Föhn winds on South Georgia and their impact on regional climate

The island of South Georgia is a small (167km long and up to 40km wide), remote subantarctic island located north of Antarctica in the Southern Ocean. South Georgia is very mountainous, with 19 peaks over 2km which roughly trends north-west to south-east. The island is also located within the centre of the belt of strong westerly winds that circumnavigate the Antarctic continent. The island acts an effective barrier to the stably stratified westerly winds that impinge upon it. If the strength of the westerlies is sufficient, strong downslope winds can develop on the lee of the island, causing dramatic temperature increases as the descending air warms adiabatically – this is known as the föhn effect.

In the last 100 years, South Georgia has undergone significant environmental changes. Average summer temperatures over South Georgia have risen by around 1oC since the 1920s. Coupled with this has been an observed increase in the rate of glacial retreat, with glaciers on the northeast side of the island (i.e. the climatological lee side) retreating at a faster rate than those on the climatological windward side. In parallel with these changes, surface westerlies have increased by about 3 m/s driven by the pronounced trend to increased positive polarity in the Southern Annular Mode (Marshall et al., 2004). Evidence of increased surface air temperatures on the lee side, asymmetrical glacial retreat and stronger air flow over the island suggest a strong link between climate change on South Georgia and an increased activity in the föhn warming process.

Although very frequent, föhn winds on the lee side of South Georgia have not previously been studied in any great detail. Data derived from observations, Automatic Weather Station measurements and high resolution model simulations with the Weather Research and Forecasting Model (WRF) down to a horizontal resolution of 0.9km are used to examine the spatial and temporal impact strong föhn winds have on the regional climate of South Georgia. A 10-year climatological analysis shows that föhn winds are very common on the island, with one warming event occurring on average every three days. Using an objective classification system based upon changes in near-surface conditions, we find that föhn winds play a significant role in controlling the overall wind and temperature regime of the island – 83% of days with a mean average 2-m temperature ≥10oC are föhn days. The occurrence of föhn events on South Georgia does not exhibit any strong seasonal variation, but more extreme events have been found to occur in the summer months, raising the near-surface air temperature to over 20oC. We also find that the occurrence of föhn events is strongly correlated to the large scale atmospheric circulation. A particularly strong föhn developed on 5th February 2013 during strong south-westerly airflow over the island. Radiosonde and LIDAR data, along with WRF simulations, are used to investigate this case study further illustrating the rapid onset and spatial extent of the leeside warming. Overall WRF satisfactorily captures the broad scale local flow and general features of föhn winds.

We conclude that strong föhn events have a significant impact on the near-surface meteorology and energy balance of glaciers, and that high resolution model simulations are required to accurately model events on South Georgia.