Foehn Winds in the McMurdo Dry Valleys, Antarctica

Foehn winds are warm, dry and gusty downslope winds resulting from the topographic modification of the airstream in the lee of mountain barriers. They are a climatological feature common to many of the world’s mountainous regions, however, detailed investigations into foehn winds in polar regions and their effects on environmental processes are rare. In the McMurdo Dry Valleys (MDVs) of Antarctica, frequent episodes of strong foehn winds are experienced. Here they cause dramatic warming at onset and are suspected to significantly affect landscape forming processes, however, no detailed scientific investigation of foehn in the MDVs has been conducted. As a result, they are often misinterpreted as adiabatically warmed katabatic winds draining from the Polar Plateau. This thesis integrates observations from surface weather stations, numerical model output from the Antarctic Mesoscale Prediction System (AMPS), hydrological data and remote sensing techniques to understand the dynamics and influences of foehn wind events in the MDVs. Results show that foehn winds in the MDVs are caused by topographic modification of south-southwesterly airflow which is channelled into the valleys from higher levels. Modelling of a winter foehn event identifies mountain wave activity similar to that associated with mid-latitude foehn winds. These events are found to be caused by strong pressure gradients over the mountain ranges of the MDVs related to synoptic-scale cyclones positioned in the Amundsen/Ross Sea region. Importantly, these results clarify that a foehn mechanism is responsible for the strong warm wind events in the MDVs and that the influence of katabatic surges from the Polar Plateau as an origin or triggering mechanism of events is minimal. A 20-year climatology of foehn winds is presented from observational records in the MDVs. The intra- and inter-annual frequency and intensity of foehn events varies in response to the position and frequency of cyclones in this region. These cyclones are well known to be influenced by the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Statistically significant relationships are found between the SAM and foehn wind frequency during the Antarctic summer and autumn months whereas ENSO only holds significant correlations with winter air temperatures in the MDVs. Foehn winds are a major climatological feature of the MDVs with their frequency and duration affecting the region’s temperature records and their trends. Accordingly, analysis of the region’s weather and climate records and predictions of future impacts of climate change on the MDVs is incomplete without consideration of foehn winds and their influence. In the past, the influence of foehn wind events on landscape processes of this polar desert has not been well understood. Hydrological data and remote sensing techniques are used to quantify the influence of foehn winds on environmental processes in the MDVs. Foehn winds frequently cause summer temperatures to rise above 0°C leading to extensive melt and thaw in the MDVs. Sublimation, stream discharge and snow persistence are shown to be significantly influenced by foehn winds and the effects of foehn are demonstrated to outlive the duration of the event. It is concluded that foehn winds in the MDVs are the major cause of contemporary landscape change. Future changes in the MDVs landscape may be linked to variability in teleconnections (e.g., SAM and ENSO) and their influence on synoptic circulation patterns that drive cyclone activity in the Ross Sea region and the foehn wind regime