rate than those on the southwest side. These asymmetrical changes are thought to be linked with the strengthening of the westerlies. If the strength of the westerlies is sufficient, downslope winds can develop on the leeside of the island causing significant temperature increases as the descending air warms adiabatically; this is known as the föhn effect. Therefore, the aim of this thesis is to investigate whether the observed asymmetric pattern of regional warming and glacier retreat are caused by the föhn warming process.
To explore the link between the föhn effect and its impact on the regional climate of South Georgia, a 10 year climatology (2003 – 2012) of föhn events is created. Using automatic weather station observations to identify abrupt changes in temperature, humidity and wind speed, it is found that föhn events are frequently observed (874 events are identified in total) with one event occurring every four days. Following this, sensitivity simulations with the Weather Research and Forecasting model for four föhn cases studies are presented, with the aim of deducing the optimal model setup for South Georgia. The model analysis is largely supported by observations, and föhn flow is well captured at high (< 3.3km) horizontal resolution. With the model optimised for South Georgia, a 21 month model run (at 0.9km resolution) produces the first ever detailed regional climatology of South Georgia. The results from this simulation illustrate the asymmetrical impact of föhn on the island’s climate. During September 2011 – August 2012, modelled föhn events produced +2oC air temperature anomalies and up to 3m water equivalent ablation at the termini of South Georgia’s north-eastern glaciers. This supports the view that föhn is partly responsible for the asymmetrical retreat of glaciers, via enhanced leeside surface warming and melting. These results support the original hypothesis that an enhancement of the föhn warming process could have implications on future asymmetrical warming and melt.