Dynamics and Variability of Foehn Winds in the McMurdo Dry Valleys Antarctica

The McMurdo Dry Valleys (“MDVs”) are the largest ice-free region in Antarctica, featuring perennially ice-covered lakes that are fed by ephemeral melt streams in the summer. The MDVs have been an NSF-funded Long-Term Ecological Research (LTER) site since 1993, and LTER research has shown that the hydrology and biology of the MDVs are extremely sensitive to small climatic fluctuations, especially during summer when temperatures episodically rise above freezing. However, the atmospheric processes that control MDVs summer climate, namely the westerly foehn and easterly sea-breeze regimes, are not well understood. The goals of this study are to (i) produce a coherent physical mechanism for the development and spatial extent of foehn winds in the MDVs, and (ii) determine aspects of large-scale climate variability responsible for intraseasonal and interannual differences in MDVs temperature. Polar WRF simulations are run for a prominent foehn case study at 500 m horizontal grid spacing to study the mesoscale components of foehn events, and 15 summers at 2 km horizontal grid spacing to analyze event and temporal variability. The Polar WRF simulations have been tailored for use in the MDVs through modifications to the input soil conditions, snow cover, land use, and sea ice. An objective foehn identification method is used to identify and categorize events, as well as validate the model against LTER AWS observations. The MDVs foehn mechanism consists of a gap wind through a topographic constriction south of the MDVs, forced by pressure differences on each side of the gap and typically set up by cyclonic flow over the Ross and Amundsen Seas. Significant mountain wave activity over the gap modulates the flow response over the MDVs themselves, and pressure-driven channeling drives foehn flow down-valley. During strongly forced events, mass accumulation east of the MDVs from flow around Ross Island is responsible for easterly intrusions, and not a thermally forced sea breeze as previously thought. A variety of ambient flow directions and associated synoptic-scale patterns can result in MDVs foehn, but adequate forcing is necessary to activate the foehn mechanism. The warmest foehn events are associated with amplified circulation patterns that are not associated with particular interannual modes of variability, but instead related to intraseasonal variability forced by the extratropical response to a stagnant MJO. Implications of the findings upon current MDVs paleoclimate theories on the existence of huge melt lakes at the LGM are also presented.