Johanna C. Speirs

Foehn Winds in the McMurdo Dry Valleys, Antarctica: The Origin of Extreme Warming Events

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 Nino 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.

Polar Eolian Sand Transport: Grain Characteristics Determined by an Automated Scanning Electron Microscope (QEMSCAN®)

QEMSCAN®, an automated scanning electron microscope, is used to provide a high-resolution analysis of eolian sands collected from Victoria Valley, McMurdo Dry Valleys, Antarctica. This technique provides a rapid, digital, quantitative morphological and mineralogical analysis of sediments, originally developed for the mining industry, which we apply for the first time to the study of eolian sand transport. Results show fine to medium-sized sands (<300 µm) are similar in shape and mineralogy throughout the hyper-arid landscape of the Victoria Valley. We relate this to the almost continuous mixing of fine-grained sediments in the mostly snow- and ice-free valley by frequent thermally induced easterly winds and less common but stronger topographically channeled southwesterly foehn winds. Analysis of local dune sands transported during easterly winds, which typically just exceed the local threshold entrainment velocity of 5.3 m s−1 (at 0.4 m), indicate preferential transport of quartz grains by these winds. Surface type was found to exhibit considerable influence over the characteristics of eolian sand transport with much larger grains carried in saltation and modified suspension above fluvio-glacial outwash surfaces than above sand dunes. Results illustrate the potential of QEMSCAN as an effective tool for multi-parameter analysis of eolian sands allowing greater insight into the controls on eolian sand transport in settings such as the Victoria Valley, Antarctica.

A Synoptic Classification of Inflow-Generating Precipitation in the Snowy Mountains, Australia

AbstractPrecipitation falling in the Snowy Mountains region of southeastern Australia provides fuel for hydroelectric power generation and environmental flows along major river systems, as well as critical water resources for agricultural irrigation. A synoptic climatology of daily precipitation that triggers a quantifiable increase in streamflow in the headwater catchments of the Snowy Mountains region is presented for the period 1958–2012. Here, previous synoptic-meteorological studies of the region are extended by using a longer-term, year-round precipitation and reanalysis dataset combined with a novel, automated synoptic-classification technique. A three-dimensional representation of synoptic circulation is developed by effectively combining meteorological variables through the depth of the troposphere. Eleven distinct synoptic types are identified, describing key circulation features and moisture pathways that deliver precipitation to the Snowy Mountains. Synoptic types with the highest precipitatio...

Drivers of precipitation stable oxygen isotope variability in an alpine setting, Snowy Mountains, Australia

Natural archives that preserve a stable isotopic signature are routinely used to reconstruct palaeoenvironmental conditions. Isotopic values of precipitation are known to be influenced by factors such as the amount and type of precipitation, moisture pathway, landscape and terrain factors, and processes associated with precipitation formation and deposition. This study investigates oxygen isotopic variability using real-time rain and snow precipitation data from a moderate altitude (<2250 m above sea level), Southern Hemisphere alpine environment, where the causes of isotopic variability are largely unknown. Previous research at Global Network of Isotopes in Precipitation sites skewed toward rain precipitation, low-altitude, predominantly coastal locations identified amount effects as the dominant explanation of isotopic variability in southern Australia. This study based on within- and between-event real-time sampling finds that the origin of moisture and terrain effects are the dominant cause of isotopic variability in this alpine region, with little evidence of amount effects. Rainfall that originated from similar Southern Ocean latitudes showed a consistent (moderate) isotopic signature (δ18O −6.5 to −8‰). Depleted isotopic signatures are associated with prefrontal activity and intense circulation such as east coast lows. Localized thunderstorms have a more neutral isotopic signature. A windward to leeward depletion (−0.5‰ δ18O) and an elevation impact (−0.5‰ δ18O 100 m−1) were found also. These results have significant implications for understanding atmospheric drivers of isotopic variability from which oxygen isotope-based palaeoclimate reconstruction is informed in regions with complex topography and geographically diverse moisture pathways such as the Australian Alps.